MINIMUM Coax Length for GPS Antennas on 650 and 430?

[RONSIM]

I am installing a GTN 650 and a GTS 430 -- I thought I had heard there is a MINIMUM coax length (RG400) for the GPS antennas---- I need to order the coax --- anyone have the information?

Thanks for the help,

Ron

[Walt]

You'll probably need the manual to install it too. But 6.5' to 35' of RG142/RG400 will do the trick. (1.5-6.5db loss).

[RONSIM]

I knew VAF would come through! --- I have the manuals, but could not find a specific length.

Much appreciated!

Ron

[Carl Froehlich]

Why is there a minimum length? This makes not sense.

Carl

[Bill Dicus]

Is specified in the install manual. According to the upgrade installation manual for the 400 W/500 W series, the GPS antenna coaxial cable must have a minimum of two shields (that is RG 400 or RG 142B). The minimum length is approximately 13 feet of RG 400, producing a loss of 3dB. It's a lot of coax! Adding any connectors or adapters (eg BNC to TNC) increases the loss so you could use less coax. The required loss ranges from 3 to 7 dB according to the manual (see 2.5.4 and 3.10.1).

[g3xpert]

Hi Ron,

Here is a link to our previous answer to this question.
http://www.vansairforce.com/communit...64&postcount=4

The GPS 20A has the same requirements for obvious reasons.
http://www.vansairforce.com/communit...5&postcount=50

Let us know if you have additional questions.

Thanks,
Steve

[BobTurner]

Originally Posted by Carl Froehlich

Why is there a minimum length? This makes not sense.

Carl

Reason, in English: Garmin made the receiver as sensitive as they could, a worthy design goal. But the antennas are mostly from non-Garmin OEM's, and some have more gain than anticipated. The extra signal strength can cause the receiver to not meet all the TSO specs, like overload, rejection of other signals, etc. Rather than re-designing the receiver to have lower sensitvity, it is easier to just insert some loss - coax, attenuator, etc.