[Draker]

The recent "how many GPS antenna" thread reminded me that I always had this question in the back of my mind but never actually asked. I have a bunch of those little black "puck" antennas on my glare shield, all from Garmin: three GA 26C antennas for GPS and one GA 24TNC for SiriusXM. At some point during my build, I think I read somewhere that the lengths of the cables are important, so I left them alone. They are so close to the instruments though, that there is a lot to bundle up:



Yuck!

Is there a definitive source of information about whether it is acceptable to cut these down to a nicer length and re-crimp new connectors? Garmin's GA 26C manual doesn't say anything about shortening, but does advise a special extension cord if you want to lengthen. Their GDL 51 manual has a short note in a table caption:

For each 1 dB gain over 24 dB, add 1 dB of attenuation into the antenna cable path between the antenna and the GDL 51R/52R. Note that gain specifications are defined at the output of connector for some antennas, e.g. GA 24 TNC. Additional attenuation will be required for any modifications made to the cabling to these external antennas.

Which hints that the length of the cable is important, but doesn't add much for laymen. I work in cell phone technology but I still consider RF and antennas to be voodoo. Is there an actual reason not to mess with the length of these cables, or is it an old wives' tale I picked up somewhere?

 

[YellowJacket RV9]

The recent "how many GPS antenna" thread reminded me that I always had this question in the back of my mind but never actually asked. I have a bunch of those little black "puck" antennas on my glare shield, all from Garmin: three GA 26C antennas for GPS and one GA 24TNC for SiriusXM. At some point during my build, I think I read somewhere that the lengths of the cables are important, so I left them alone. They are so close to the instruments though, that there is a lot to bundle up:



Yuck!

Is there a definitive source of information about whether it is acceptable to cut these down to a nicer length and re-crimp new connectors? Garmin's GA 26C manual doesn't say anything about shortening, but does advise a special extension cord if you want to lengthen. Their GDL 51 manual has a short note in a table caption:



Which hints that the length of the cable is important, but doesn't add much for laymen. I work in cell phone technology but I still consider RF and antennas to be voodoo. Is there an actual reason not to mess with the length of these cables, or is it an old wives' tale I picked up somewhere?

My understanding is that the system is tuned for a very specific cable length; changing it would result in reduced performance.

 

[BobTurner]

The only one I’m aware of is that for certain Garmin TSO’d gps navigators, the OEM antenna had too much gain, and the receiver was unable to adequately reject certain off frequency signals. The crazy fix was to specify a minimum loss between the antenna and receiver, usually accomplished by having excess coax.

 

[Ironflight]

I can report that they work fine if you shorten them by only an inch or two - I have cut the end off to feed through a small hole (for appearance sake) and then crimped on a new end. Of course, they take a totally diffferent part number end (than for RG-400), and a different crimper!

 

[AZSteve053]

Background:
GPS antenna's that output RF, as in use a coax cable (not a RS-232 puck) generally (98%) contain a Low Noise Amplifier (LNA) to achieve the lowest noise figure possible, these LNA's require DC power which is supplied on the coax cable - typically 5vdc and yes you can measure it with a DVM or Simpson meter at the receiver antenna jack. The GPS receivers in our planes must live/function in the plane with a multitude of other signals which radiate from all types of antenna's - and because of this the receivers are designed & tested to operate correctly in this environment with some margins. The primary frequencies for GPS L1 in in the 1.575 Ghz area and these frequencies are very susceptible to loss when carried thru any coax cable - typically measured in db/loss per ft or 100ft (RG400 which is a pretty good cable loses about 21db/100ft @ GPS Freq's).
Real Answer:
So - Yes you can shorten cables, but if you just put a connector on the new end what level signal will you have ? You can use various techniques to figure this out - actual measurement before & after or if the coax cable exact spec's are known you can compute it. Think of it this way - shortening by a small amount (say a foot or 2) is almost a slam dunk as Paul Dye just mentioned, shortening by say 3 or more feet and your into a gray area. Ok - Now you shortened alot & have Too much signal, just add an attenuator in series EXCEPT you'll lose the DC path necessary to drive the LNA. There are devices that can be plumbed in: think DC blocks, Bias Tee etc (not cheap). Becomes complicated.
Alternate suggestion: Read/Investigate equipment info in depth, try to identify antenna manufacture & part number - then call the manufacture and see if they have a different antenna with desired cable length having the same signal output.
Hope this helps...