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GPS antenna question ?

Larry DeCamp

Well Known Member
Instructions for GA35 antenna to GPS20A specifies minimum 6.5 ft of cable. So I have a 7' RG400 . Instructions say you can coil the cable to fit the space allowed. Is there any problem anticipated with 6 FT of cable with 3 strands coiled in tie-wraps ??
 
You can always make a cable 18” and try it out...
Only if you are willing to troubleshoot time and time again. The requirement is clear and it is also clear that it does not have to be an uncoiled/distance.

To the OP, coiling it is not an issue.
 
Well...heck...

Actually, Garmin specifies the amount of attenuation (in db) -- 1.5 to 6.5db; if you look at the specs for RG-400 and the TNC/BNC connectors at the L1 frequency (1575MHz) that's how they get to 6.5ft minimum. (See 190-01007-C0, Rev.2, Page 4-2, Section 4.1.1)

And it does say "should", not "shall", so I chose to ignore it - figuring that I could always add a loop of coax to the mix if it didn't work.

Turned out that it wasn't necessary, both RV-7s of mine with this mounting are doing well -- and no issues with ADS-B GPS parameters (NIC, NACp, NACv, SIL, & SDA)

Attached pic was the initial power on while the airplane was in my driveway next to the garage and under a tree -- it's much better in flight.
 

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The reason for the minimum length is not insufficient GPS signal. It’s to meet the spec on rejection of interfering signals on certain frequencies. You won’t know if your short cable was the best thing to do until or if you encounter the max interfering signal that the box is still supposed to function with, and it doesn’t work. And without sophisticated instruments the only real test is if you immediately plug in longer coax and it solves the issue of not working.
The same thing goes with hidden gps antennas. Unlike ILS signals, which are highly standardized, GPS signals vary greatly in strength. And they’re weak to start with! You never know how good or not your hidden gps antenna is until you fly into truly marginal reception conditions, and your gps drops out while the guy you’re in formation with-with an external antenna-continues to receive his gps data.
 
The reason for the minimum length is not insufficient GPS signal. It’s to meet the spec on rejection of interfering signals on certain frequencies. You won’t know if your short cable was the best thing to do until or if you encounter the max interfering signal that the box is still supposed to function with, and it doesn’t work. And without sophisticated instruments the only real test is if you immediately plug in longer coax and it solves the issue of not working.
The same thing goes with hidden gps antennas. Unlike ILS signals, which are highly standardized, GPS signals vary greatly in strength. And they’re weak to start with! You never know how good or not your hidden gps antenna is until you fly into truly marginal reception conditions, and your gps drops out while the guy you’re in formation with-with an external antenna-continues to receive his gps data.

Yes - you are correct.

Both installations passed the acceptance testing as specified by Garmin in the 190-01007-A3, and 190-01007-C0 Install manuals.

Guess I'm lucky.
 
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Yes - you are correct.

Both installations passed the acceptance testing as specified by Garmin in the 190-01007-A3, and 190-01007-C0 Install manuals.

Guess I'm lucky.

Actually, you don’t know yet if you’re lucky or not. The tests mentioned do not and cannot duplicate marginal or worst case scenarios.
 
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