GNX-375 Minimum GPS Antenna Cable Length?

[Ironflight]

It’s Sunday, and I hate bothering my avionics friends on their weekend….but that’s what forums are for, right?

Hey folks - double check me, because I am tired of reading install manuals, and might have missed it…. In the good old days, when we were all installing the magic 430 and 430W boxes, there was a MINIMUM length for the GPS antenna cable. We sometimes had a roll of RG-400 under the panel to make that work. I’m looking for an antenna location on the F1 project for the 375, and couldn’t find anything about a minimum cable length, so either I have missed it, or Garmin got smart and designed the thing to not care.

Anyone find/know of a limitation on this?

Paul

 

[msstahl]

It’s Sunday, and I hate bothering my avionics friends on their weekend….but that’s what forums are for, right?

Hey folks - double check me, because I am tired of reading install manuals, and might have missed it…. In the good old days, when we were all installing the magic 430 and 430W boxes, there was a MINIMUM length for the GPS antenna cable. We sometimes had a roll of RG-400 under the panel to make that work. I’m looking for an antenna location on the F1 project for the 375, and couldn’t find anything about a minimum cable length, so either I have missed it, or Garmin got smart and designed the thing to not care.

Anyone find/know of a limitation on this?

Paul

The Garmin 175 that I installed last year stated a minimum cable length of 6’. With that being said I’m testing a 2’ cable length and so far so good.

 

[Webb]

If memory serves, my 750 stated 6’ and the antenna no closer than 2’ to antennas that emit like a com antenna.

 

[Dan 57]

The Garmin 175 that I installed last year stated a minimum cable length of 6’. With that being said I’m testing a 2’ cable length and so far so good.

Didn't test a 2' version, but remember reading about that 6' requirement somewhere... in that extensive (approaching 1'000 pages) manual.
Since GPS is super important for Garmin boxes, I "just" looped the wire to +/- reach the min required length on my 375 install... might be life dependent on a LPV approach.

 

[BobTurner]

Remember the reason for this is that the antenna (an OEM device) actually had too much gain. If you don’t want excess coax you can always insert some connectors in series (each one having a few tenths of a dB loss) to meet the loss spec.

 

[Kazomike]

Minimum length is approximately 6.5 feet. From GNX 375 TSO Installation Manual, Rev 3, 190-02207-04

6.4.2 GPS Antenna Cable
Once the antenna mounting position has been prepared, route the coax cable from the antenna to the GNX 375.
Proper selection of coax cable and assembly of connectors is critical to GPS signal performance. Cable loss from the
GPS/SBAS antenna must be between 1.5 dB and 6.5 dB in order to maintain proper rejection to interference signals.
Coaxial connectors and adapters, such as TNC to BNC, add additional loss to the cable and should be considered when
computing the cable loss. A typical loss of 0.2 dB can be used for each connection. To maintain integrity of the SBAS
signal, the GPS antenna coaxial cable must have a minimum of two shields (e.g., RG-400 or RG-142B).

NOTE
If RG-142B or RG-400 is used, 1.5 dB equates to a length of approximately 6.5 feet of cable with a
connector on each end. RG-142B or RG-400 cable can be used as long as the length is less than 35 feet.
For longer lengths, use low-loss double or triple shielded 50 ohm coax.
For very short runs, where the loss is less than 1.5 dB, additional cable should be used to increase the loss to within 1.5
dB to 6.5 dB. This additional cable may be coiled, taking into account the minimum bend radius of the cable.

 

[Walt]

Remember the reason for this is that the antenna (an OEM device) actually had too much gain. If you don’t want excess coax you can always insert some connectors in series (each one having a few tenths of a dB loss) to meet the loss spec.

Might better off with a real 3db RF attenuator than a string of connectors tied together

 

[Ironflight]

Minimum length is approximately 6.5 feet. From GNX 375 TSO Installation Manual, Rev 3, 190-02207-04

6.4.2 GPS Antenna Cable
Once the antenna mounting position has been prepared, route the coax cable from the antenna to the GNX 375.
Proper selection of coax cable and assembly of connectors is critical to GPS signal performance. Cable loss from the
GPS/SBAS antenna must be between 1.5 dB and 6.5 dB in order to maintain proper rejection to interference signals.
Coaxial connectors and adapters, such as TNC to BNC, add additional loss to the cable and should be considered when
computing the cable loss. A typical loss of 0.2 dB can be used for each connection. To maintain integrity of the SBAS
signal, the GPS antenna coaxial cable must have a minimum of two shields (e.g., RG-400 or RG-142B).

NOTE
If RG-142B or RG-400 is used, 1.5 dB equates to a length of approximately 6.5 feet of cable with a
connector on each end. RG-142B or RG-400 cable can be used as long as the length is less than 35 feet.
For longer lengths, use low-loss double or triple shielded 50 ohm coax.
For very short runs, where the loss is less than 1.5 dB, additional cable should be used to increase the loss to within 1.5
dB to 6.5 dB. This additional cable may be coiled, taking into account the minimum bend radius of the cable.

Tanks for the reply and the reference!

That’s exactly the language I was looking for, and remember from the old days. However….I am looking at the Install manual Rev 9, and no such language exists, nor are their requirements (that I can see) for antenna cable. It does have a table of compatible antennas, but nothing about cable lengths. I wonder if they just decided it didn’t make a difference? Or….there is a different manual that is up to date that is for the antenna installation….

 

[Ironflight]

Found it! Thanks for everyone who chimed in this afternoon…I finally got back to my real computer with the full Adobe on iOS, did a search for “cable, and the GPS antenna cable requirements (in Rev 9) are in section 4.1.5. its pretty much the same as before - they want 1.5 dB loss, and recommend cable lengths between 6.5 feet and 35 feet. Additional information is that a connector is worth about .2 dB loss - so you’d have top have a bit of a string of connectors. I guess that as an operations engineer, I immediately divide 6.5’ by 1.5 to get the loss per foot of the plain cable….. or maybe 6.5 minus .2 minus .2 (one connector on each end) and THEN divide 6.5 by 1.1….

There’s a reason why when we aero engineering students showed up for our ONE required electronics class way back when, we were welcomed to “electrical engineering for dummies!!”

 

[Majorpayne317641]

Found it! Thanks for everyone who chimed in this afternoon…I finally got back to my real computer with the full Adobe on iOS, did a search for “cable, and the GPS antenna cable requirements (in Rev 9) are in section 4.1.5. its pretty much the same as before - they want 1.5 dB loss, and recommend cable lengths between 6.5 feet and 35 feet. Additional information is that a connector is worth about .2 dB loss - so you’d have top have a bit of a string of connectors. I guess that as an operations engineer, I immediately divide 6.5’ by 1.5 to get the loss per foot of the plain cable….. or maybe 6.5 minus .2 minus .2 (one connector on each end) and THEN divide 6.5 by 1.1….

There’s a reason why when we aero engineering students showed up for our ONE required electronics class way back when, we were welcomed to “electrical engineering for dummies!!”

What I did was also added connectors for the bulk head with a bulk head connector and the math worked to being very close to a 6ft length even though it’s only about 2ft long.

 

[Walt]

What I did was also added connectors for the bulk head with a bulk head connector and the math worked to being very close to a 6ft length even though it’s only about 2ft long.

A bunch of connectors will increase VSWR vs a single run of RG400, and don’t forget that GA35 is DC powered using the coax.