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  #1  
Old 03-03-2018, 07:12 PM
sblack sblack is offline
 
Join Date: Dec 2009
Location: Montreal
Posts: 1,267
Default Com antenna on rollbar

I want to try to put my com antenna on the roll bar between the main bar and the brace that goes forward at the bottom. I was thinking of fabbing up a ground plane and supporting it by adel clamps off the 2 tubes. I am not sure how big the ground plane has to be. I am using a lynx microsystems whip antenna.

Has anybody used this approach with reasonable results?
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  #2  
Old 03-04-2018, 01:50 AM
Tom Martin Tom Martin is offline
 
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This is not a good idea. Your radio needs a large area as a ground plane. I tried it with resulting damage to other electrical equipment when transmitting.
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  #3  
Old 03-04-2018, 07:33 AM
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Carl Froehlich Carl Froehlich is offline
 
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While the phrase “ground plane” is often used, a better phrase is “counterpoise”.

The standard aircraft 1/4 wave comm antennas needs a counterpoise. This can be aircraft structure, another 1/4” long wire (making a vertical dipole) or other. In your case if you mount the antenna on the roll bar you have a counterpoise.

But - there a significant practical shortfalls to your mounting proposal and I predict you will not be happy with the outcome. Aircraft comm antennas are easy to make work in the standard mounting locations. Is there reason why you don’t want to take the standard approach?

Carl
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Old 03-04-2018, 01:35 PM
sblack sblack is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
While the phrase “ground plane” is often used, a better phrase is “counterpoise”.

The standard aircraft 1/4 wave comm antennas needs a counterpoise. This can be aircraft structure, another 1/4” long wire (making a vertical dipole) or other. In your case if you mount the antenna on the roll bar you have a counterpoise.

But - there a significant practical shortfalls to your mounting proposal and I predict you will not be happy with the outcome. Aircraft comm antennas are easy to make work in the standard mounting locations. Is there reason why you don’t want to take the standard approach?

Carl
I would like to have all my antennas internal to minimize drag. I know there is not a huge drag penalty for a com antenna by itself, but if you look at those people who have tweaked and tuned their airplanes to increase their performance the first thing they do is to fair in everything possible and get rid of any protruding things that they possibly can. Between the transponder, elt and com antennas there is a non negligible drag penalty.
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  #5  
Old 03-04-2018, 01:36 PM
sblack sblack is offline
 
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Quote:
Originally Posted by Tom Martin View Post
This is not a good idea. Your radio needs a large area as a ground plane. I tried it with resulting damage to other electrical equipment when transmitting.
What equipment did you damage? This is the first time I have heard anything like that.
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  #6  
Old 03-04-2018, 02:06 PM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by sblack View Post
I would like to have all my antennas internal to minimize drag. I know there is not a huge drag penalty for a com antenna by itself, but if you look at those people who have tweaked and tuned their airplanes to increase their performance the first thing they do is to fair in everything possible and get rid of any protruding things that they possibly can. Between the transponder, elt and com antennas there is a non negligible drag penalty.
Ok - but perhaps you can achieve your objectives via a different path. For example:
- Wingtip comm antenna - it will work far better than one on the rollbar.
- ELT antenna under the eppenagge fairing (I did this on two planes - works well).
- Blade transponder antenna - the drag from this antenna at even the best RV speed would be hard to measure in a instrumented wind tunnel.

But when you do all this I suspect your find traditional drag reduction efforts to have better results. For example:
- Really make sure your rigging is right. I’ve seen a lot of RVs with rigging that simply scream of added drag.
- Engine cooling air. Do you have too much? Is the air your are taking in cooling the engine or just leaking by and adding drag?
- Have you adjusted the HS angle of incidence? If you cruise around with the elevator trailing edges down from perfect neutral, your HS is inducing too much downward force. Careful here - a 0.040” shim under the HS forward spar makes a big difference.

Carl
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  #7  
Old 03-04-2018, 02:21 PM
sblack sblack is offline
 
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All of those other things are important too. Drag reduction is usually the sum of many small things. I am a aero engineer in my day job and I have spent many months in wind tunnels. Drag reduction is generally a very tedious process involving many small details that are difficult to quantify. Trim drag and cooling drag can be bigger contributors, but excressance drag (lumps and bumps sticking out, or poorly fitting fairings etc) is also important. So I am trying to build as much drag reduction into the project as I can.
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  #8  
Old 03-05-2018, 01:15 PM
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n82rb n82rb is offline
 
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i would be concerned with RF energy right behind my head.......

bob burns
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  #9  
Old 03-05-2018, 04:12 PM
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Xkuzme1 Xkuzme1 is offline
 
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Put the Bob Archer inner wingtip antenna in your wingtip. They work pretty good.

Very easy to install.
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  #10  
Old 03-06-2018, 06:25 AM
ABRV4 ABRV4 is offline
 
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Scott,
I use a copper foil antenna adhered to the inside of the left gear leg. I believe I got it from Vans. I've used it for over 10 years and it works fine. Cheap and no additional drag.
Alan
RV-4
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