GA35 GPS Antenna Mounting

[rapid_ascent]

I'm in the process of making a doubler for my GA35 which I plan to mount on the top of the aft fuselage. My question has to do with the actual mounting method for the attachment of the mounting screws. I had planned to use floating platenuts as I have seen other use and in fact Stein sells a doubler kit with this type of platenut. When I went to read the mounting instructions (yes this time I did) it calls out use of a simple flat washer and locking nut for the mounting screws. This is unlike the GA57X doubler instructions which describe the use of floating platenuts as a mounting option. So, is the use of platenuts considered non-compliance with the installation instructions. BTW this antenna is for my GTN650xi. I realize as experimental builders we can probably do whatever we want but I was more curious on a technicality basis.

 

[terrye]

Not sure of the "non-conformance" aspect, but using the recommended washers and locknuts means you need two people to install on the aft fuselage. Also don't think the floating platenuts are necessary. I used 4 x non-floating corner platenuts riveted to a mounting plate. Laid out and drilled accurately and checked with the antenna before installation. I'll tape the mounting plate to the doubler and install the antenna mounting screws from the outside.

 

[rapid_ascent]

I was referring to what would be required for standard GA non-experimental aircraft when I said non-conformance. This antenna will be used for an IFR navigator. I like to know the rules even if I decide to break them.

 

[Raiz]

I once installed an antenna with floating nut plates but it failed the DC ground resistance requirement. It was about 5 times higher than the 2.5 milliohms specified by Garmin. I don't know how much of a problem this would have been in practice, but I swapped the nut plates out and used regular nuts and star washers, which easily met the requirement.

 

[rapid_ascent]

I'm curious if you had the antenna mounted over a painted surface or bare/alodined aluminum? I could see that happening if it was a painted surface otherwise I would think the base of the antenna to the skin would be sufficient.

 

[Avanza]

NOTE
GPS/SBAS antenna cable loss must be between 1.5 dB and 6.5 dB in order to maintain proper rejection of interference signals. If RG-142B or RG-400 is used, it must be between 6.5 and 35 feet long to meet the cable loss requirement. For longer lengths, use low-loss double or triple-shielded, 50 ohm coaxial cable.
No closer than 2 feet from any VHF COM antenna or any other antenna that may emit interference.
A good ground is essential for acceptable performance.

I had to install a notch filter on both COM-radios and metal braided sleeving
due to interference on GPS reception when transmitting.
Garmin P/N 330-00067-00.

Good luck

 

[rapid_ascent]

I've tried to separate my COM and GPS/SXM coax routes so I'll see how that goes. I did have to run the COM 2 coax in the same bundle when it routed up the firewall though. Also, both of my COM antennas are on the bottom of the plane. Time will tell if I need any additional isolation.

Garmin does have the 2.5 milliohm ground spec so I'll check that when I have the antenna installed.