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Com antenna distance

mototopo

mototopo

Well Known Member
I just finished fabricating the antenna reinforcing plates to install two Rami bent Com antenna under the seat floor.
Unfortunatelly, I initially choose to mount them in the third bay (the same bay where there is the crotch strap support) to have a nice flat rib web to rivet the doubler flange for added strengt. Only when finished I realized I don't have the 2 feet distance between the antennas as indicated on the installation manual (the actual distance is about 18 inches).

Since I haven't yet drilled the floor, I can still mount them elsewhere, but I'd like to know if the reduced distance can cause problems "in the real world".
One more doubt: if I move the antennas one bay outer, is the distance from the edge of the floor enough to provide a good ground plane?
 
As far as radio performance, most likely they will work fine there.

However, depending on the radio and other factors, you most likely will get bleed over. If you plan to use the "split" mode that some audio panels have, most likely you will have issues.
 
mototopo said:
....

Since I haven't yet drilled the floor, I can still mount them elsewhere, but I'd like to know if the reduced distance can cause problems "in the real world".
One more doubt: if I move the antennas one bay outer, is the distance from the edge of the floor enough to provide a good ground plane?
Click to expand...

The distance to the edge of the floor does not really matter, the lower wing skins will provide the ground plane.
 
Claudio,

I am building a 7 and sitting on the same decision. I have not built the stiffeners, but have twin bent whip comant's on hand as well. I was planning on the next bay out board of the crotch strap, just for the reason of congestion, as well as distance between coms. Is there any reason you avoided that bay?
 
A good rule of thumb is to have the antennae no closer than the length of the longest antenna apart from each other.
 
Thanks to everyone. I'll install them one bay out than the current chosen position.

BillL said:
Claudio,

I am building a 7 and sitting on the same decision. I have not built the stiffeners, but have twin bent whip comant's on hand as well. I was planning on the next bay out board of the crotch strap, just for the reason of congestion, as well as distance between coms. Is there any reason you avoided that bay?
Click to expand...

I fabricated an L-shaped doubler with a 1/2" flange on one side for added strenght. The idea was to rivet this flange to a rib web and since the ribs where the crotch strap is installed have the webs pointed inward, the installation looked easy and clean. On the outer bay, I have the rib's flange on the way and I'll have to fabricate a different kind of doubler (I hope my description is enough clear... I don't know ho to insert an image to better explain my decision).
 
Antenna spacing

I don't believe there is a hard and fast rule for antenna spacing, but yes I agree with Norman CYYJ that the length of the longest element used (1/4 wavelength at the lowest frequency) is a good guide to reduce not only interference but also radiation pattern. IMO the power output of the transmitting radio is more of consideration to factor in (higher power = more interference to adjacent/close proximity radios)
 
Positiioning com antennas

I'm not sure but I thought that com antennas should be mounted in a verticle position in order to be most effective. Appreciate feedback.
 
az_gila said:
The distance to the edge of the floor does not really matter, the lower wing skins will provide the ground plane.
Click to expand...

True, but only if the wing and fuselage skins are electrically bonded together (which most are not).
 
Walt said:
True, but only if the wing and fuselage skins are electrically bonded together (which most are not).
Click to expand...

I bet after a very short time the screws at the fus. lower wing skin joint will make that bonding in a good enough fashion for RF...:)

If not, removal of a very small amount of paint will ensure bonding.
 
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