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nuckolls.bob(at)cox.net
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 Posted: Mon Mar 10, 2008 8:25 am Post subject: "Coat Hanger" antenna |
 |
|
At 07:59 AM 3/8/2008 -0800, you wrote:
| Quote: | No offense to Ernest or Bob, the old coat hanger
antennas that you terminated the coax with crimp on
lugs has not been used in production airplanes since
the 50's or early 60's.
Coat hanger antenna
<http://www.aircraftspruce.com/catalog/avpages/av534.php>http://www.aircraftspruce.com/catalog/avpages/av534.php
What Eric says about "EM field energy", which I recall
bits from physics and armature radio, the coax,
BNC connector is way more efficient. That inch of
exposed shield/core & important lost insulation does
count. The "coat hanger antenna connections are
subject to corrosion and fatigue way more than a BNC
connector. Just my opinion.
|
But without quantification. The advantages of the
modern antennas are mainly mechanical. They don't
twist in the insulator. In high-dollar models there
are p-static immunities due to the DC grounded
fabrication . . . and they look sexier and have lower
maintenance costs.
| Quote: |
No one is seriously using the coat hanger wire antenna
on new OEM aircraft any more.
|
. . . for the reasons stated plus some others but
none related to observable performance.
| Quote: | "Testimonials" that they work in the plane or bench are
great, but unless you do a test on the airframe in an
antenna test chamber (EMF / RF anechoic chamber),
we are guessing. Besides performance there is the
reliability of the installation.
Just from an installation standpoint, spend the $124
for the real antenna and leave the $50 coat hanger
antenna for the closet. Antenna energy, non-ionizing
radiation is EMF energy at high frequencies. It needs
"ducting" to be most effective.
|
???? don't know what this "ducting" stuff is.
| Quote: |
Does the "strip-it crimp-it" antenna connection work?
Yes it "works", but gosh ughaaa, ugly.
We're talking about 5-8 watts of energy and communications
of 5 to 50 mile, line of sight, listening for even more powerful
transmitters, usually without obstacles. We can get away
with a weak antenna.
|
"Weak" is un-quantified. However, just like concerns about
"iron poor blood" it may help sell alternative antennas.
| Quote: |
The coat hanger works, but its not ideal. We have better
ways. If I was restoring a classic Beech or something, yea
I'd keep the coat hanger antenna. Other wise coat hangers
are for the closet (pun intended).
$110-$150 for newer antenna design
<http://www.aircraftspruce.com/catalog/avpages/av17.php>http://www.aircraftspruce.com/catalog/avpages/av17.php
PS: Some have experienced RFI with unshielded antenna
connections thru gauges and avionics.
|
Now this IS a potential effect because the little
chunk of antenna inside the aircraft is indeed a
radiator.
However, in the age of plastic airplanes (both experimental
and T/C) and reasonable adherence to DO-160 radiated
and conducted susceptibility issues, potential victims
are not going to go T/U due to the small increase in
energy at the panel due to errant radiation. Field
strengths for your VHF comm while transmitting are really
strong in the cockpit (tens of volts per meter) even
with idealized antenna installations.
Bottom line is that we're fabricating OBAM aircraft.
Yes, there are sexier, more convenient and probably
more 'efficient' antennas to be considered over the
stone-simple, rod and feed-thru insulator antennas
that were quite popular 40 years ago. It all comes
down to the builder's preferences for trading his/her
personal expenditure of $time$ versus purchasing
the product of someone else's $time$ for a more
sophisticated design.
Performance risks for going the low-dollar, DIY
route are low and easily managed.
Bob . . .
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gmcjetpilot
Joined: 04 Nov 2006
Posts: 170
|
| Posted: Tue Mar 11, 2008 4:40 pm Post subject: "Coat Hanger" antenna |
|
|
| Quote: | From: "Robert L. Nuckolls, III" <nuckolls.bob(at)cox.net (nuckolls.bob(at)cox.net)>
Subject: Re: "Coat Hanger" antenna
|
>>At 07:59 AM 3/8/2008 -0800, you wrote:
>>No offense to Ernest or Bob, the old coat hanger
>>antennas that you terminated the coax with crimp on
>>lugs has not been used in production airplanes since
>>the 50's or early 60's.
>>
>>Coat hanger antenna
>><http://www.aircraftspruce.com/catalog/avpages/av534.php>http://www.aircraftspruce.com/catalog/avpages/av534.php
>>
>>What Eric says about "EM field energy", which I recall
>>bits from physics and armature radio, the coax,
>>BNC connector is way more efficient. That inch of
>>exposed shield/core & important lost insulation does
>>count. The "coat hanger antenna connections are
>>subject to corrosion and fatigue way more than a BNC
>>connector. Just my opinion.
>But without quantification. The advantages of the
>modern antennas are mainly mechanical. They don't
>twist in the insulator. In high-dollar models there
>are p-static immunities due to the DC grounded
>fabrication . . . and they look sexier and have lower
>maintenance costs.
Bob its more than sexy it is indeed that last inch of exposed
dielectric, often close to some conductive material, even
in a composite plane (don't they need a ground plane for a 1/4
wave dipole?) that is the issue, at least to me.
>>No one is seriously using the coat hanger wire antenna
>>on new OEM aircraft any more.
>for the reasons stated plus some others but
>none related to observable performance.
Bob I have to take your word for it. However in my experience
I have seen these antennas SRW go up and up and up with time
because of dissimilar metal corrosion and bad crimps. Like I
said most pilots only demand short range line-O-sight
communication, not DX comm. The term observable
performance should include maintenance and durability in
my humble opinion.
>>"Testimonials" that they work in the plane or bench are
>>great, but unless you do a test on the airframe in an
>>antenna test chamber (EMF / RF anechoic chamber),
>>we are guessing. Besides performance there is the
>>reliability of the installation.
>>
>>Just from an installation standpoint, spend the $124
>>for the real antenna and leave the $50 coat hanger
>>antenna for the closet. Antenna energy, non-ionizing
>>radiation is EMF energy at high frequencies. It needs
>>"ducting" to be most effective.
>>
>>???? don't know what this "ducting" stuff is.
Well Wikipedia to the rescue:
http://en.wikipedia.org/wiki/Coaxial_cable
I use the word "ducting" in an informal way to describe how
a coaxial cable propagates elect mag wave energy.
"Coaxial lines solve this problem by confining the
electromagnetic wave to the area inside the cable,
between the center conductor and the shield. The
transmission of energy in the line occurs totally through
the dielectric inside the cable between the conductors."
"ducting" - was a casual my way of describing how coaxial
propagates EMW. Where open wire transmission lines, like
you have that inch before the "coat hanger" antenna, two
parallel wires, have the property where the electromagnetic
wave propagating down the line, extends into the space
surrounding it, an undesirable characteristic. You say the
loss is small? OK I guess, sure but every little bit helps
or hurts. The fact is radios are so good with filters and
excellent sensitivity and stability, a less than optimal
antenna can be some what tollerated.
>>Does the "strip-it crimp-it" antenna connection work?
>>Yes it "works", but gosh ughaaa, ugly.
>>
>>We're talking about 5-8 watts of energy and communications
>>of 5 to 50 mile, line of sight, listening for even more powerful
>>transmitters, usually without obstacles. We can get away
>>with a weak antenna.
>"Weak" is un-quantified. However, just like concerns about
>"iron poor blood" it may help sell alternative antennas.
True it is unqualified, but we could get into dB or signal strength,
but all I am saying is in Aviation we are talking about strong
signals at short distances, most of the time, period. However if
you want real long range communications at distance than it
does matter. Yes "matter" is an unqualified term as well.
>>The coat hanger works, but its not ideal. We have better
>>ways. If I was restoring a classic Beech or something, yea
>>I'd keep the coat hanger antenna. Other wise coat hangers
>>are for the closet (pun intended).
>>
>>$110-$150 for newer antenna design
>><http://www.aircraftspruce.com/catalog/avpages/av17.php>http://www.aircraftspruce.com/catalog/avpages/av17.php
>>
>>PS: Some have experienced RFI with unshielded antenna
>>connections thru gauges and avionics.
>Now this IS a potential effect because the little
>chunk of antenna inside the aircraft is indeed a
>radiator.
So true Bob.
>However, in the age of plastic airplanes (both experimental
>and T/C) and reasonable adherence to DO-160 radiated
>and conducted susceptibility issues, potential victims
>are not going to go T/U due to the small increase in
>energy at the panel due to errant radiation. Field
>strengths for your VHF comm. while transmitting are really
>strong in the cockpit (tens of volts per meter) even
>with idealized antenna installations.
>
>Bottom line is that we're fabricating OBAM aircraft.
>Yes, there are sexier, more convenient and probably
>more 'efficient' antennas to be considered over the
>stone-simple, rod and feed-thru insulator antennas
>that were quite popular 40 years ago. It all comes
>down to the builder's preferences for trading his/her
>personal expenditure of $time$ versus purchasing
>the product of someone else's $time$ for a more
>sophisticated design.
Well sexier I guess, but cost is only times 2 and when you
have a $1000 or $3000 radio, $60 more is a drop in da bucket.
>Performance risks for going the low-dollar, DIY
>route are low and easily managed.
>
>Bob
Well I guess we disagree with the last part, but I agree
making antennas and testing them is FUN! VHF on a plane
is pretty easy, and I have no problem with a DIY, however
as I said in my opinion, if you are going to buy an antenna,
like most builders, get a $120 antenna with a BNC connector.
My focus is all towards metal planes, composites have more
flexibility to experiment with internal antennas.
A BNC will be more secure & robust. You can have BNC
problems and other issues, but keeping the coaxial cable
intact to the last inch, is not totally trivial.
With a composite plane the connection will be on the "back
side of the ground plane? Right? There will be signal loss and
reflected RF into the airplane, IMHO.
Good discussion. The proof is in the eating of the pudding.
Fly a plane with both antennas and test them against a
ground station for both TX and RX. I think you will see a
difference in one antenna getting a weak incoming signal
and getting out a stronger signal. Of course bent whips
are a compromise. The radiating element should be as
perpendicular to the ground plane as possible but most
pilots like the looks and lower drag of the bent whip for a
little more SRW. Also the "Blade" high speed antennas
have "wider" band width. There are good enough antennas
and better, best antennas. It is definitely esoteric but for
just secure connection the BNC has it over the crimp
and screw exposed twin lead approach, at least in the
VHF band of freq's, in my opinion. Old ways are still
good but there is a reason new methods & connections
where developed. It may take more sensitive lab equip
to notice but I can't believe it makes NO difference, but
than you say you tested it, and I have not. All I can go
by is when trouble shooting poor radio performance on
old planes, the first place I look is at those old corroded
connections on the "coat hanger".
Cheers George
Be a better friend, newshound, and [quote][b]
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nuckolls.bob(at)cox.net
Guest
|
| Posted: Wed Mar 12, 2008 6:29 am Post subject: "Coat Hanger" antenna |
|
|
At 05:30 PM 3/11/2008 -0700, you wrote:
| Quote: | >From: "Robert L. Nuckolls, III"
<<mailto:nuckolls.bob(at)cox.net>nuckolls.bob(at)cox.net>
>Subject: Re: "Coat Hanger" antenna
>>At 07:59 AM 3/8/2008 -0800, you wrote:
>>No offense to Ernest or Bob, the old coat hanger
>>antennas that you terminated the coax with crimp on
>>lugs has not been used in production airplanes since
>>the 50's or early 60's.
>>
>>Coat hanger antenna
>><<http://www.aircraftspruce.com/catalog/avpages/av534.php>http://www.aircraftspruce.com/catalog/avpages/av534.php>http://www.aircraftspruce.com/catalog/avpages/av534.php>http://www.aircraftspruce.com/catalog/avpages/av534.php
>>
>>What Eric says about "EM field energy", which I recall
>>bits from physics and armature radio, the coax,
>>BNC connector is way more efficient. That inch of
>>exposed shield/core & important lost insulation does
>>count. The "coat hanger antenna connections are
>>subject to corrosion and fatigue way more than a BNC
>>connector. Just my opinion.
>But without quantification. The advantages of the
>modern antennas are mainly mechanical. They don't
>twist in the insulator. In high-dollar models there
>are p-static immunities due to the DC grounded
>fabrication . . . and they look sexier and have lower
>maintenance costs.
Bob its more than sexy it is indeed that last inch of exposed
dielectric, often close to some conductive material, even
in a composite plane (don't they need a ground plane for a 1/4
wave dipole?) that is the issue, at least to me.
|
Sure. But let's consider the "loaded" antenna. An antenna that's
physically too short for efficient operation at the desired frequency.
The ideal, full sized quarter-wave mobile antennas operating at say 14 Mhz
would LIKE to have about 17.5 feet of antenna on the bumper. See:
http://tinyurl.com/2dyfss
This is not practical on a moving vehicle so some means of electrically
lengthening a "too short" antenna is called for. This is done in a variety
of ways but all involve adding some reactive component (inductive or
capacitive)
to the antenna.
My last HF mobile installation featured a 4' stub mast, an adjustable
center loading coil and an 8' whip antenna above all this. Here's
a modern example of this style of artificial antenna lengthening:
http://www.hiqantennas.com/images/MVC-462.jpg
Let's examine a practical example where there's value
in making an antenna shorter while minimizing effects on
performance. A few years ago I got a call from the production
line on some new ELT transmitters that were shutting down
with a high SWR fault. Here's the antennas in question:
http://tinyurl.com/2u5c8u
The original system integrator installed dual antennas
(125.5 and 406 Mhz) under the fiberglas 'instep cap'
of the vertical fin. There were some composite
air ducts with metallic components also located
in the fin cap. These not only made it necessary to
fold the longer antenna over, proximity of the metal
further de-tuned it. See:
http://tinyurl.com/38symp
Older versions of the ELT transmitter would tolerate
this degradation of antenna performance, the new ones would
not.
I wrote a white paper suggesting a variety of options.
The elegant solution was a top-loaded antenna physically
short enough to mount under the fin cap and avoid
close proximity to the duct-metal.
http://tinyurl.com/2vv5e4
I crafted an exemplar antenna that was only 9" tall
and took it to the lab for comparison with a full
sized 24" antenna. It's radiation angle was higher
and maximum radiation levels were down by less than
2 dB from the full sized quarter wave. Entirely
satisfactory for this installation.
We could have considered some other scenarios. See:
http://tinyurl.com/3aaocs
There are an infinite number of combinations of
top, center and base loading that would minimize
SWR at the frequency of interest. However, all versions
other than top-loading increased fabrication labor
and produced a lower performance.
Let's take the ideas illustrated above and consider
when one chooses to use the stone-simple, rod and
feed-thru antenna.
Open conductors have inductance. As a rule of thumb,
we ball-park an open wire (and in this case, an extension
of the antenna base below the ground plane) at 20 nH/inch.
Assuming 1.5" of bare conductors total, we can expect
something on the order of 30nH of series inductance at
the base of the antenna. This works out to about 25 ohms
of reactance at 130 Mhz. A full length, 24" antenna's
center frequency would shift downward due to this loading
inductance.
A purest would get out the antenna analyzer and trim
the antenna's physical length (shift toward capacitive
reactance) to balance reactive effects of the base loading
inductance.
The next time I have occasion to get the test ground
plane out, I'll take a look at the effects of exposed
conductors at the base of the stone-simple antenna
and report the amount of shortening required to shift
the antenna's center frequency back to the design
optimum.
These exposed conductors ARE at the base of the
antenna where the highest currents are encountered
which go hand-in-hand with the highest radiation.
So indeed, we are squirting some RF into the aircraft's
interior spaces that would not be present with other
antennas . . . but in practice this is seldom a problem
either for EMC issues or overall performance.
| Quote: |
>>No one is seriously using the coat hanger wire antenna
>>on new OEM aircraft any more.
>for the reasons stated plus some others but
>none related to observable performance.
Bob I have to take your word for it. However in my experience
I have seen these antennas SRW go up and up and up with time
because of dissimilar metal corrosion and bad crimps.
|
Absolutely! But connectors corrode too . . . and
suffer from bad craftsmanship at assembly. I tossed
out the large-area, low-pressure ground terminal supplied
with the stone-simple antenna and substituted a PIDG
terminal properly bonded to the skin with a 10-32 screw
torqued up tight. Similarly, I made sure that the
PIDG terminal at the base of the antenna had the
super-mash on it between two nuts. If you don't
have gas-tight joints, they will degrade with time.
| Quote: | Like I
said most pilots only demand short range line-O-sight
communication, not DX comm. The term observable
performance should include maintenance and durability in
my humble opinion.
|
To be sure. The DIY antennas do have cost of ownership issues
that are strongly affected by craftsmanship of the original
installation. But if one understands the potential 'gotchas'
with ANY antenna and exercises due diligence for craftsmanship,
there's good value to be realized from either of these
antennas.
't know what this "ducting" stuff is.
| Quote: |
Well Wikipedia to the rescue:
<http://en.wikipedia.org/wiki/Coaxial_cable>http://en.wikipedia.org/wiki/Coaxial_cable
I use the word "ducting" in an informal way to describe how
a coaxial cable propagates elect mag wave energy.
"Coaxial lines solve this problem by confining the
electromagnetic wave to the area inside the cable,
between the center conductor and the shield. The
transmission of energy in the line occurs totally through
the dielectric inside the cable between the conductors."
"ducting" - was a casual my way of describing how coaxial
propagates EMW. Where open wire transmission lines, like
you have that inch before the "coat hanger" antenna, two
parallel wires, have the property where the electromagnetic
wave propagating down the line, extends into the space
surrounding it, an undesirable characteristic. You say the
loss is small? OK I guess, sure but every little bit helps
or hurts. The fact is radios are so good with filters and
excellent sensitivity and stability, a less than optimal
antenna can be some what tollerated.
|
Understand.
| Quote: |
>"Weak" is un-quantified. However, just like concerns about
>"iron poor blood" it may help sell alternative antennas.
True it is unqualified, but we could get into dB or signal strength,
but all I am saying is in Aviation we are talking about strong
signals at short distances, most of the time, period. However if
you want real long range communications at distance than it
does matter. Yes "matter" is an unqualified term as well.
|
Wouldn't argue that the stone-simple antenna does not
represent the best-we-know-how-to-do. But neither is
a Toyota starter bolted to a Lycoming on an adapter
plate carved out on a bandsaw.
I would recommend that any builder take advantage of
the convenience of pre-fabricated, optimized antennas.
But I think we would be remiss as teachers if we did not
define the simple-ideas that control performance
and service life of the DIY approaches. I would not
discourage anyone from doing the best they can with
what they have to work with . . . whether by choice or
circumstance.
<snip>
| Quote: |
>Bottom line is that we're fabricating OBAM aircraft.
>Yes, there are sexier, more convenient and probably
>more 'efficient' antennas to be considered over the
>stone-simple, rod and feed-thru insulator antennas
>that were quite popular 40 years ago. It all comes
>down to the builder's preferences for trading his/her
>personal expenditure of $time$ versus purchasing
>the product of someone else's $time$ for a more
>sophisticated design.
Well sexier I guess, but cost is only times 2 and when you
have a $1000 or $3000 radio, $60 more is a drop in da bucket.
|
Agreed. But one can do the stone-simple antenna with
self procured components for much less than the $60
sale price . . . and there are very few builders who
have $3000 tied up in a radio.
| Quote: |
>Performance risks for going the low-dollar, DIY
>route are low and easily managed.
>
>Bob
Well I guess we disagree with the last part, but I agree
making antennas and testing them is FUN! VHF on a plane
is pretty easy, and I have no problem with a DIY, however
as I said in my opinion, if you are going to buy an antenna,
like most builders, get a $120 antenna with a BNC connector.
My focus is all towards metal planes, composites have more
flexibility to experiment with internal antennas.
A BNC will be more secure & robust. You can have BNC
problems and other issues, but keeping the coaxial cable
intact to the last inch, is not totally trivial.
|
True. We could build a bracket to mount the connector
but it needs to be attached to the airframe with rivets,
not by assembly forces of the ceramic feed-thru insulator.
The problems of establishing gas-tight joints at both
the ground plane and at the antenna feed-point are the
same for both metal and plastic airplanes. See:
http://tinyurl.com/yu38rz
I would hope that the efforts I made toward that goal
over 35 years ago paid off in trouble-free performance
over the lifetime of the installation. The last time
I had occasion to install a rod-n-cone antenna was
on a wrecked straight-tail, C182 during the rebuild.
It was a belly mounted antenna for a #2 nav/com.
The antenna survived the accident but the belly skin
was wrinkled and got replaced.
| Quote: |
With a composite plane the connection will be on the "back
side of the ground plane? Right? There will be signal loss and
reflected RF into the airplane, IMHO.
|
True. The effects are non-zero but for most users
are not operationally significant.
| Quote: |
Good discussion. The proof is in the eating of the pudding.
Fly a plane with both antennas and test them against a
ground station for both TX and RX. I think you will see a
difference in one antenna getting a weak incoming signal
and getting out a stronger signal. Of course bent whips
are a compromise. The radiating element should be as
perpendicular to the ground plane as possible but most
pilots like the looks and lower drag of the bent whip for a
little more SRW. Also the "Blade" high speed antennas
have "wider" band width. There are good enough antennas
and better, best antennas. It is definitely esoteric but for
just secure connection the BNC has it over the crimp
and screw exposed twin lead approach, at least in the
VHF band of freq's, in my opinion. Old ways are still
good but there is a reason new methods & connections
where developed. It may take more sensitive lab equip
to notice but I can't believe it makes NO difference,
|
. . . no perceivable difference from the pilot's
seat. We often get wrapped around a performance axle
trying to tweak things based on test measurements
that return no operational benefits. I had one
instructor tell me there was benefit after a
balked landing to turn off the alternator so that we
could get more horsepower to the prop! Intuitively
we know there's a measurable difference but
the practical benefits are someplace between zero and not
much.
| Quote: | . . . but than you say you tested it, and I have not.
|
All of the testing has been in the lab were we
do indeed see quantifiable differences. But in
practice, tens of thousands of these antennas have
flown and offered the user good return on investment
too.
| Quote: | All I can go
by is when trouble shooting poor radio performance on
old planes, the first place I look is at those old corroded
connections on the "coat hanger".
|
All very much on point . . . and I would suggest that
any readers of this List make note of the fact that
some installations are more deserving of preventative
maintenance than others. Use of good assembly practice
when making up the joints will forestall if not eliminate
future maintenance events.
Bob . . .
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