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bobair(at)me.com
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 Posted: Wed Jan 26, 2011 1:36 pm Post subject: Comm Antenna & SWR: More Information |
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Bob N., et al,
I've been doing some reading re my antenna issue and here's what I've found so far (in no particular order).
Cobham/Comant has some good information on installing their antennas. See
Installation Guides (including PDF of AC43.13-2B):
http://www.cobham.com/about-cobham/avionics-and-surveillance/about-us/satcom/fullerton/installation-guide.aspx
FAQs
http://www.cobham.com/about-cobham/avionics-and-surveillance/about-us/satcom/fullerton/faqs.aspx
Data Sheet for the subject Comm Antenna that I'm using (Comant CI-122)
http://www.cobham.com/media/9104/ci%20122%20data%20sheet.pdf
Cobham/Comant's installation instructions follow AC43.13-2B fairly closely and, in fact, refer to that FAA Advisory Circular in several places.
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance?
The ground plane is also primed with "rattle-can" self-etching primer. But as Bob N. supposed, I do have an access hole in the ground plane for the antenna BNC connector centered among the four 10-32 stainless machine screws in the antenna base. I used a washer stack to get as close as possible to the cored composite shell thickness and the hardware is clamped on the antenna base, through the non-cored fiberglass exterior skin, the washer stack and the ground plane. The ground plane metal is "bright" under the washers. Cobham/Comant endorses this approach (see FAQ #6 at http://www.cobham.com/about-cobham/avionics-and-surveillance/about-us/satcom/fullerton/faqs.aspx#171).
Cobham/Comant says the resistance between the ground plane and the antenna connector should be 3 milliohms (see http://www.cobham.com/media/5051/compositeaircraftinstallation.pdf). My ohmmeter only reads to 0.1 Ohm and I can't measure lower than that without making up one of Bob N's four-wire ohmmeters.
Antenna choice, revisit?
Cobham/Comant isn't very positive about bent whip antennas for Comm 1. They say "Bent whips may not provide the best VSWR because of proximity to the ground plane (aircraft skin), which can cause reflections, plus they lack overall height. Remember, VHF antennas “like” to be tall and straight for best operation." See FAQ #2.
Other than the antenna selection issue itself, this reading leads me to three additional areas to investigate: ground plane symmetry, ground plane size, and antenna mounting location on ground plane.
Re ground plane symmetry...
AC43.13-2B, paragraph 310(b) on page 27 says that "...ground plane symmetry is critical." There is no reason or engineering data given but its a fairly direct statement. After re-running the tests with my 24" x 34" practice ground plane, I may cut it back to 24" x 24" to see what VSWR readings I get with that size.
Re ground plane size...
I understand that, in theory, the ideal ground plane is of infinite dimension. (For an aside, see quote below.) Bob N. says the practical ideal is a disc with a radius equal to one-quarter of the wavelength. An on-line calculator told me that the wavelength at 122.95 MHz is 8'. One fourth of that is 2'. But that means the idealized disc should have a diameter of 4'. My ground plane is only 24" wide -- half of what it should be. It isn't feasible to put a ground plane 4' wide in my plane -- most of it would have to curl up the sides of the fuselage which would defeat the purpose anyhow. Moreover, AC43.13-2B, paragraph 310(b), page 27, says most antennas require a ground plane size of approximately 24" x 24".
Re antenna mounting location on ground plane...
In addition to the issue of the symmetry of the ground plane itself, will it make a difference in VSWR if the antenna base is not mounted in the center of the ground plane. As I mentioned in my earlier post, I mounted the base about 1/3 of the distance along the long axis so that the entire bent whip would be "covered" by the ground plane. But if, electrically, the bent whip really behaves like the straight piece of 12 AWG wire Bob N. used in his experiment, then that seems to indicate that the antenna base needs to be installed in the center of the ground plane, whether the ground plane is square or rectangular.
Next steps...
I'm finishing up the final terminations on my feed lines and will jury-rig an external ground plane in order to re-do the tests outside the airplane as Bob N. suggested. I hope to have the new test data to you in the next couple of days.
Bonus data point re VOR Antenna installation direction...
Cobham/Comant says the dipoles should be pointed forward. See http://www.cobham.com/about-cobham/avionics-and-surveillance/about-us/satcom/fullerton/faqs.aspx#171
Do VOR/GS V-Dipoles such as the CI 158C need to be “pointed” forward or aft ? For best results, the radiating elements should be directed forward, as the emitted pattern needs to “see” forward and downward. However, some aircraft owners believe they provide a better appearance if the radiators are pointed aft. If mounted in this manner, the installer should flight check for proper antenna performance.
(I installed my pointing aft because it looks better. Now I'll have to pay extra attention on the flight check.)
Best regards,
Bob Falstad
GlaStar N248BF
"In theory, there is no difference between theory and practice. In practice, there is." (I've heard this attributed to various people, most often Yogi Berra.)
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jan 26, 2011 7:18 pm Post subject: Comm Antenna & SWR: More Information |
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Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance?
Not much . . . if any. Let's consider the way
a ground plane works. In an earlier post I
explained how the sum total of 4 'prefect'
radials routed 80% plus of energy delivered
to the end of the coax to be delivered to
the antenna.
Now, if you draw say 100 equally spaced lines
from the antenna location on your ground plane
out to the edge, we can see that the minimum 'radial'
length is on the order of 12". No single 12" radial
is very useful . . . but there's a lot of them.
The longest radial goes out to 2 or more corners.
They might be as short as 20' but perhaps longer
if the antenna is not directly centered on the sheet.
[img]cid:.0[/img]
Obviously, the ground plane is not "ideal". But
then lots of satisfactory antenna installations
were 'grounded' to small plates welded to the
structure of a rag-n-tube airplane. Terrible
radials.
Your ground plane sheet is blessed with a few
pretty good radials and lots of not so good
but the SUM TOTAL of their effects at the base
of the antenna is substantial . . . and probably
quite adequate.
With respect to 'measuring' resistance between
the antenna base and the airframe, know that
very few installations get such treatment.
Practiced installers know that certain processes
ALWAYS yield good bonding. Further, those same
processes yield joints with good longevity in
service.
If your coax is good, the connectors are properly
installed AND you apply reliable processes for
getting high pressure connection between antenna
base and ground plane, an micro-ohmmeter test will
be quite predictable and therefore redundant.
Antenna choice, revisit?
Cobham/Comant isn't very positive about bent whip antennas for Comm 1. They say "Bent whips may not provide the best VSWR because of proximity to the ground plane (aircraft skin), which can cause reflections, plus they lack overall height. Remember, VHF antennas “like” to be tall and straight for best operation." See FAQ #2.
Not true. It's the current flowing in about the bottom
20% of the antenna element that controls most of
an antenna's performance. In fact, it's common practice
to mechanically shorten an antenna with "capacity
hats" that act strongly in low current, high voltage
portion of the antenna element.
Bending the top of the antenna horizontal has little
effect on radiation pattern but will lower the frequency
of resonance.
Other than the antenna selection issue itself, this reading leads me to three additional areas to investigate: ground plane symmetry, ground plane size, and antenna mounting location on ground plane.
Go for the bonding . . . get that right and you're
98% done.
Re ground plane symmetry...
AC43.13-2B, paragraph 310(b) on page 27 says that "...ground plane symmetry is critical." There is no reason or engineering data given but its a fairly direct statement. After re-running the tests with my 24" x 34" practice ground plane, I may cut it back to 24" x 24" to see what VSWR readings I get with that size.
Please don't do this. AC43-13 was NOT crafted by
skilled practitioners of the arts or science of
building airplanes . . . That's another story.
See page 2 of
http://www.aeroelectric.com/articles/Getting_Started.pdf
Re ground plane size...
I understand that, in theory, the ideal ground plane is of infinite dimension.
That's the 'perfect'. . . the 'ideal' is more practical
(For an aside, see quote below.) Bob N. says the practical ideal is a disc with a radius equal to one-quarter of the wavelength. An on-line calculator told me that the wavelength at 122.95 MHz is 8'. One fourth of that is 2'. But that means the idealized disc should have a diameter of 4'. My ground plane is only 24" wide -- half of what it should be. It isn't feasible to put a ground plane 4' wide in my plane -- most of it would have to curl up the sides of the fuselage which would defeat the purpose anyhow. Moreover, AC43.13-2B, paragraph 310(b), page 27, says most antennas require a ground plane size of approximately 24" x 24".
Which supports the assertions I made earlier about
AC43-13 . . . 24 x 24 is a practical 'ideal' ground
plane for 174 Mhz given that the 4 best radials are
only 17" long.
Re antenna mounting location on ground plane...
In addition to the issue of the symmetry of the ground plane itself, will it make a difference in VSWR if the antenna base is not mounted in the center of the ground plane. As I mentioned in my earlier post, I mounted the base about 1/3 of the distance along the long axis so that the entire bent whip would be "covered" by the ground plane. But if, electrically, the bent whip really behaves like the straight piece of 12 AWG wire Bob N. used in his experiment, then that seems to indicate that the antenna base needs to be installed in the center of the ground plane, whether the ground plane is square or rectangular.
Next steps...
I'm finishing up the final terminations on my feed lines and will jury-rig an external ground plane in order to re-do the tests outside the airplane as Bob N. suggested. I hope to have the new test data to you in the next couple of days.
A good experiment . . . if you're really curious and
want to do it.
Bob . . .
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BobsV35B(at)aol.com
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| Posted: Wed Jan 26, 2011 7:48 pm Post subject: Comm Antenna & SWR: More Information |
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Good Evening All,
This discussion about ground planes has forced me to send another one of my way off subject messages! <G>
Some of you may be aware of the speed modifications that Mike Smith was selling for Bonanzas a few years ago. While Bonanzas are certainly NOT homebuilt aircraft, Mike's work was very much in the realm of what homebuilders do. He guaranteed a speed increase of at least ten knots on any Bonanza that used his full set of speed kits.
In his quest for speed, he took EVERYTHING off the top of the fuselage and as much off the belly as humanly possible. He even installed a VHF communication antenna in a fiberglass tail cone. It was horizontally oriented rather than vertical and the ground plane was no more than six inches by eight inches.
Did it work? Some folks found that it was a perfectly usable solution and are still using it regularly. Others have either eliminated it or added another stock antenna on the belly.
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought.
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman 3977A
Do Not Archive
In a message dated 1/26/2011 9:19:32 P.M. Central Standard Time, nuckolls.bob(at)aeroelectric.com writes:
| Quote: |
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance?
|
[quote][b]
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dsvs(at)ca.rr.com
Guest
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| Posted: Wed Jan 26, 2011 8:08 pm Post subject: Comm Antenna & SWR: More Information |
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One more antenna that is not done right and still works fine. The ELT on many RV’s is horizontal and inside the tail cone fairing. Or in the baggage area with only the top few inches not close to metal. Neither of these install techniques have good ground planes nor the correct orientation but they work with both the 121.5 and the 406 versions of ELT
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of BobsV35B(at)aol.com
Sent: Wednesday, January 26, 2011 7:44 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Comm Antenna & SWR: More Information
Good Evening All,
This discussion about ground planes has forced me to send another one of my way off subject messages! <G>
Some of you may be aware of the speed modifications that Mike Smith was selling for Bonanzas a few years ago. While Bonanzas are certainly NOT homebuilt aircraft, Mike's work was very much in the realm of what homebuilders do. He guaranteed a speed increase of at least ten knots on any Bonanza that used his full set of speed kits.
In his quest for speed, he took EVERYTHING off the top of the fuselage and as much off the belly as humanly possible. He even installed a VHF communication antenna in a fiberglass tail cone. It was horizontally oriented rather than vertical and the ground plane was no more than six inches by eight inches.
Did it work? Some folks found that it was a perfectly usable solution and are still using it regularly. Others have either eliminated it or added another stock antenna on the belly.
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought.
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman 3977A
Do Not Archive
In a message dated 1/26/2011 9:19:32 P.M. Central Standard Time, nuckolls.bob(at)aeroelectric.com writes:
| Quote: |
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance? http://www.matronics.com/Navigator?AeroElectric-List |
01234567
[quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Jan 27, 2011 8:20 am Post subject: Comm Antenna & SWR: More Information |
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| Quote: |
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought. |
And good data it is.
Folks often get their airplane antenna issues stirred into
a stew of some good and not-so-good data about "antenna
struggles". The majority of data in the wild comes from
folks wanting to watch tv from a station 60 miles away,
a ham operator who wishes to extract .5 microvolt signals
from a universe of noise, and the two way radio company
attempting to satisfy a fleet customer with 100 vehicles
that wander afar from home base.
All of these situations might gain a measure of improvement
by fine tuning swr, radiation patterns, coax losses,
installation losses, etc.
THEN you have airplanes. Airplanes almost never offer
opportunities to install the best antennas we know how
to do for reasons unique to airplanes. Weird shapes,
not enough acreage, aerodynamic buffeting, aerodynamic
drag, and general ugliness on an otherwise "fast" looking
airplane.
The really cool thing about airplane antennas is that
a wet string hung out the window would probably
suffice for 90% of the aviator's communications needs.
We got into a discussion on the List many years ago
about the relative utility of the new line of miniature
transceivers with puny transmitters . . . 1 to 2 watts.
There was a lot of concern for overall communications
performance. However, most worries were grounded in
memories of having witnessed John Wayne's harrowing
experiences exacerbated by too week a radio signal while
wrestling with 5 bad guys and a string of simultaneous
systems failures.
The environmental influences on aviation signal
strength are (1) line of sight limitations due to curvature
of the earth, (2) path loss (attenuation as a function
of distance and frequency) and (3) environmental
noise at the receive end. It turns out that a couple of
1 watt transceivers talking to each other on rudimentary
antennas had a range on the order of 1000 miles in free space.
As a practical matter, we rarely need to talk with
a facility that is more than 50 miles away. For an
airplane at 2000 AGL talking to a facility with 40'
high antennas, line-of-sight distance is 70+
miles. Bottom line: the NEED for high power
and finely tuned performance in antennas/feedlines
for the way we use radios in our airplanes is
exceedingly rare.
This discussion about SWR, etc is certainly
valuable for understanding the simple-ideas that
drive system performance. At the same time, I hope
that Bob doesn't spend a great deal of time seeking
the holy grail of 1:1 SWR, perfectly circular radiation
patterns and zero loss coax.
Nearly a century of practical experience has demonstrated
that (1) the grail is not attainable and (2) wouldn't
make an observable difference if it were attained.
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Jan 27, 2011 8:22 am Post subject: Comm Antenna & SWR: More Information |
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At 11:05 PM 1/26/2011, you wrote:
| Quote: | One more antenna that is not done right and still works fine. The
ELT on many RV's is horizontal and inside the tail cone fairing. Or
in the baggage area with only the top few inches not close to
metal. Neither of these install techniques have good ground planes
nor the correct orientation but they work with both the 121.5 and
the 406 versions of ELT
|
Excellent point. This feeds directly into the assertions
I made in the post a few minutes ago. The satellite flying
overhead has no line-of-sight limitations nor is it plagued
with line noise, ignition noise and strong local radio
stations. It's as close as you can get to the ideal
receiving environment.
This is a large component of the recipe for success
in spite of poorly crafted or even crash-damaged
beacon antennas.
Bob . . .
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dsvs(at)ca.rr.com
Guest
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| Posted: Thu Jan 27, 2011 3:49 pm Post subject: Comm Antenna & SWR: More Information |
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Hi Bob,
Don Panier at www.deltapopaviation.cpm makes and sells com, nav, adsb aprs
and transponder antennas that take their connection to the ground plane from
the mounting screws. The antenna does not have any metal contact on the
base other than the mounting screws. These work well in composite and metal
ac.
--
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dsvs(at)ca.rr.com
Guest
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| Posted: Thu Jan 27, 2011 4:05 pm Post subject: Comm Antenna & SWR: More Information |
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Make that www.deltapopaviation.com
--
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Thu Jan 27, 2011 4:07 pm Post subject: Comm Antenna & SWR: More Information |
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The only ground plane I know of which is close to infinite is salt water. Fibre glass boats commonly use the ocean itself as a ground plane for radio... Believe me they can have problems too.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert L. Nuckolls, III
Sent: January 26, 2011 10:44 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Comm Antenna & SWR: More Information
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance?
Not much . . . if any. Let's consider the way
a ground plane works. In an earlier post I
explained how the sum total of 4 'prefect'
radials routed 80% plus of energy delivered
to the end of the coax to be delivered to
the antenna.
Now, if you draw say 100 equally spaced lines
from the antenna location on your ground plane
out to the edge, we can see that the minimum 'radial'
length is on the order of 12". No single 12" radial
is very useful . . . but there's a lot of them.
The longest radial goes out to 2 or more corners.
They might be as short as 20' but perhaps longer
if the antenna is not directly centered on the sheet.
[img]cid:image001.jpg(at)01CBBE61.979439D0[/img]
Obviously, the ground plane is not "ideal". But
then lots of satisfactory antenna installations
were 'grounded' to small plates welded to the
structure of a rag-n-tube airplane. Terrible
radials.
Your ground plane sheet is blessed with a few
pretty good radials and lots of not so good
but the SUM TOTAL of their effects at the base
of the antenna is substantial . . . and probably
quite adequate.
With respect to 'measuring' resistance between
the antenna base and the airframe, know that
very few installations get such treatment.
Practiced installers know that certain processes
ALWAYS yield good bonding. Further, those same
processes yield joints with good longevity in
service.
If your coax is good, the connectors are properly
installed AND you apply reliable processes for
getting high pressure connection between antenna
base and ground plane, an micro-ohmmeter test will
be quite predictable and therefore redundant.
Antenna choice, revisit?
Cobham/Comant isn't very positive about bent whip antennas for Comm 1. They say "Bent whips may not provide the best VSWR because of proximity to the ground plane (aircraft skin), which can cause reflections, plus they lack overall height. Remember, VHF antennas “like” to be tall and straight for best operation." See FAQ #2.
Not true. It's the current flowing in about the bottom
20% of the antenna element that controls most of
an antenna's performance. In fact, it's common practice
to mechanically shorten an antenna with "capacity
hats" that act strongly in low current, high voltage
portion of the antenna element.
Bending the top of the antenna horizontal has little
effect on radiation pattern but will lower the frequency
of resonance.
Other than the antenna selection issue itself, this reading leads me to three additional areas to investigate: ground plane symmetry, ground plane size, and antenna mounting location on ground plane.
Go for the bonding . . . get that right and you're
98% done.
Re ground plane symmetry...
AC43.13-2B, paragraph 310(b) on page 27 says that "...ground plane symmetry is critical." There is no reason or engineering data given but its a fairly direct statement. After re-running the tests with my 24" x 34" practice ground plane, I may cut it back to 24" x 24" to see what VSWR readings I get with that size.
Please don't do this. AC43-13 was NOT crafted by
skilled practitioners of the arts or science of
building airplanes . . . That's another story.
See page 2 of
http://www.aeroelectric.com/articles/Getting_Started.pdf
Re ground plane size...
I understand that, in theory, the ideal ground plane is of infinite dimension.
That's the 'perfect'. . . the 'ideal' is more practical
(For an aside, see quote below.) Bob N. says the practical ideal is a disc with a radius equal to one-quarter of the wavelength. An on-line calculator told me that the wavelength at 122.95 MHz is 8'. One fourth of that is 2'. But that means the idealized disc should have a diameter of 4'. My ground plane is only 24" wide -- half of what it should be. It isn't feasible to put a ground plane 4' wide in my plane -- most of it would have to curl up the sides of the fuselage which would defeat the purpose anyhow. Moreover, AC43.13-2B, paragraph 310(b), page 27, says most antennas require a ground plane size of approximately 24" x 24".
Which supports the assertions I made earlier about
AC43-13 . . . 24 x 24 is a practical 'ideal' ground
plane for 174 Mhz given that the 4 best radials are
only 17" long.
Re antenna mounting location on ground plane...
In addition to the issue of the symmetry of the ground plane itself, will it make a difference in VSWR if the antenna base is not mounted in the center of the ground plane. As I mentioned in my earlier post, I mounted the base about 1/3 of the distance along the long axis so that the entire bent whip would be "covered" by the ground plane. But if, electrically, the bent whip really behaves like the straight piece of 12 AWG wire Bob N. used in his experiment, then that seems to indicate that the antenna base needs to be installed in the center of the ground plane, whether the ground plane is square or rectangular.
Next steps...
I'm finishing up the final terminations on my feed lines and will jury-rig an external ground plane in order to re-do the tests outside the airplane as Bob N. suggested. I hope to have the new test data to you in the next couple of days.
A good experiment . . . if you're really curious and
want to do it.
Bob . . .
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Thu Jan 27, 2011 4:26 pm Post subject: Comm Antenna & SWR: More Information |
|
|
Again you have to ask is the signal going full force in the direction you want? It makes no sense to set up a properly tuned antenna in what is essentially a faraday cage.
An ELT wants to be seen. That means an omni-directional output. With the antenna in the tailcone you may not be seen at an airport two miles directly behind your plane. You chances of being seen by a station in front of you are even less.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Don
Sent: January 27, 2011 12:36 AM
To: aeroelectric-list(at)matronics.com
Subject: RE: Comm Antenna & SWR: More Information
One more antenna that is not done right and still works fine. The ELT on many RV’s is horizontal and inside the tail cone fairing. Or in the baggage area with only the top few inches not close to metal. Neither of these install techniques have good ground planes nor the correct orientation but they work with both the 121.5 and the 406 versions of ELT
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of BobsV35B(at)aol.com
Sent: Wednesday, January 26, 2011 7:44 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Comm Antenna & SWR: More Information
Good Evening All,
This discussion about ground planes has forced me to send another one of my way off subject messages! <G>
Some of you may be aware of the speed modifications that Mike Smith was selling for Bonanzas a few years ago. While Bonanzas are certainly NOT homebuilt aircraft, Mike's work was very much in the realm of what homebuilders do. He guaranteed a speed increase of at least ten knots on any Bonanza that used his full set of speed kits.
In his quest for speed, he took EVERYTHING off the top of the fuselage and as much off the belly as humanly possible. He even installed a VHF communication antenna in a fiberglass tail cone. It was horizontally oriented rather than vertical and the ground plane was no more than six inches by eight inches.
Did it work? Some folks found that it was a perfectly usable solution and are still using it regularly. Others have either eliminated it or added another stock antenna on the belly.
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought.
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman 3977A
Do Not Archive
In a message dated 1/26/2011 9:19:32 P.M. Central Standard Time, nuckolls.bob(at)aeroelectric.com writes:
| Quote: |
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance? http://www.matronics.com/Navigator?AeroElectric-List http://forums.matronics.com http://www.matronics.com/contribution |
012345678901234567
[quote][b]
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
|
| Posted: Thu Jan 27, 2011 4:26 pm Post subject: Comm Antenna & SWR: More Information |
|
|
The important thing with com antennas is good omnidirectional ( all directions ) coverage. A horizontally polarized antenna in a tailcone, outside of having too small a ground plane, will have a dead area in front of the plane and another one behind it. I can’t see how this can be considered good for airplanes heading into or away from a control zone.
Many large aircraft actually have antennae above and below the fuselage to allow communication with other aircraft as well as ground.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of BobsV35B(at)aol.com
Sent: January 27, 2011 12:14 AM
To: aeroelectric-list(at)matronics.com
Subject: Re: Comm Antenna & SWR: More Information
Good Evening All,
This discussion about ground planes has forced me to send another one of my way off subject messages! <G>
Some of you may be aware of the speed modifications that Mike Smith was selling for Bonanzas a few years ago. While Bonanzas are certainly NOT homebuilt aircraft, Mike's work was very much in the realm of what homebuilders do. He guaranteed a speed increase of at least ten knots on any Bonanza that used his full set of speed kits.
In his quest for speed, he took EVERYTHING off the top of the fuselage and as much off the belly as humanly possible. He even installed a VHF communication antenna in a fiberglass tail cone. It was horizontally oriented rather than vertical and the ground plane was no more than six inches by eight inches.
Did it work? Some folks found that it was a perfectly usable solution and are still using it regularly. Others have either eliminated it or added another stock antenna on the belly.
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought.
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman 3977A
Do Not Archive
In a message dated 1/26/2011 9:19:32 P.M. Central Standard Time, nuckolls.bob(at)aeroelectric.com writes:
| Quote: |
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance? http://www.matronics.com/Navigator?AeroElectric-List |
01234567
[quote][b]
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BobsV35B(at)aol.com
Guest
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| Posted: Thu Jan 27, 2011 5:28 pm Post subject: Comm Antenna & SWR: More Information |
|
|
Good Evening Noel,
I don't think anyone will disagree with what you say, but the point is that a lot of good competent and concientuous folks have found that the crummy location does work acceptably. You and I can agree it is not good, but if it works for others, are we supposed to tell them it is not allowed?
As 'Lectric Bob has said, chances are a wet noodle will work most of the time.
While I know folks who are still using the stick in the tail cone, I know more who have given up on it and gone to a more conventional antenna.
As Always.It All Depends! <G>
Do Not Archive
Happy Skies,
Old Bob
In a message dated 1/27/2011 6:27:12 P.M. Central Standard Time, noelloveys(at)yahoo.ca writes:
| Quote: |
The important thing with com antennas is good omnidirectional ( all directions ) coverage. A horizontally polarized antenna in a tailcone, outside of having too small a ground plane, will have a dead area in front of the plane and another one behind it. I can’t see how this can be considered good for airplanes heading into or away from a control zone.
Many large aircraft actually have antennae above and below the fuselage to allow communication with other aircraft as well as ground.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of BobsV35B(at)aol.com
Sent: January 27, 2011 12:14 AM
To: aeroelectric-list(at)matronics.com
Subject: Re: AeroElectric-List: Comm Antenna & SWR: More Information
Good Evening All,
This discussion about ground planes has forced me to send another one of my way off subject messages! <G>
Some of you may be aware of the speed modifications that Mike Smith was selling for Bonanzas a few years ago. While Bonanzas are certainly NOT homebuilt aircraft, Mike's work was very much in the realm of what homebuilders do. He guaranteed a speed increase of at least ten knots on any Bonanza that used his full set of speed kits.
In his quest for speed, he took EVERYTHING off the top of the fuselage and as much off the belly as humanly possible. He even installed a VHF communication antenna in a fiberglass tail cone. It was horizontally oriented rather than vertical and the ground plane was no more than six inches by eight inches.
Did it work? Some folks found that it was a perfectly usable solution and are still using it regularly. Others have either eliminated it or added another stock antenna on the belly.
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought.
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman 3977A
Do Not Archive
In a message dated 1/26/2011 9:19:32 P.M. Central Standard Time, nuckolls.bob(at)aeroelectric.com writes:
| Quote: |
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance? http://www.matronics.com/Navigator?AeroElectric-List |
01234567
8 |
[quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Jan 27, 2011 6:46 pm Post subject: Comm Antenna & SWR: More Information |
|
|
| Quote: | The antenna does not have any metal contact on the
base other than the mounting screws. These work well in composite and metal
ac.
|
Virtually ALL comm antennas make ground plane
connections only through their mounting screws.
I've seen installation manuals suggest that the
skin of the airplane under the base of an antenna
be cleaned to the bare metal . . . ostensibly for
the purpose of improving on the antenna to airplane
bonding.
Bonding is achieved by HIGH PRESSURE joints in
metallic fasteners. I crafted this drawing about
5 years ago. . .
http://aeroelectric.com/Pictures/Antenna/Comm_Antenna_Installation.gif
I DID suggest that the stock gasket shipped with
many antennas be discarded and that the base be
moisture sealed to the skin with a bead of RTV.
However, you'll not an emphasis on clean metal
circles around each of the mounting fasteners
where we'll exploit the pressure that cranking
down on the nut will offer.
Stray outside the clamp-up radius around each
bolt very far, and any notion of long term, high
quality boding between the parts becomes a fantasy.
Bob . . .
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nuckolls.bob(at)aeroelect
Guest
|
| Posted: Thu Jan 27, 2011 6:53 pm Post subject: Comm Antenna & SWR: More Information |
|
|
At 07:12 PM 1/27/2011, you wrote:
| Quote: | The important thing with com antennas is good omnidirectional ( all
directions ) coverage.
|
"Good" is non-quantified. I've seen horizontal pattern
plots for comm antennas that had large (10db or more)
variations around the compass . . . but these antennas
proved adequate to the performance necessary.
| Quote: | A horizontally polarized antenna in a tailcone, outside of having
too small a ground plane, will have a dead area in front of the
plane and another one behind it.
|
???? The VOR antenna whiskers are horizontally polarized
dipole with major lobes fore and aft with nulls off to the
sides. Further, dipole antennas require no ground plane.
| Quote: | I can't see how this can be considered good for airplanes heading
into or away from a control zone.
|
No such "null" exists fore and aft.
| Quote: |
Many large aircraft actually have antennae above and below the
fuselage to allow communication with other aircraft as well as ground.
|
???? Antenna placement is largely a matter of separation
between the antennas for similar systems . . . and judicious
use of real estate. Once an airplane is airborne, relative
differences between top and bottom mounted antennas is
insignificant.
Bob . . .
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dsvs(at)ca.rr.com
Guest
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| Posted: Thu Jan 27, 2011 7:43 pm Post subject: Comm Antenna & SWR: More Information |
|
|
Thanks Bob. I was led to believe that the antenna and skin needed tp be
paint free and making good contact. This is what the instructions that came
with my commant and Avidyne Ryan antennas had to say. Glad that I mow
understand the actual requirements.
--
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Jan 27, 2011 8:18 pm Post subject: Comm Antenna & SWR: More Information |
|
|
ELT's are designed to be seen by satellites. Until
the present 405 Mhz with superior locating capability
came along the
I recall hearing a report about 20 years ago that
for EVERY downed airplane located, ELT was a factor
in a very small percentage of those finds . . . about
6% if I recall correctly. Don't know how much better
the stats are now.
In any case installation of ELT antennas is a
"gee I hope this works when and if they
need it" kind of design task. There is no
place on an airplane that is immune from
crash-induced damage. Equipment placement
is based on the study of crash history.
So when on short final to the rocks, one
should endeavor to crash like most other
folks crashed such that their ELT survived.
Shadowing to the extent that some ground
based facility two miles away doesn't hear
it is the very least of concerns.
| Quote: |
One more antenna that is not done right and still works fine. The ELT on many RV’s is horizontal and inside the tail cone fairing. Or in the baggage area with only the top few inches not close to metal. Neither of these install techniques have good ground planes nor the correct orientation but they work with both the 121.5 and the 406 versions of ELT |
On what basis were these installations said
to "work fine". Have these installations
been reviewed by the ELT manufacturer? I
can't imagine anyone signing off on an
installation that didn't provide the customary
ground plane combined with a good view of the
sky assuming the wreckage was still upright.
[img]cid:.0[/img]
Here's the ELT antennas on a Beechjet. Both
are installed under a fiberglas fairing just
forward of the vertical fin.
[img]cid:.1[/img]
The 121.5 antenna had to be folded over under the
fiberglas to clear some ductwork. This distortion
caused the ELT to go into "hi SWR shutdown".
[img]cid:.2[/img]
I made the suggestion that we build a top-hat
loaded vertical that could be tuned for that
location and wouldn't have to be bent over under
the fairing.
Certification time-line issues shot that idea down.
They just widened the window on the SWR monitor
to prevent the shutdown. But even with this band-aid
approach, close attention was paid to adequate
radiation performance and patterns.
If one hopes to achieve better than 6% probability
of the ELT being useful, one should be wary
of pronouncements of "works fine" when deviating
from TSO approved manufacturer's recommendations.
Horizontal polarization isn't a big thing but not
having a ground plane and clearest possible view
of the sky is problematic.
Bob . . .
| Quote: |
From: owner-aeroelectric-list-server(at)matronics.com [ mailto:owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com)] On Behalf Of BobsV35B(at)aol.com
Sent: Wednesday, January 26, 2011 7:44 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Comm Antenna & SWR: More Information
Good Evening All,
This discussion about ground planes has forced me to send another one of my way off subject messages! <G>
Some of you may be aware of the speed modifications that Mike Smith was selling for Bonanzas a few years ago. While Bonanzas are certainly NOT homebuilt aircraft, Mike's work was very much in the realm of what homebuilders do. He guaranteed a speed increase of at least ten knots on any Bonanza that used his full set of speed kits.
In his quest for speed, he took EVERYTHING off the top of the fuselage and as much off the belly as humanly possible. He even installed a VHF communication antenna in a fiberglass tail cone. It was horizontally oriented rather than vertical and the ground plane was no more than six inches by eight inches.
Did it work? Some folks found that it was a perfectly usable solution and are still using it regularly. Others have either eliminated it or added another stock antenna on the belly.
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought.
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman 3977A
Do Not Archive
In a message dated 1/26/2011 9:19:32 P.M. Central Standard Time, nuckolls.bob(at)aeroelectric.com writes:
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance?
0
1
2
3
4
5
6
No virus found in this message.
Checked by AVG - www.avg.com
01/27/11 |
Bob . . .
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nuckolls.bob(at)aeroelect
Guest
|
| Posted: Thu Jan 27, 2011 8:26 pm Post subject: Comm Antenna & SWR: More Information |
|
|
At 10:36 PM 1/27/2011, you wrote:
| Quote: |
Thanks Bob. I was led to believe that the antenna and skin needed tp be
paint free and making good contact. This is what the instructions that came
with my commant and Avidyne Ryan antennas had to say. Glad that I mow
understand the actual requirements.
|
It's not uncommon for folks who specialize in
one technology to be weak in peripheral technologies.
Anyone who has worked in the EMC lab to identify and
control the transfer of RF energy understands that
large area, low pressure, open-to-atmosphere interfaces
between two metallic surfaces is NOT a bond of any useful
kind. Gas tight over lifetime of the airplane is
the goal.
If one could "tack weld" the base of an antenna
to the skin of the airplane in a half-dozen places
around the edge, the RF bonding guys would be
delighted. The next best thing is to make sure the
conduction footprints around the mounting bolts
pretend like they're welded to the airplane
Bob . . .
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
|
| Posted: Fri Jan 28, 2011 11:58 am Post subject: Comm Antenna & SWR: More Information |
|
|
The stick in the tail reminds me of when I was a kid my dad had shares in a company which carried out the seal hunt using a spotter plane and a helicopter. Daily reports from the front (the harvesting area) were sent form the spotter plane via HF. The plane had a trailing “Wet Noodle” antenna which could not be heard when he was flying toward our town.. If the plane flew a direction 90 deg to that we copied him perfectly.
With a com radio it is important that everyone be able to hear you and the reverse too... You must be able to hear everyone else in your vicinity... That is hard to do with a horizontally p[olarized antenna at those frequencies.
As for telling people they can’t do that... That’s not my job but I sure don’t mind telling them of some of the problems they probably will have with a specific set up.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of BobsV35B(at)aol.com
Sent: January 27, 2011 9:53 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Comm Antenna & SWR: More Information
Good Evening Noel,
I don't think anyone will disagree with what you say, but the point is that a lot of good competent and concientuous folks have found that the crummy location does work acceptably. You and I can agree it is not good, but if it works for others, are we supposed to tell them it is not allowed?
As 'Lectric Bob has said, chances are a wet noodle will work most of the time.
While I know folks who are still using the stick in the tail cone, I know more who have given up on it and gone to a more conventional antenna.
As Always.It All Depends! <G>
Do Not Archive
Happy Skies,
Old Bob
In a message dated 1/27/2011 6:27:12 P.M. Central Standard Time, noelloveys(at)yahoo.ca writes:
| Quote: |
The important thing with com antennas is good omnidirectional ( all directions ) coverage. A horizontally polarized antenna in a tailcone, outside of having too small a ground plane, will have a dead area in front of the plane and another one behind it. I can’t see how this can be considered good for airplanes heading into or away from a control zone.
Many large aircraft actually have antennae above and below the fuselage to allow communication with other aircraft as well as ground.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of BobsV35B(at)aol.com
Sent: January 27, 2011 12:14 AM
To: aeroelectric-list(at)matronics.com
Subject: Re: Comm Antenna & SWR: More Information
Good Evening All,
This discussion about ground planes has forced me to send another one of my way off subject messages! <G>
Some of you may be aware of the speed modifications that Mike Smith was selling for Bonanzas a few years ago. While Bonanzas are certainly NOT homebuilt aircraft, Mike's work was very much in the realm of what homebuilders do. He guaranteed a speed increase of at least ten knots on any Bonanza that used his full set of speed kits.
In his quest for speed, he took EVERYTHING off the top of the fuselage and as much off the belly as humanly possible. He even installed a VHF communication antenna in a fiberglass tail cone. It was horizontally oriented rather than vertical and the ground plane was no more than six inches by eight inches.
Did it work? Some folks found that it was a perfectly usable solution and are still using it regularly. Others have either eliminated it or added another stock antenna on the belly.
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought.
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman 3977A
Do Not Archive
In a message dated 1/26/2011 9:19:32 P.M. Central Standard Time, nuckolls.bob(at)aeroelectric.com writes:
| Quote: |
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance? http://www.matronics.com/Navigator?AeroElectric-Listhttp://forums.matronics.comhttp://www.matronics.com/contribution ====================================List href="http://www.matronics.com/Navigator?AeroElectric-List">http://www.matronics.com/Navigator?AeroElectric-List |
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Float Flyr
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Location: Campbellton, Newfoundland
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| Posted: Fri Jan 28, 2011 12:38 pm Post subject: Comm Antenna & SWR: More Information |
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???? The VOR antenna whiskers are horizontally polarized
dipole with major lobes fore and aft with nulls off to the
sides. Further, dipole antennas require no ground plane.
VOR is not a transmitting antenna. The VOR transmitters are on the ground
and are also horizontally polarized. The receive lobes of the antennae are
part insignificant as aircraft do not approach the VOR sideways.
No such "null" exists fore and aft.
Experience as well as radio theory tells me is sure does exist on the tip of
horizontally polarized signals.
| Quote: | Many large aircraft actually have antennae above and below the
fuselage to allow communication with other aircraft as well as ground.
|
???? Antenna placement is largely a matter of separation
between the antennas for similar systems . . . and judicious
use of real estate. Once an airplane is airborne, relative
differences between top and bottom mounted antennas is
insignificant.
Then why do large Aircraft often have two com antennae? One on top,
Sometimes in the vert stab and one below. The reason for the two antennae
is because of shading of the signal to stations directly below the plane
(upper antenna) or shading of a other aircraft more or less directly above
the plane (Belly position antenna)
For some reason Bob I think we are off the same page again.
Noel
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Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Fri Jan 28, 2011 12:57 pm Post subject: Comm Antenna & SWR: More Information |
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True the ELT antenna should be seen first and foremost by satellites. But they must also be able to be seen by search aircraft. The new 406 ELTs do a much better job of being seen by the satellites (Assuming an open area) and are also more linear than the old 121.5 units thus making them superior for the task they perform.
From here I’m going to go on a bit of a tangent.
The new units are programmed to a specific aircraft. The first problem I see with this is as soon as a signal is received it can be identified as a single seat something or the other and not all the resources that could be put in the field will be used. This is selective response and I for one am against it.
The next item is because each unit is coded to a particular aircraft it becomes impossible to lend a unit out while one is in for mundane battery changing. The AMO I worked for often loaned out ELTs because we really didn’t want to see our customers going off into the wilds nude of the security an ELT can offer. Commercial aircraft are allowed to operate without an ELT on board forup to 90 days if the unit is in for maintenance. Several years ago a party of executives crashed about two miles off the end of one runway at Goose Bay, Labrador in a snow storm. Their ELT had legally been removed for scheduled maintenance. The passengers did survive the crash but froze to death. The wreckage was found several months later when the snow started to melt.
Finally pilots will often monitor 121.5 enroute. This is a nice security feature in that a down plane can be found even before a search starts.
My preference is a unit which has both frequencies activated and then using something like the SPOT system as a backup.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert L. Nuckolls, III
Sent: January 27, 2011 11:44 PM
To: aeroelectric-list(at)matronics.com
Subject: RE: Comm Antenna & SWR: More Information
ELT's are designed to be seen by satellites. Until
the present 405 Mhz with superior locating capability
came along the
I recall hearing a report about 20 years ago that
for EVERY downed airplane located, ELT was a factor
in a very small percentage of those finds . . . about
6% if I recall correctly. Don't know how much better
the stats are now.
In any case installation of ELT antennas is a
"gee I hope this works when and if they
need it" kind of design task. There is no
place on an airplane that is immune from
crash-induced damage. Equipment placement
is based on the study of crash history.
So when on short final to the rocks, one
should endeavor to crash like most other
folks crashed such that their ELT survived.
Shadowing to the extent that some ground
based facility two miles away doesn't hear
it is the very least of concerns.
One more antenna that is not done right and still works fine. The ELT on many RV’s is horizontal and inside the tail cone fairing. Or in the baggage area with only the top few inches not close to metal. Neither of these install techniques have good ground planes nor the correct orientation but they work with both the 121.5 and the 406 versions of ELT
On what basis were these installations said
to "work fine". Have these installations
been reviewed by the ELT manufacturer? I
can't imagine anyone signing off on an
installation that didn't provide the customary
ground plane combined with a good view of the
sky assuming the wreckage was still upright.
[img]cid:image001.jpg(at)01CBBF0F.1D6F5000[/img]
Here's the ELT antennas on a Beechjet. Both
are installed under a fiberglas fairing just
forward of the vertical fin.
[img]cid:image002.jpg(at)01CBBF0F.1D6F5000[/img]
The 121.5 antenna had to be folded over under the
fiberglas to clear some ductwork. This distortion
caused the ELT to go into "hi SWR shutdown".
[img]cid:image003.jpg(at)01CBBF0F.1D6F5000[/img]
I made the suggestion that we build a top-hat
loaded vertical that could be tuned for that
location and wouldn't have to be bent over under
the fairing.
Certification time-line issues shot that idea down.
They just widened the window on the SWR monitor
to prevent the shutdown. But even with this band-aid
approach, close attention was paid to adequate
radiation performance and patterns.
If one hopes to achieve better than 6% probability
of the ELT being useful, one should be wary
of pronouncements of "works fine" when deviating
from TSO approved manufacturer's recommendations.
Horizontal polarization isn't a big thing but not
having a ground plane and clearest possible view
of the sky is problematic.
Bob . . .
From: owner-aeroelectric-list-server(at)matronics.com [ mailto:owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com)] On Behalf Of BobsV35B(at)aol.com
Sent: Wednesday, January 26, 2011 7:44 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: Comm Antenna & SWR: More Information
Good Evening All,
This discussion about ground planes has forced me to send another one of my way off subject messages! <G>
Some of you may be aware of the speed modifications that Mike Smith was selling for Bonanzas a few years ago. While Bonanzas are certainly NOT homebuilt aircraft, Mike's work was very much in the realm of what homebuilders do. He guaranteed a speed increase of at least ten knots on any Bonanza that used his full set of speed kits.
In his quest for speed, he took EVERYTHING off the top of the fuselage and as much off the belly as humanly possible. He even installed a VHF communication antenna in a fiberglass tail cone. It was horizontally oriented rather than vertical and the ground plane was no more than six inches by eight inches.
Did it work? Some folks found that it was a perfectly usable solution and are still using it regularly. Others have either eliminated it or added another stock antenna on the belly.
Personally, I have never flown a Bonanza that was equipped with the tail cone antenna, but I have personally spoken to a pilot who has flown his that way in heavy IFR operations for over twenty years. Just how good do antennas really NEED to be?
Mike also was a big exponent of using a set of VHF Navigation blades mounted as far back on the Bonanza fuselage as possible. The manufacturer of the antennas said they were way too close to the tail feathers to work properly. I have personally mounted several blade antennas in that position and they have all worked very well.
Just some data for further thought.
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman 3977A
Do Not Archive
In a message dated 1/26/2011 9:19:32 P.M. Central Standard Time, nuckolls.bob(at)aeroelectric.com writes:
Additional detail about my ground plane installation.
My ground plane isn't symmetrical (and, contrary to my earlier post, it is actually 24" x 34", not 24" x 36" -- I had cut 2" off the length so that it would fit in the space I had available). I also didn't mention that I cut two slots in the ground plane so that it would fit over a couple of supports for the floors in my baggage compartment. These slots are ~3" x 1/2" and are about 10" aft of the antenna base and about 4" either side of the fuselage centerline. Will these slots in the ground plane degrade the ground plane's performance?
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No virus found in this message.
Checked by AVG - www.avg.com
01/27/11
Bob . . .
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Noel Loveys
Kitfox III-A
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