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doramsey(at)gmail.com
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 Posted: Fri Nov 18, 2011 9:00 am Post subject: Antennae continuity |
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Probably a dumb question but I will ask it anyway. I always check for continuity and shorts any time I build a coax connection. But I got the bright idea today that I should also check the continuity from the panel end of the cable to the com antennae itself. Got good continuity between shield and the antennae screws and no continuity between the center conductor and the antennae rod itself. I am guessing that this is due to their being an insulator between the internal antennae wire and the external metal rod of the antennae. Is that correct? I know the cable was good (or at least tested good) before I connected it to the com antennae. I had always assumed that the com antennae rod was a conductive part of the transmit center conductor. [quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Fri Nov 18, 2011 4:15 pm Post subject: Antennae continuity |
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At 10:56 AM 11/18/2011, you wrote:
Probably a dumb question but I will ask it anyway. I always check
for continuity and shorts any time I build a coax connection. But I
got the bright idea today that I should also check the continuity
from the panel end of the cable to the com antennae itself. Got good
continuity between shield and the antennae screws and no continuity
between the center conductor and the antennae rod itself. I am
guessing that this is due to their being an insulator between the
internal antennae wire and the external metal rod of the
antennae. Is that correct? I know the cable was good (or at least
tested good) before I connected it to the com antennae. I had always
assumed that the com antennae rod was a conductive part of the
transmit center conductor.
I presume we're talking about a manufactured
antenna . . . what brand and p/n? What you've
described not common in my experience . . . but not
impossible to imagine.
Every DIY antenna makes a direct connection between
the coax center conductor and the bottom of the
antenna mast. The 'pure' 1/4-wave antenna supported
over the 'ideal' ground plane has a base impedance
on the order of 20 ohms. This tells us that direct
connection of a 50 ohm coax feed line will produce
a best SWR of at least 2:1 that rises either side
of resonance for the antenna.
Many VHF/UHF antennas seen on the ground feature drooping
radials. Bending the radials downward has a doubly
beneficial effect. The feed point impedance rises.
At about 45 degrees of droop, the impedance approaches
50 ohms thus providing a better SWR. It also has the
effect of lowering the radiation angle for the antenna
thus improving performance for talking/listening to
stations just above the horizon.
VHF antennas for airplanes are exceedingly limited with
respect to optimization for lowest SWR. Fortunately,
VHF communications are light of sight. A few watts and
a wet string for an antenna will let you talk as far
as you commonly need to talk . . . a distance that
seldom extends out the horizon. So the mediocre 1/4-wave
whisker feed directly with a 50 ohm coax offers
no observable degradation of performance.
Occasionally, a design goal for a new antenna
design calls for lower SWR. It may not let you
talk or listen further, but it does look 'better'
in the sales literature. A variety of matching
techniques can be used to bring the load
impedance of the finished antenna closer to
that of the feed line. If that network includes
capacitors . . . it would explain the loss of DC
continuity from coax center conductor to antenna
mast. For any such matching circuit to exist,
there must be a sufficient volume within a base
fairing to house the components.
I've not seen antennas with matching networks
crafted with capacitors . . . but it's possible.
It's also possible that continuity measured from
center conductor to ground in an inductively compensated
antenna might approach zero ohms leading the observer
to think that it's 'shorted'.
Continuity tests for open feed lines are always
informative. But in cases of confusing ohmmeter
measurements to the antenna itself, checking the
SWR of an installed antenna is the true test
for proper function.
Bob . . .
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doramsey(at)gmail.com
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| Posted: Fri Nov 18, 2011 11:43 pm Post subject: Antennae continuity |
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Bob, the antennae is a RA Miller AV-17. More information...when I removed the coax and held my multimeter probe to the center connection and checked continuity to the antennae rod, I still get no continuity. So either the antennae is bad or there is something inherent in the design that electrically isolates the external skin of the rod.
On Fri, Nov 18, 2011 at 7:10 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
[quote] --> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>
At 10:56 AM 11/18/2011, you wrote:
Probably a dumb question but I will ask it anyway. I always check for continuity and shorts any time I build a coax connection. But I got the bright idea today that I should also check the continuity from the panel end of the cable to the com antennae itself. Got good continuity between shield and the antennae screws and no continuity between the center conductor and the antennae rod itself. I am guessing that this is due to their being an insulator between the internal antennae wire and the external metal rod of the antennae. Is that correct? I know the cable was good (or at least tested good) before I connected it to the com antennae. I had always assumed that the com antennae rod was a conductive part of the transmit center conductor.
I presume we're talking about a manufactured
antenna . . . what brand and p/n? What you've
described not common in my experience . . . but not
impossible to imagine.
Every DIY antenna makes a direct connection between
the coax center conductor and the bottom of the
antenna mast. The 'pure' 1/4-wave antenna supported
over the 'ideal' ground plane has a base impedance
on the order of 20 ohms. This tells us that direct
connection of a 50 ohm coax feed line will produce
a best SWR of at least 2:1 that rises either side
of resonance for the antenna.
Many VHF/UHF antennas seen on the ground feature drooping
radials. Bending the radials downward has a doubly
beneficial effect. The feed point impedance rises.
At about 45 degrees of droop, the impedance approaches
50 ohms thus providing a better SWR. It also has the
effect of lowering the radiation angle for the antenna
thus improving performance for talking/listening to
stations just above the horizon.
VHF antennas for airplanes are exceedingly limited with
respect to optimization for lowest SWR. Fortunately,
VHF communications are light of sight. A few watts and
a wet string for an antenna will let you talk as far
as you commonly need to talk . . . a distance that
seldom extends out the horizon. So the mediocre 1/4-wave
whisker feed directly with a 50 ohm coax offers
no observable degradation of performance.
Occasionally, a design goal for a new antenna
design calls for lower SWR. It may not let you
talk or listen further, but it does look 'better'
in the sales literature. A variety of matching
techniques can be used to bring the load
impedance of the finished antenna closer to
that of the feed line. If that network includes
capacitors . . . it would explain the loss of DC
continuity from coax center conductor to antenna
mast. For any such matching circuit to exist,
there must be a sufficient volume within a base
fairing to house the components.
I've not seen antennas with matching networks
crafted with capacitors . . . but it's possible.
It's also possible that continuity measured from
center conductor to ground in an inductively compensated
antenna might approach zero ohms leading the observer
to think that it's 'shorted'.
Continuity tests for open feed lines are always
informative. But in cases of confusing ohmmeter
measurements to the antenna itself, checking the
SWR of an installed antenna is the true test
for proper function.
Bob . . .
====================================
om" target="_blank">www.aeroelectric.com
ooks.com" target="_blank">www.buildersbooks.com
et="_blank">www.homebuilthelp.com
="_blank">http://www.matronics.com/contribution
le, List Admin.
====================================
-List" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List
====================================
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le, List Admin.
="_blank">http://www.matronics.com/contribution
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[b]
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nuckolls.bob(at)aeroelect
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| Posted: Sat Nov 19, 2011 6:23 am Post subject: Antennae continuity |
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At 01:39 AM 11/19/2011, you wrote:
Bob, the antennae is a RA Miller AV-17. More information...when I
removed the coax and held my multimeter probe to the center
connection and checked continuity to the antennae rod, I still get no
continuity. So either the antennae is bad or there is something
inherent in the design that electrically isolates the external skin of the rod.
Good data point . . . and given that the data
does not comport with things we understand about
'simple' antennas, it might prompt further
experiments to satisfy a curiosity . . . and
to confirm the antenna's performance.
Do you, or somebody you know, have access to
an antenna analyzer or perhaps a Bird thruline
watt meter?
Bob . . .
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doramsey(at)gmail.com
Guest
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| Posted: Sat Nov 19, 2011 6:47 am Post subject: Antennae continuity |
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Bob, I have sent a note out to my friends to see if there is one available but I dont have one. Would it be a reasonable check to simply hook up my hand held radio to the antennae and see if I can pick up traffic. With no antenna, I would think I would hear nothing.
On Sat, Nov 19, 2011 at 9:19 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
[quote] --> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>
At 01:39 AM 11/19/2011, you wrote:
Bob, the antennae is a RA Miller AV-17. More information...when I removed the coax and held my multimeter probe to the center connection and checked continuity to the antennae rod, I still get no continuity. So either the antennae is bad or there is something inherent in the design that electrically isolates the external skin of the rod.
Good data point . . . and given that the data
does not comport with things we understand about
'simple' antennas, it might prompt further
experiments to satisfy a curiosity . . . and
to confirm the antenna's performance.
Do you, or somebody you know, have access to
an antenna analyzer or perhaps a Bird thruline
watt meter?
Bob . . .
====================================
om" target="_blank">www.aeroelectric.com
ooks.com" target="_blank">www.buildersbooks.com
et="_blank">www.homebuilthelp.com
="_blank">http://www.matronics.com/contribution
le, List Admin.
====================================
-List" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List
====================================
http://forums.matronics.com
====================================
le, List Admin.
="_blank">http://www.matronics.com/contribution
====================================
[b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Nov 19, 2011 9:53 am Post subject: Antennae continuity |
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At 08:44 AM 11/19/2011, you wrote:
| Quote: | Bob, I have sent a note out to my friends to see if there is one
available but I dont have one. Would it be a reasonable check to
simply hook up my hand held radio to the antennae and see if I can
pick up traffic. With no antenna, I would think I would hear nothing.
|
That's a good toe-in-the-water test but without
side-by-side comparison with a known good antenna
it's pretty "coarse" data.
Being a manufactured antenna by a reputable firm,
it's highly unlikely that the antenna is bad.
I wouldn't loose any sleep over it. This discussion
is more academic than practical but it helps
our readers achieve a better sense of how all these
simple-ideas fit together.
Bob . . .
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doramsey(at)gmail.com
Guest
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| Posted: Sat Nov 19, 2011 11:31 am Post subject: Antennae continuity |
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I set up handheld as a scanner for local frequencies and it came in reasonable loud and clear when aircraft flew overhead (and that is with airplane in the hanger). I did a test with a neighbor and his handheld and he could not really tell the difference between the standard whip antenna, the antennae that is the subject of this discussion and my second com antennae which is a Bob Archer antenna buried in my vertical stabilizer. So it seems it is working as an antenna, but if I can chase down some test equipment, I will have a better idea how good of an antenna it is.
On Sat, Nov 19, 2011 at 12:50 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
[quote] --> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>
At 08:44 AM 11/19/2011, you wrote:
| Quote: | Bob, I have sent a note out to my friends to see if there is one available but I dont have one. Would it be a reasonable check to simply hook up my hand held radio to the antennae and see if I can pick up traffic. With no antenna, I would think I would hear nothing.
|
That's a good toe-in-the-water test but without
side-by-side comparison with a known good antenna
it's pretty "coarse" data.
Being a manufactured antenna by a reputable firm,
it's highly unlikely that the antenna is bad.
I wouldn't loose any sleep over it. This discussion
is more academic than practical but it helps
our readers achieve a better sense of how all these
simple-ideas fit together.
Bob . . .
====================================
om" target="_blank">www.aeroelectric.com
ooks.com" target="_blank">www.buildersbooks.com
et="_blank">www.homebuilthelp.com
="_blank">http://www.matronics.com/contribution
le, List Admin.
====================================
-List" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List
====================================
http://forums.matronics.com
====================================
le, List Admin.
="_blank">http://www.matronics.com/contribution
====================================
[b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Nov 19, 2011 11:43 am Post subject: Antennae continuity |
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At 01:27 PM 11/19/2011, you wrote:
| Quote: | I set up handheld as a scanner for local frequencies and it came in
reasonable loud and clear when aircraft flew overhead (and that is
with airplane in the hanger). I did a test with a neighbor and his
handheld and he could not really tell the difference between the
standard whip antenna, the antennae that is the subject of this
discussion and my second com antennae which is a Bob Archer antenna
buried in my vertical stabilizer. So it seems it is working as an
antenna, but if I can chase down some test equipment, I will have a
better idea how good of an antenna it is.
|
Good sleuthing. If you are able to get some
test equipment, do a 'sweep' over the frequency
range of interest at, say 0.5 Mhz steps and
record the data. Do the Archer antenna too.
Take your measurements from the cockpit . . .
i.e. at the transceiver end of the feed line.
It will be interesting to plot that data
for comparison with the data Don supplied
us. But if you've got 'better' things to
do with your $time$ for achieving or keeping
your airplane capable of flight, I wouldn't
put this endeavor very far up on the list
of things to do.
Bob . . .
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