Testing GPS Antenna

[hugues1811]

i have a garmin gps map 192 telling me more and more often i have a poor signal antenna. i borrow a garmin gpsmap 492 (same connection) and it don't even receive signal from the antenna. before, when its happen i clean the bnc connector and it work fine after that. My question is how can i test the antenna (how man ohm), specially the cable becaus there a bent just after the bnc and i move often the gps (the other side of the cable go in the mushroom antenna. second question, can i cut the end of the cable and weld a new bnc connector. and last question, is gps antenna active or passive and what is composed, a piece of wire, a winding, a clistron or an ignitron (iwas an eletronician but it was a ver long time ago
Thanks

[El Rubio]

The antenna is probably a Garmin GA29 or GA30 and is active. The good thing is it is just an antenna and receive amplifier, not the complete GPS receiver like some. This means replacing it would be much cheaper. The amp circuit requires around 5vdc. You can measure this at the BNC connector on the main unit. If your antenna has a connector at the base of it, you can measure it there also. I have measured some common GPS antennas with a multimeter and read around 200 ohms, but I can't say for certain if yours would read close to that or not. You can replace the BNC connector, and from your description it sounds like that may be the problem. You could measure the ohms while bending the cable near the BNC and tell if you have a break there. BNC would be best crimped on instead of soldered, and you can put a 90 degree adapter or connector to take the tension off where the cable is bent now.

Whenever I replace a connector where there is tension, or it gets disconnected a lot, I put 3 layers of heat shrink tubing at the base. This stiffens up the cable there and makes it last a long time. If it's exposed to the weather, it should be weather proofed too.

[hugues1811]

Thank you very much Rubio, this help me a lot, but all mushroom antenna are active? also, my gps stand in my cockpit, underneath my bimini, i saw on ebay a gps antenna , it is an oval piece of plastic about 4inch by 1inch and on the botom there are a bnc at 90 deg. then i can connect it directly to the back of the gps, no need to reroute a coax cable. i don't like crimping and preferer soldering then i put a coat of liquid tape to protect of humidity and finally a shrinking tube. Good idea the 90 deg elbow, i am going to buy one

[smac999]

there should be a screen with gps status and signal. watch the screen and wiggle the connecter and cable and see if it drops out. (the screen may take a few secs to change so wiggle slowly)