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Antenna Question

M

MTBengel

Active Member
I plan on installing my (secondary) radio antenna on the top, mid-fuselage. The skin is a bit thin back there... will a 6"x9" (.032) backup plate provide enough rigidity? I would also like to mount my ELT antenna about a foot behind it. Since the ELT antenna would not be operating (outside a crash), is this an issue?
 
Even though the ELT antenna is not transmitting, it will still interfere with the com antenna signal. There should be 2' between the two.
 
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Thanks guys... always better to ask before drilling holes. Any thoughts on the backup plate and rigidity mid fuselage for the COM pole antenna?
 
Resurrecting my antenna theory here:

The middle of the aviation band (123.0 MHz) has a 1/4 wavelength of about 24 inches. That means the the nearby antenna receives and re- radiates the signal arriving back at the transmitting antenna 180 degrees out of phase, which creates a null. I would avoid any distance between 21-27 inches if you can.

This is how old style directional TV antennas worked... It was a parasitic array of precisely spaced passive elements.

Anyone with more knowledge is free to comment...

Don
 
MTBengel said:
I plan on installing my (secondary) radio antenna on the top, mid-fuselage. The skin is a bit thin back there... will a 6"x9" (.032) backup plate provide enough rigidity? I would also like to mount my ELT antenna about a foot behind it. Since the ELT antenna would not be operating (outside a crash), is this an issue?
Click to expand...

I installed a Comant blade antenna in a simliar location on my -6A. I fashioned a doubler that went between the hat sections; this doubler has flanges which attach to the hat sections while the doubler itself is riveted to the skin underneath the antenna.

This has worked well; it's rigid and it conforms to the principles laid out in AC43.13.
 
Thanks all, this really helps. One more question... what would be the closest acceptable distance to place the ELT antenna from the vertical stabilizer? To get 24" away from the Com antenna the ELT antenna would end up about 10" from the stabilizer.
 
Mark,

No hard answer for that question. Intuitively, I don't think 10 inches would be a problem. Wherever you mount it there will theoretically be some blanking in that direction. The closer to the stab the larger the angle of blanking, but my gut says it would be minimal. You could probably get as close as 6 inches, as long as the slipstream doesn't bend the antenna back and allow it to hit and rub on the stab.

How much does it bend? Drive down the freeway with it stuck out the window and see what it does at 70 or so. Real scientific huh?

On my -8 there is little real estate for an ELT antenna, so I used a "rubber-duckey" antenna mounted just behind the rear passenger's right shoulder.

Don
 
MTBengel said:
Thanks all, this really helps. One more question... what would be the closest acceptable distance to place the ELT antenna from the vertical stabilizer? To get 24" away from the Com antenna the ELT antenna would end up about 10" from the stabilizer.
Click to expand...

10" is too close for the simple reason that in flight it will bend back and scratch your VS. I've seen an otherwise nice RV with scratches on both sides of the VS from the ELT antenna.

A number of guys mount them longitudinally under the empennage faring.
 
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