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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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 Posted: Wed Jul 01, 2020 5:04 am Post subject: IS a COM ANTENNA GROUND PLANE NECESSARY |
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A friend of mine is building a Kitfox S7. It is 99 percent completed. He
installed the com antenna in the vertical stabilizer which has steel tube and
wood ribs and fabric cover which has been professionally painted. It is
definitely not an ideal location. A concern that I have is that there is no
ground plane. The antenna base is mounted to a wooden rib. All that is
visible or accessible is the BNC connector and coax. He installed the antenna
there because someone else did so on their plane and claimed that it worked.
Questions:
1. Should he try transmitting on the com radio to see how well the antenna
works and the range? Or could the radio be damaged by reflected power?
2. Can a ground plane be made by removing the outer insulation from the
RG-58 coax and soldering two wires to the shield near the BNC connector?
One of the two wires would droop down and forward, the other would
droop down and aft.
3. Would a new antenna work if installed inside of the steel tube fuselage?
4. Is he better off installing a new antenna on the outside of the fuselage?
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Joe Gores |
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ceengland7(at)gmail.com
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| Posted: Wed Jul 01, 2020 7:27 am Post subject: IS a COM ANTENNA GROUND PLANE NECESSARY |
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On 7/1/2020 8:04 AM, user9253 wrote:
| Quote: |
A friend of mine is building a Kitfox S7. It is 99 percent completed. He
installed the com antenna in the vertical stabilizer which has steel tube and
wood ribs and fabric cover which has been professionally painted. It is
definitely not an ideal location. A concern that I have is that there is no
ground plane. The antenna base is mounted to a wooden rib. All that is
visible or accessible is the BNC connector and coax. He installed the antenna
there because someone else did so on their plane and claimed that it worked.
Questions:
1. Should he try transmitting on the com radio to see how well the antenna
works and the range? Or could the radio be damaged by reflected power?
2. Can a ground plane be made by removing the outer insulation from the
RG-58 coax and soldering two wires to the shield near the BNC connector?
One of the two wires would droop down and forward, the other would
droop down and aft.
3. Would a new antenna work if installed inside of the steel tube fuselage?
4. Is he better off installing a new antenna on the outside of the fuselage?
--------
Joe Gores
There are a lot of successful comm antenna installations within steel
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tube fuselages. That might be the easiest path forward, since getting a
long enough ground plane 'whisker' aft of the tail mounted antenna might
be a problem.
On the other hand, I suspect that any modern comm radio will protect
itself from damage, so if he doesn't care about max range
transmission/reception, he could just try what he's got. Also, there are
lots of ways to make/add ground plane whiskers, even if he ended up with
something sub optimal and an asymmetrical coverage pattern. A couple of
fittings and adapters should give him a way to add whiskers without
cutting into the coax.
https://www.google.com/search?q=1/4+wave+antenna+ground+plane&rlz=1C1CHBF_enUS843US843&sxsrf=ALeKk02Mio8mukK9rYmhrx1EsHM2zf4bpw:1593614869209&source=lnms&tbm=isch&sa=X&ved=2ahUKEwia9L-BpqzqAhVDmeAKHVLdD7wQ_AUoAnoECA0QBA&biw=1920&bih=975
Charlie
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jul 01, 2020 8:19 am Post subject: IS a COM ANTENNA GROUND PLANE NECESSARY |
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At 08:04 AM 7/1/2020, you wrote:
| Quote: | --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
A friend of mine is building a Kitfox S7. It is 99 percent completed. He
installed the com antenna in the vertical stabilizer which has steel tube and
wood ribs and fabric cover which has been professionally painted. It is
definitely not an ideal location. A concern that I have is that there is no
ground plane. The antenna base is mounted to a wooden rib. All that is
visible or accessible is the BNC connector and coax. He installed the antenna
there because someone else did so on their plane and claimed that it worked.
Questions: |
Hmmm . . . 'works' is unqualified . . . a wet string will
suffice for communications at some level . . .
| Quote: | 1. Should he try transmitting on the com radio to see how well the antenna
works and the range? |
That's the 'acid test' but first . . .
| Quote: | | Or could the radio be damaged by reflected power? |
by all means he should MEASURE the SWR over the full
range of comm frequencies. 2:1 or better over 108 to 136
is usually possible with optimized vhf comm antennas.
3:1 is 'useable' but anything greater is cause for
re-evaluation of the installation.
| Quote: | 2. Can a ground plane be made by removing the outer insulation from the
RG-58 coax and soldering two wires to the shield near the BNC connector?
One of the two wires would droop down and forward, the other would
droop down and aft. |
what kind of antenna are we talking about? A simple whip?
how did it get attached to the airframe . . . is the coax
connector 'grounded' to the steel-tube? If so, nothing
else you can 'add' will make any difference. Proximity
to conductive structure is your driving condition and
will make for a really 'un-round' radiation pattern.
| Quote: | | 3. Would a new antenna work if installed inside of the steel tube fuselage? |
again, 'works' is non-qualified. Optimized antennas will
first accept the transmitted energy with minimum losses.
Second, it will have as close to a omni-directional
radiation pattern as practical. Researching the first
is easy . . . go measure it. The second is not so easy.
We used to run radiation pattern tests on antennas already
flying by putting test equipment in the airplane and
then flying a couple 360 circles about 50 miles out while
measuring the signal received on a calibrated receiver
in the airplane. Not easy/cheap to do.
Most departures from optimally mounted 1/4-wave
whips still 'work' . . . and as long as the pilot
is satisfied, details as to radiation patterns
don't mater much. SWR is easy to check and is
your first clue as to operational efficiency.
If an antenna does not readily accept transmitted
energy, radiation patterns are somewhat irrelevant.
| Quote: | | 4. Is he better off installing a new antenna on the outside of the fuselage? |
I wouldn't hold out for an antenna inside the
fuselage being markedly better than inside the
vertical fin . . . but that's a WAG. Instrumented
testing is the only way to make that call with
engineered confidence.
Bob . . .
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Wed Jul 01, 2020 9:54 am Post subject: Re: IS a COM ANTENNA GROUND PLANE NECESSARY |
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Yes, the antenna is a simple 1/4 wave whip. The antenna base is mounted in
a wood rib. The antenna base does not touch the steel tube airframe but is
3/4 inch away from a horizontal steel tube and about 6 inches away from a
vertical steel tube, not good I know.
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Joe Gores |
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Fri Jul 17, 2020 4:54 pm Post subject: Re: IS a COM ANTENNA GROUND PLANE NECESSARY |
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I took my SWR meter to the airport today and checked my friend's com antenna. The SWR was 3.8
Now he knows that his antenna installation will not work very well.
He is considering mounting a new antenna on top of the fuselage.
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Joe Gores |
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jimkale(at)roadrunner.com
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| Posted: Fri Jul 17, 2020 9:21 pm Post subject: IS a COM ANTENNA GROUND PLANE NECESSARY |
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At 3.8 SWR the transmitter is in danger of over heating to the point of meltdown!!!!!
Sent from my iPhone
| Quote: | On Jul 17, 2020, at 8:12 PM, user9253 <fransew(at)gmail.com> wrote:

I took my SWR meter to the airport today and checked my friend's com antenna. The SWR was 3.8
Now he knows that his antenna installation will not work very well.
He is considering mounting a new antenna on top of the fuselage.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=497356#497356
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Mon Aug 03, 2020 4:26 am Post subject: Re: IS a COM ANTENNA GROUND PLANE NECESSARY |
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My friend abandoned the antenna embedded in the vertical stabilizer on his
Kitfox. He purchased a new antenna and mounted it atop the fuselage just aft
of the cargo area. The antenna base is mounted to a steel plate that is part
of the tubular steel airframe. The SWR was 1.95 when transmitting on
122.75. The ground plane could probably be improved if necessary. The
wings were folded back during the SWR test. The aluminum flaperons were
within 6 inches of the antenna. Question: Do nearby metal objects
(flaperons) affect the SWR? If so, the the SWR test could be repeated when
the wings are unfolded.
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Wed Aug 12, 2020 5:41 am Post subject: Re: IS a COM ANTENNA GROUND PLANE NECESSARY |
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One would think that the Kitfox steel tube fuselage frame would make a
good ground plane, but it does not. With the antenna mounted to a flat
plate that is welded to the top of the fuselage, the SWR was 2.4 or worse.
I then connected a 22 inch long wire to one of the antenna mounting screws and
extended it horizontally and forward over the open baggage area. Surprise!
The SWR greatly improved. Below are the numbers. My friend might
experiment by adding additional ground plane radials.
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FREQ _ SWR
118 _ 2.30
119 _ 1.78
120 _ 1.49
121 _ 1.24
122 _ 1.20
123 _ 1.30
124 _ 1.35
125 _ 1.39
126 _ 1.59
127 _ 1.82
128 _ 1.95
129 _ 1.99
130 _ 2.00
131 _ 1.99
132 _ 2.00
133 _ 2.10
134 _ 2.20
135 _ 2.50
136 _ 3.10
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finn.usa(at)gmail.com
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| Posted: Thu Aug 13, 2020 6:24 am Post subject: IS a COM ANTENNA GROUND PLANE NECESSARY |
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Using the nanoVNA, I played with an old homemade comm antenna, 1/4
sticking up, three "ground plane" wires 120 degrees apart and bent downward.
It was amazing to me how the impedance and thus SWR changed with bending
the ground plane wires up or down a tiny bit.
So, it's not just about adding ground plane wires, but also about how
they slope down.
Of course, I don't know what impedance-matching network may be hiding in
the RAMI antenna..
Finn
On 8/12/2020 11:14 AM, Robert L. Nuckolls, III wrote:
| Quote: | The feedpoint impedance of an un-compensated
1/4 wave vertical is on the order of 30 ohms
at resonance . . . so the BEST expected SWR
over the range of interest would be about what
you're seeing 1.20:1
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