[flickroll]

I am most likely going to add a G 430W to my -8 which will join up with a G SL30 and G 327 xponder. My current antenna configuration is a bent whip on the belly, and directly opposite that antenna is a blade type transponder antenna. I'm thinking about putting the second comm antenna where the transponder antenna currently is, and mounting the blade transponder antenna elsewhere on the belly. Does anyone have any idea on how close a transponder antenna can be to a comm antenna? And will having the bent whips within 2 - 3 feet of each other be a cause for concern? thanks

 

[the_other_dougreeves]

You'll be better off keeping the COMM antennas separate.

XPDR antenna has requirements in terms of how close it can be to the actual transponder. Some separation between the COMM and XPDR antennas won't hurt.

TODR

 

[flickroll]

You'll be better off keeping the COMM antennas separate.

That's exactly what I'm planning to do, but on an -8 they'd be about 2' apart which is pretty close. For those with dual bent whips on the belly of an -8, how has the performance been? I suspect that modern radios are less susceptible to problems than those of older vintage.

Has anyone mounted a transponder antenna to a wing inspection panel? An avionic tech friend of mine says that a transponder antenna should not be close to the comm antennas, so my plan to mount the xponder antenna aft of the comm antennas may not work so well.

Thanks

 

[RVG8tor]

What I did

I just went through this decision process on my -8A. I put the two VHF up front just behind the wing spar, have seen this on many 8s and no one seems to have issues with them in this configuration. I have the GTX-327 and the book says to keep the transponder antenna 3' from other antennas and at least 3' from the actual transponder box. There is also a guide in the install manual about cable types and maximum lengths to keep the signal lose acceptable. I have RG400 and this limits you to 6 feet. Oddly some showed 8 feet in an earlier version of the install manual.

These constraints make it difficult to find a spot for the transponder antenna on the RV-8, if you stay under the cable length then you are too close to the VHF antennas. I chose to keep the transponder antenna the 3 feet from the VHF antennas and accept the longer than recommended length of RG-400. There are more expensive types of cable that will work, so my plan is if I have issues using the RG-400 then I will replace it with the higher price stuff.

I put my transponder antenna just aft of the rear seat bulkhead just left of center but still between the center baggage floor support ribs. I think you could just add the VHF to the rear location and still be OK, hopefully someone flying with that type of configuration will chime in and tell you how they have worked for them. There are pictures on my log, the link is below. I am not flying so not sure how this will work but I have a configuration similar to others.

Cheers

 

[gvouga]

One Data Point

I am most likely going to add a G 430W to my -8 which will join up with a G SL30 and G 327 xponder. My current antenna configuration is a bent whip on the belly, and directly opposite that antenna is a blade type transponder antenna. I'm thinking about putting the second comm antenna where the transponder antenna currently is, and mounting the blade transponder antenna elsewhere on the belly. Does anyone have any idea on how close a transponder antenna can be to a comm antenna? And will having the bent whips within 2 - 3 feet of each other be a cause for concern? thanks

I have 2 Comant (CI-122 I think) bent whip antennas on the belly of my -7A. The base of each antenna is mounted about 6" behind and below the control sticks when viewed from under the seat pans. The spacing is about 2'. My Transponder antenna is mounted on the leftside of the fuse directly under the whip of the COM antenna.

I know this probably would not be the preferred mounting location for the transponder antenna since it is under (or above depending on how you look at it) the COM whip, but it works fine. I added the COM antenna later and just decided to leave the transponder antenna in place to see if it worked. No issues with either the transponder or COM's. Actually, the COM's have incredible range compared to what I'm used to in other planes. I routinely hear and talk with people over 100miles away.

Greg
RV-7A

 

[Ironflight]

Another data point for you Jim,

My little transponder antenna is mounted on the left side lower wing-to-fuselage fairing. That is actually outside the fuselage, but attached to the belly skin that sticks out to close the gap between the fuse and wing. I mounted it just forward of the aft spar. That put it far away from my single comm antenna (just aft of main spar, about 4" right of centerline) and I have had no interference problems. Occasionally, I think of putting another bent-whip comm antenna on the belly to replace my wingtip comm (for #2), bu then I think about pulling up the floor to do it, and find something else better to do...

Paul

 

[flickroll]

Good discussion guys, thanks for the input. Another constraint that I have is that I wish to install dual AHRS just forward of the fuselage bulkhead where the static ports are located (608??). The AHRS units need to be away from a transponder antenna, so mounting the antenna aft of the rear seat back may not work for me. I just dusted off my G327 installation manual and it says to keep the xponder antenna at least 3' from a communications antenna, although in my current configuration my transponder antenna is 2' from the comm antenna and both the xponder and comm work well. I bet with today's avionics it's not nearly as critical as it used to be. This is more or less confirmed by 'non standard' installations (including mine) that appear to work well.

 

[gvouga]

Good discussion guys, thanks for the input. Another constraint that I have is that I wish to install dual AHRS just forward of the fuselage bulkhead where the static ports are located (608??). The AHRS units need to be away from a transponder antenna, so mounting the antenna aft of the rear seat back may not work for me. I just dusted off my G327 installation manual and it says to keep the xponder antenna at least 3' from a communications antenna, although in my current configuration my transponder antenna is 2' from the comm antenna and both the xponder and comm work well. I bet with today's avionics it's not nearly as critical as it used to be. This is more or less confirmed by 'non standard' installations (including mine) that appear to work well.

I've also seen several -8's with the dual bent whips on the belly under the pilot seat and a transponder antenna in front of the main wing spar. I have heard this setup works well, but I don't have first hand knowledge.

My AHRS from AFS is mounted on the first bulkhead behind the baggage compartment in my -7A. Roughly 4' from my transponder and COM antennas on the belly. I've had no issues.

Greg
RV-7A

 

[Mel]

Rule of Thumb!

A good general rule of thumb for antennas is that no other antenna should be closer than the antenna length at any point. In other words, a 24" antenna should be no closer than 24" to any other antenna measured at any point.