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-   -   Second VHF antenna (https://vansairforce.net/community/showthread.php?t=200022)

olegusan 10-12-2021 12:11 AM

Second VHF antenna
 
Hi all,

I'm going to install a second radio in my RV12 and trying to figure out the best place for the second antenna.

Can anyone share their experience with the subject?

12vaitor 10-12-2021 03:16 PM

I added my second comm antenna on top of the tailcone behind the ADAHRS location. In addition to a doubler plate at the antenna location, I added some J-channel reinforcement between the frames. The skin thickness seemed too thin to handle the drag load specified by the antenna manufacturer. Similar problem to the ADARHS mounting area that required a doubler to stiffen the mount.

The other problem is running another coax cable down the center channel, I ended up adding another set of pass through bushings to handle the cables for the Garmin GPS antenna, NAV/LOC antenna, and Comm 2.

https://flic.kr/p/2mAcd3T

John Salak
RV12 N896HS

rcarsey 10-12-2021 08:35 PM

I'm following this. I have a ham radio antenna (146Mhz) that I've been meaning to install, but haven't pulled the trigger on yet. Unsure of how close is too close .. the frequency is close enough to aviation band that it can be an issue.

Maybe if I put it out on a wing, I won't have to install a rudder trim tab :)

RFSchaller 10-12-2021 09:57 PM

I installed a second Com and put the antenna on top of the fuselage about 3 feet behind the rear window.

turbo 10-13-2021 12:04 PM

I have two coms and an ils/vor on the bottom spaced about 3 feet apart, vor at the tail end. :)

RFSchaller 10-13-2021 01:08 PM

When I installed the cat whiskers for a VOR/ILS I drilled out the blind rivets securing the fiberglass vertical stabilizer cap, mounted the whiskers on top and replaced the cap over the whiskers using screws and nut plates in place of the rivets.

gemiller 10-31-2021 08:31 PM

You will generally find that a second comm antenna should be mounted on the opposite side of the fuselage from the first comm antenna. If that is not possible, 4ft separation is what I would recommend. The comm radios have very sharp filters so the only real risk is jamming the receive comm with the transmit comm when they are tuned with little frequency separation ( a few MHz). I have seen dual comm antennas spaced much closer than 4 feet and I suspect for those installations the user rarely, if ever, tuned the two radios to nearly the same frequency.

My experience comes from 40 years of designing antennas for aircraft at a large aircraft manufacuring company.

Good luck.

Cumulo 10-31-2021 11:14 PM

Quote:

Originally Posted by gemiller (Post 1565388)
You will generally find that a second comm antenna should be mounted on the opposite side of the fuselage from the first comm antenna. If that is not possible, 4ft separation is what I would recommend. The comm radios have very sharp filters so the only real risk is jamming the receive comm with the transmit comm when they are tuned with little frequency separation ( a few MHz). I have seen dual comm antennas spaced much closer than 4 feet and I suspect for those installations the user rarely, if ever, tuned the two radios to nearly the same frequency.

My experience comes from 40 years of designing antennas for aircraft at a large aircraft manufacturing company.

Good luck.

Agreed.

Once upon a time I had an FAA repair station. A 206 Stationair drove up and asked me to check out a comm problem. It was the stock factory set-up with 2 rather robust Cessna ARC transceivers - usually got about 12 watts of transmitter power on the meter. Well, I unknowingly pulled the wrong comm cable and measured about 3 watts on the wrong antenna! Two resonant antennas can exhibit a surprising amount of coupling even at 4 feet.

So, the ~4 feet of separation of the comm antennas on nearly all Cessnas is marginally close .

I like one one the top, one on the bottom as well.

Ron

Richard Connell 11-01-2021 05:52 PM

I notice youre in Melbourne.
Area freq on ground is pretty ordinary in a lot of places and antennae on the belly are near useless.
Id def put the second on the top.
I had 2 belly mounted com antennas on my RV7 and have one top and bottom for my RV10
Cheers

gemiller 11-02-2021 06:42 PM

The smaller fuselage diameter of the RV12 relative to the RV7 will provide better radiation pattern coverage for the bottom mount antenna. The antenna mounting location on the bottom may be significantly blocked by the wing. I have found that I have had no problem communicating on the ground with my (new to me) RV12. My antenna is mounted aft of the wings.

I also performed a numerical simulation to determine the improved isolation provided by a top/bottom mount combination versus a bottom/bottom or top/top. The technical number is about 13 dB or (20 times) isolation improvement.


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