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gereed75

Well Known Member
How big of a ground plane do these antenna need to be effective?? It's about a 21" long bent whip that came with my new ACK (both 121.5 and 406) being installed to replace the old unit.

I have long suspected that the popular two places to put these (under the emp fairing or somewhere under the canopy in the baggage compartment) provide insufficient ground planes for effective operation.

Trying to do it better this time around. Antenna guys???? Thanks
 
Gary,
I am not antenna guy but I had an experience :) My inspector specifically noted prior to inspection that he won't be pleased to see ELT antenna under empennage fairing. He cautioned that putting very important antenna "somewhere" in the baggage compartment is not a good idea either.
 
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Mine is under fiberglass cabin cover on 4" wide aluminum crossbrace behind rh rear seat. I figured it would work better than under emp frg. If it gets damaged during a crash it probably won't matter if we get help. If we go down in trees I feel it has a better chance of remaining intact inside the cabin.
 
Placing ELT antenna

the ground plane should be 21inches in diameter for 121.5 or 11inches for 406. Ground planes for monopole whip are best at 1/4 wave radius. Formula is 492/freq. In MHz=1/2 wave length in feet. Remember that the key to the ELT working is the signal radiating up to the sky since that's where the satellite is for locating the 406 and airplanes searching will be looking for 121.5 from above you. Under the canopy is fine using an angle mount attached to the crossing memer back of the seat with the whip tilted back tip towards the baggage bulkhead. Use GOOD coax RG400!!

Tyler
N174MT
K4NLM
 
What are the odds that the ground plane gets crumpled or the antenna gets dislodged in a crash?

If the airframe is grounded and is buried/resting in the ground, would the actual earth function as a ground plane albeit more than likely in the wrong orientation?
 
AC 43 has specs for ground planes in 2B. But make it as big a practical because the size they quote makes for a difficult install. The ideal plane should be the length of the antenna in all directions, but as said before this is not practical. Keep the antenna as far as possible from comm antennas too, I've had to troubleshoot comm signals getting re radiated through the elt antenna because of their close proximity to the comm antenna.
 
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