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  #11  
Old 03-14-2021, 04:12 PM
HAL Pilot's Avatar
HAL Pilot HAL Pilot is offline
 
Join Date: Nov 2015
Location: Las Vegas NV
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The F-01479 Skin piece is quite thick I suspect it does not need a doubler. It is probably a good Idea to mount the antenna before mounting the Horizontal Stab as there would be room to reach in from above. However it looks like if you do have the Tail feathers on you can still reach in through the two holes in the front part of the Aft deck.

.
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Last edited by HAL Pilot : 03-14-2021 at 04:20 PM.
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  #12  
Old 03-15-2021, 07:40 AM
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rockitdoc rockitdoc is offline
 
Join Date: May 2020
Location: Fort COllins, CO
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Quote:
Originally Posted by HAL Pilot View Post
The F-01479 Skin piece is quite thick I suspect it does not need a doubler. It is probably a good Idea to mount the antenna before mounting the Horizontal Stab as there would be room to reach in from above. However it looks like if you do have the Tail feathers on you can still reach in through the two holes in the front part of the Aft deck.

.
I called tech. They said a doubler is recommended. We put in a 0.032" piece reaching in from the top as you suggested. It wasn't too bad. Dimpling the skin was the hardest part, but we managed with the Cleaveland close quarters tool using steel nails as mandrels. I broke every one but they made acceptable dimples. It's a two person job to rivet, but now I can rest assured the antenna will not be vibrating back there.
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  #13  
Old 03-15-2021, 08:37 AM
rapid_ascent rapid_ascent is offline
 
Join Date: Mar 2008
Location: Dublin, CA
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Vans sells a small doubler for the transponder type antenna. I bought two of them. Mine are installed just aft of the firewall. I couldn't reach to do the riveting singlehanded so I rigged up my backriveting plate with my floor jack pressing the plate up against the bottom skin of the plane. I backriveted them from the inside. Worked like a charm. You might be able to do something similar.
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  #14  
Old 03-17-2021, 11:19 AM
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bjdecker bjdecker is offline
 
Join Date: Jun 2005
Location: Georgetown, TX
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Quote:
Originally Posted by Walt View Post
Actually power loss is quite high at these freq's, Garmin recommends max 2 db loss in the STC IM (approx 9' RG400 with 2 connectors) for new generation xpdrs with high power. The older xpdrs were 1.5db max loss.

A 3db loss means only half the power from the unit actually makes it to the antenna.

(RG400 has a loss of approx 18.5db per 100' and each connector is avg at .2 db)

If you have 15' of RG400 on your xpdr (approx 3 db loss) you may have trouble passing the 91.413 check due to low power and sensitivity not to mention your range will be reduced. If you need this long of a cable you should use a lower loss coax.

Recommended ground plane for xpdr antenna is 8" dia not 2.8"
Walt,

Thank you as always for keeping us honest. However, I do have a couple of points that need to be clarified/understood.

1/4 wave monopole antenna design use the formula of L = 234/Freq -> e.g. 234/1090, with 135 for the legs to achieve the correct impedance in air. I see in the Part 23 AML for the GTX 3x5, Garmin is calling for 12" for the ground plane; what gives here?

I also see the 2.0dB call out in the AML for maximum loss which is about 9' of RG-400 (18.5dB/100ft, + ~.2dB for each BNC). In the RV-14 with a panel mount GTX 3x5 it looks like you need about 16' of feedline. Van's specs RG-400 for this installation. What did you do in this case? Remote mount the 3x5 or use other feedline? Or just take the extra .5dB loss and ignore the AML...

Cheers!
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  #15  
Old 03-17-2021, 03:23 PM
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Walt Walt is offline
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
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Quote:
Originally Posted by bjdecker View Post
Walt,

Thank you as always for keeping us honest. However, I do have a couple of points that need to be clarified/understood.

1/4 wave monopole antenna design use the formula of L = 234/Freq -> e.g. 234/1090, with 135 for the legs to achieve the correct impedance in air. I see in the Part 23 AML for the GTX 3x5, Garmin is calling for 12" for the ground plane; what gives here?

I also see the 2.0dB call out in the AML for maximum loss which is about 9' of RG-400 (18.5dB/100ft, + ~.2dB for each BNC). In the RV-14 with a panel mount GTX 3x5 it looks like you need about 16' of feedline. Van's specs RG-400 for this installation. What did you do in this case? Remote mount the 3x5 or use other feedline? Or just take the extra .5dB loss and ignore the AML...

Cheers!
Yes if you actually do the calculation at 960mhz (ADS-B in) comes out to about 6" minimum. Maybe Garmin prefers a 1/2 wavelength which would be 12".

If I was mounting the xpdr in the panel I would definitely find a location up front for the antenna, the other option (which I usually do) is remote mount the xpdr in the rear). No I would not accept the 1db loss for the extra 6 ft of cable.
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Last edited by Walt : 03-17-2021 at 03:28 PM.
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