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Author Topic:   GPS Antenna Transmission Line Splicing
BritSti posted 05-14-2007 05:05 PM ET (US)   Profile for BritSti   Send Email to BritSti  
[Seeks] signal quality experience with cutting and soldering new connector onto an existing GPS cable.

I recently installed a Navman 5500 chartplotter with an external antenna. Mounted the antenna on my Montauk 170's grab rail, cut the connector off, drilled two small holes in the stainless steel rails to route the cable into the the console interior.

Now this looks great and in using a new crimp on/solder SMA connector (SM Electronics) I have a working GPS signal. It gets a fix fairly quickly while stationary, but once moving the gps signal seems to drop out fairly quickly.

I feel like I have degraded the signal quality with the new connector. [Seeks] any recommendations on GPS antennas that may be more effective than the OEM Navman, such as Gilsson?

Chuck Tribolet posted 05-14-2007 10:44 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I'd blame the connector, not the antenna. SMCs are hard to
do right. And Navman has a good rep.

Also, at least with Garmin, that would void your warranty on
the antenna. I asked them about this a while back, but since
the connector was a BNC (and Shake has some nice BNC conectors),
and the unit was off warranty, I lopped about 10' of coax
off, and signal strength got better, maybe. It sure didn't
get any worse, and I have a bit more space in my console.


where2 posted 05-14-2007 11:18 PM ET (US)     Profile for where2  Send Email to where2     
Having once asked Standard Horizon whether adjusting the cable length on one of their GPS chart plotters was advisable, they suggested that the length of the GPS antenna cable was part of the design equation, changing the length would degrade the signal.

If you are experiencing a loss of signal when the engine is running, I would begin to suspect inerference from an adjacent gauge or engine harness wiring. Specifically, I'd look at the tachometer signal lead. It may also help to ground the grab rail segment you ran the antenna cable through (I would test it with an alligator test lead and see if it helps, if so, install a more permanent grounding lead to the grab rail).

GreatBayNH posted 05-15-2007 10:29 AM ET (US)     Profile for GreatBayNH  Send Email to GreatBayNH     
I cut my Garmin external antenna cable down to less than 2 feet. It works fine.


Chuck Tribolet posted 05-15-2007 07:23 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Cable lengths are critical at long wave lengths (low
frequencies). GPS runs at 1.5 GHz. The wave length is on
the order of eight inches.


BritSti posted 05-18-2007 02:19 PM ET (US)     Profile for BritSti  Send Email to BritSti     
Thanks for the advice here, ended up ordering a two more SMA connectors, a crimp and a clamp style type and will have another go at cutting the line and adding the connector. SMA connections do seem tough to do right given the inner and outer conductor and the need for the right crimp tool and die.

I would say I am eliminating about 15 feet of cable doing it this way; an internal antenna would have been a whole lot easier...

Ordered a Glisson antenna as a backup, as the last time I went out, all looked well, but the GPS dropped off right away with any real speed.

Is anyone familiar with the HDOP reading you can see on you chartplotters "satellite screen", the lower the better correct?


jimh posted 05-18-2007 11:54 PM ET (US)     Profile for jimh  Send Email to jimh     
HDOP = horizontal dilution of position
roloaddict posted 05-20-2007 11:28 PM ET (US)     Profile for roloaddict  Send Email to roloaddict     
The lower The HDOP, the better.

Does you rail mount vibrate when underway? Bad vibration will cause problems with the Fix.

I have run various GPS antennas on various cable lengths, even the wrong impedance cable ( 75 ohm vs 50 ohm due to the circumstances beyond control ). Usually you see a loss of SNR or signal quality.

Since you last messed with the connectors I would check those. Your connection is for the signal as well as a DC loop for power. You antenna maybe losing power due to a poor connection on the center pin or the shield.

Good luck.

roloaddict posted 05-20-2007 11:30 PM ET (US)     Profile for roloaddict  Send Email to roloaddict     

The shorter cables, with the right impedance, will increase your SNR levels.

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