Comm antenna ground plane

[dhmoose]

I'm not well versed with the inner workings of antennas so I wanted to pose a question to the group. I'm about to install a comm antenna on the top of the tailcone just behind the junction of the fiberglass cabin cover and the aluminum of the tailcone. It'll be on the aluminum...but I wanted to make sure that the performance of the antenna (ground plane?) isn't degraded by the proximity to the fiberglass junction.

Any thoughts?

Thanks,
David

 

[BobTurner]

It's not the fiberglass but the lack of a conductor. Yes, there will be an effect. How much is the question. If you can move it back from the fiberglass by 10" that will give you a quarter wavelength of aluminum, and you should not notice much degradation. You can always add some aluminum or copper screen under the fiberglass, electrically tied to the tailcone.

 

[dhmoose]

Hmmm

Bob, Trouble with moving it 10" back is that the comm antenna will then be 2' away from the ELT antenna (manufacturer recommends 3'). I'm not sure how important the 3' rule is for the ELT considering the ELT antenna transmission quality will only be important if things have gone really wrong

I think I'll consider installing a ground plane into the fiberglass and make it electrically connected to the tailcone. I'm not sure how to do that, but I'll do some homework. Thanks!
David

 

[BobTurner]

I'd put the metal screen on the inside, on the ceiling of the baggage area.
Do you have a gps antenna? Try to keep the com as far from it as possible.
Yes, ELT antenna can affect the com transmission/reception.

 

[dhmoose]

Can't

I have the Aerosport overhead panel up there so I can't access the area for the ground plane. What about digging out some of the cabin cover glass from the outside, then glassing over it? And what metal screen material are you referring to for the job?

 

[Bill.Peyton]

I mounted mine back a slight amount rear of the FG to establish a sufficient ground plane. I have compared reception with the belly comm numerous times in flight and not seen any appreciable difference between the two. On the ground, the upper antenna definitely works better for surface communication.

One thing you need to consider is the top of the fuse at that juncture is not flat. If you simply make a doubler for the skin and rivet the doubler to the skin you will distort it. I anchored the antenna support doubler as shown in the photo below.

I then created a shim out of .062 and tapered it so that follows the contour of the outer skin that rides only between the doubler and the antenna. Now the surface of the outer skin is not distorted by the antenna nor the doubler. The antenna is rock solid using this scheme.

I would not be concerned about the ELT antenna proximity. The only time that it would be operational is in a crash. In addition, they are so far apart in frequency I can't imagine how it would matter (unless your using a 121.5 ELT of course). BTW, I also mounted my ELT antenna inside the Aerosport overhead console. It's mounted horizontally to the rear baggage bulkhead and protrudes through into the overhead console. Yes, I know it's supposed to be vertically polarized, but who know how the plane will end oriented up in an accident.

 

[Walt]

Ideally for optimum performance the ground plane should be about 43” in diameter for the VHF antenna (1/4 wave length).

This being impratical many times, try to make it as large as possible, a good rule of thumb is the radius should be at least the length of the antenna.

Many of the tuned antennas are about 18" long so 36" diameter should be the minumum. This also happens to be the recommended distance between comm antennas.

 

[Auburntsts]

Like you, I have a com antenna mounted a couple of inches aft of the cabin-tailcone seam. What I opted to do was to extend the ground plane onto the cabin by embedding 3 strips of copper foil tape in the top on the outside. The strips run from the mounting location onto the top --1 strip runs forward parallel to the longitudinal axis and the other 2 run out at 45 deg.

 

[Wayne Gillispie]

I installed a 24" diameter, 016" aluminum ground plane on the bottom side of cabin cover above rear seats. Works great, especially on the ground.

 

[dhmoose]

Not optimal

For optimum performance the ground plane should be about 46? in diameter for the VHF antenna.

That's a lot of real estate! And I don't think it exists on the tailcone. I suppose "less then optimal" will be the result or I need to pick a new location.

 

[dhmoose]

I mounted mine back a slight amount rear of the FG to establish a sufficient ground plane. I have compared reception with the belly comm numerous times in flight and not seen any appreciable difference between the two. On the ground, the upper antenna definitely works better for surface communication.

One thing you need to consider is the top of the fuse at that juncture is not flat. If you simply make a doubler for the skin and rivet the doubler to the skin you will distort it. I anchored the antenna support doubler as shown in the photo below.

I then created a shim out of .062 and tapered it so that follows the contour of the outer skin that rides only between the doubler and the antenna. Now the surface of the outer skin is not distorted by the antenna nor the doubler. The antenna is rock solid using this scheme.

I would not be concerned about the ELT antenna proximity. The only time that it would be operational is in a crash. In addition, they are so far apart in frequency I can't imagine how it would matter (unless your using a 121.5 ELT of course). BTW, I also mounted my ELT antenna inside the Aerosport overhead console. It's mounted horizontally to the rear baggage bulkhead and protrudes through into the overhead console. Yes, I know it's supposed to be vertically polarized, but who know how the plane will end oriented up in an accident.

I think I'll follow your lead Bill. Compromise on the ELT proximity to the comm but get a good ground plane for the comm antenna. And I know what you mean with the non-flat surface. I took that into account for the ELT install and will do it again for the comm.

I liked Todd's copper foil idea too...but I'm not sure how to cover the foil reliable to prevent paint problems.

Thanks to all for your help!
David

 

[Auburntsts]

I liked Todd's copper foil idea too...but I'm not sure how to cover the foil reliable to prevent paint problems.

What I did was to sand out shallow channels into the cabin top , laid in the foil, and covered it with micro. Time will tell if this approach works ......

 

[bob888]

comm antenna location

What's wrong with the belly, under rear seats? Much better ground plane available without these other mods.

 

[J Twilbeck]

What's wrong with the belly, under rear seats? Much better ground plane available without these other mods.

Nothing. I mounted mine in this location (not flying yet, although many RV-10's mount in the same location). The concern is reception when your on the ground receiving clearance or other transmissions. It may not be as clear than an antenna mounted on top of the aircraft.

 

[Bill.Peyton]

The only issue I have with the belly antenna are occasional complaints from Ground and Tower when I forget and use the lower radio/antenna combination.

 

[Wayne Gillispie]

What's wrong with the belly, under rear seats? Much better ground plane available without these other mods.

If we have an extremely hard landing or take it off in the trees, maybe the top one will survive. $25 and 3 hrs of extra build time was worth it to me.