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Grounding radio antenna and transponder antenna

 
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Argonaut36



Joined: 19 May 2019
Posts: 20

PostPosted: Sun Sep 15, 2019 7:46 am    Post subject: Grounding radio antenna and transponder antenna Reply with quote

On my airplane the transponder antenna (or better, the ground plane of the transponder antenna, is grounded to the tubular frame, right where the negative cable from the battery is connected. On the contrary, the radio antenna is not properly grounded; it is mounted on a removable panel on the bottom of the airplane; this panel is attached with machine screws and sheet metal screws to other airplane panels. But none of these panels is properly bonded to the tubular frame of the aircraft.
Do the transponder and radio antennas need to be grounded and, if so, what is the proper way of doing this?
Thanks


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user9253



Joined: 28 Mar 2008
Posts: 1436
Location: Riley TWP Michigan

PostPosted: Sun Sep 15, 2019 11:19 am    Post subject: Re: Grounding radio antenna and transponder antenna Reply with quote

Antennas do not necessarily have to be grounded to the airframe. But antennas do need a properly sized ground plane for the frequency. Usually the ground plane radius is greater than the antenna length. The larger the ground plane radius, the better.

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Argonaut36



Joined: 19 May 2019
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PostPosted: Mon Sep 16, 2019 7:10 am    Post subject: Re: Grounding radio antenna and transponder antenna Reply with quote

Thanks for your reply.
As far as the size of the ground plane, I am constrained by the small dimensions of the airplane. The ground plane should be larger, but there is no much that I can do about it.
I try to improve everything else in order to get more range and less static. When you say that ‘antennas do not necessarily have to be grounded to the airframe’, you seem to imply that it would be better to ground the antenna, even if it is not indispensable. In my case, because of the ground plane issue, I would take any improvements, doesn’t matter how small. Could you please confirm that, ideally, radio antennas should be grounded to the airframe and provide a reference for that (book, website, paper…).
Thanks


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user9253



Joined: 28 Mar 2008
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Location: Riley TWP Michigan

PostPosted: Mon Sep 16, 2019 6:25 pm    Post subject: Re: Grounding radio antenna and transponder antenna Reply with quote

Bob N. posted today
"One or more clamps should secure the
ground plane to structure . . . not for electrical
'bonding' but for mechanical support.
"
In my opinion (I am no antenna expert), it is not necessary to connect an antenna ground plane to the airframe. However, the airframe could be used as a ground plane if of sufficient size and shape.
Radio frequency current and DC current behave entirely differently. What is an open circuit to DC could be a short circuit for radio frequency current.

An antenna ground plane does not necessarily have to be straight or flat. Nor does it have to be a sheet of metal. Curved strips of wire or metal tape that radiate outward from the antenna base could serve as a ground plane. Experiment to find something that works.
http://www.aeroelectric.com/Reference_Docs/Antenna/
-
"ARRL Antenna Book" The 2 meter band is close to the aviation band.
-
https://ia801600.us.archive.org/7/items/TheArrlAntennaBook/Hall-TheArrlAntennaBook.pdf[


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Argonaut36



Joined: 19 May 2019
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PostPosted: Mon Sep 16, 2019 7:44 pm    Post subject: Re: Grounding radio antenna and transponder antenna Reply with quote

Many thanks for the additional information provided. I will go through the documents linked to your post.

Please note that I have included additional information on my antenna set up in my reply to the post of Bob.


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ceengland7(at)gmail.com
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PostPosted: Wed Sep 18, 2019 7:59 am    Post subject: Grounding radio antenna and transponder antenna Reply with quote

If the copper tape isn't soldered or otherwise electrically bonded to the rest of the metal ground plane (panel), then it's not doing anything productive. You could get what you were hoping for by using strips of aluminum; even something really thin like strips cut from aluminum flashing material. They could be riveted on one end to the aluminum doubler shown in the pics, and glued with clear 'sensor safe' rtv to the plexi, and screwed at the other end using the plexi mounting screws. 
Having said that, the antenna still won't be centered in the ground plane. You could add strips going the other direction using similar techniques. The 'ideal' ground plane extends out from the base as far as the antenna height. As few as 4 equally spaced radial arms can get the job done pretty effectively. 
Having said *that*, are you sure your problems are purely transmission range? Tube/rag a/c are notoriously noisy in the cockpit. You may have as much a problem with *audio* signal to noise ratio as with transmission distance. Ability to accurately describe comm deficiencies is pretty rare, even for controllers. If you think that could be a possibility, we can expand on that. 
Charlie
(sound tech in one of my previous lives)
On Tue, Sep 17, 2019 at 9:45 PM Argonaut36 <fmlibrino(at)msn.com (fmlibrino(at)msn.com)> wrote:

Quote:
--> AeroElectric-List message posted by: "Argonaut36" <fmlibrino(at)msn.com (fmlibrino(at)msn.com)>

Thanks to both for the additional comments.
I reply to the questions of Bob as follows:
•       I do not have the old coax any more
•       I have never used a hand-held radio in this airplane
•       The radio performance is ok for Class D operations, but not so good for Class B and Class C, when you need to communicate from further out and clarity of communications is even more important
•       As far as additional input, I can tell you that the radio harness was replaced without appreciable changes
I am posting a file that includes 2 pictures of my panel/antenna.  The copper strips are just taped (not soldered) and there is no communing disk.  The strips do not extend laterally, because of the limited width of the panel.  I assumed that, as we normally fly towards the radio station we are talking to, that would be kind of acceptable.
Could you please make comments on my copper foil strips and elaborate a little further on how the set up described by Bob is in his book could be implemented on my panel (keeping the panel removable from the airplane)?
Note: in my previous post with the dimensions of the ground plane I have just disregarded the copper foil strips and I have assumed that the section of panel behind the plexigas window and the double plate work together as a ground plane.
Thanks




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Argonaut36



Joined: 19 May 2019
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PostPosted: Wed Sep 18, 2019 7:06 pm    Post subject: Re: Grounding radio antenna and transponder antenna Reply with quote

Thanks to All for the interesting comments and suggestions.
In this post I would like to address the comments of Charlie on noise in the cockpit. I need a little more time to write a second post on the antenna solutions that have been proposed for testing (coax and external copper foil). I may have a couple of questions.
Yes, the problem is not just range. It is also noise. ATC says that my transmissions are weak and with a lot of static noise, but still acceptable. As I wrote in an earlier post, that is not the case if I go to Class B or C airspace and I need to call from far out.
My impression is that I receive better than I transmit. The noise level in the cockpit is very high and when I press the push-to-talk button, the static noise that I hear in the sidetone is louder than my voice.
I use what is considered a very good noise canceling microphone that I periodically replace. My understanding is that the noise wears out the microphone. I have also flown with a Bose headset; transmissions were better, but only marginally.


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