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Topside antenna and the magnetometer (GMU 11)

Riessen

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I plan to install a second com antenna on top of the tailcone (sometime post-certification, of course…) and Garmin’s documents and proximity table identify various sources and types of electrical interference, like unshielded conductors, motors, etc, but no concern or mention about com antennas.

Does anyone have experience with the GMU 11 and distance/interference from com antennas? Is it likely or possible?

Additionally, regarding measuring antenna separation… measured from tip or base? Such as distance from the vertical stab?
 
Installation of a COM-antenna on TOP of the tail cone will not work.
The reason is that the COM-antenna require a ground plain of at least 2 ft.
Referring to the IM-manual rev AQ, "The GMU 11 is an extremely sensitive three-axis magnetic sensor".
With that in mind, it should be installed in a position where there is NO other
electrical devices, cables or magnetic components or HF fields.
If no such place exists, a compromise have to be made. Move nearby cables
or equipment as far away as possible. Try to achieve a distance of 2 ft to other installations as far as possible. The GMU 11 is what gives course guidance to your EFIS. A VHF radio throws out 6-10 W to the antenna, a strong field that is known to interfere with other electrical installation in an aircraft, particularly those that are sensitive.
If you find that there is no possibility to move a cable, use a shielded one.

Good luck
 
Is the tail off e of an rv12 composite or something? If it’s aluminum like all the other RVs I don’t see why an antenna on top wouldn’t work. The whole tail cone becomes the ground plane.

OP- I can’t help you specifically re com antennas, but on my RV 7 the magnetometer is on top of the aft deck under the empennage fairing. It’s only about 6” from the ELT antenna and it passed the interference check easily.

Maybe it be a different story if you’re actually broadcasting but even so, there are tons of RV7s and 9s and whatnot with the magnetometer back in that location (recommended by garmin) and a com antenna on top of the tail cone no more than probably 18” or so forward and ther seem to work ok
 
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I added a second comm with an antenna mounted on top of the tail cone about half way between the canopy and vertical stab. Works fine. My Cherokee has both comm antennas similarly mounted with no issues. Do yourself a favor and buy an antenna that incorporates a balun.
 
Oops! Never mind the comment on the balun. I was thinking of the cat whiskers I installed at the same time for my VOR/ILS. The rest applies. The comments on the ground plane are true up to a point. The larger the ground plane the more omnidirectional the Xmit/rcv pattern. Since the fuselage is longer than its width the pattern will allow best reception along the longitudinal axis of the fuselage, but you’ll still have adequate sensitivity to the sides.
 
What did you work out?

It has been nearly a year since your original post. I am making the same plans to add a second com on the RV-12iS I am building.

I am wondering what you finally worked out?
 
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