GPS Antenna Cable

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kevnurse

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I am trying to fix my GPS (old Garmin 120), which stopped receiving satellite signals. The power/data cable connection is fine and the GPS starts up as expected. However, it cannot find any satellites.

The antenna BNC connector on the back was corroded and the central core had perished. So, I bought a new BNC connector, which I fitted to the cable correctly. However, still no satellites. Now the interesting thing. I checked the core of the cable to ensure that it is insulated from the copper braid (ground). It was not insulated. I cut the cable back a bit to get away from any end of cable damage, but it still wasn't insulated. Then, with the cable disconnected, I checked the inside of the instrument to see if the core is supposd to be insulated from the outer braid. Inside the instrument, at the point where the core and the braid are soldered on to the pcb terminals they make a circuit (through my tester). I've no reason to suspect that the gps instrument is faulty. It was working fine until the antenna connection corroded away.

Now I'm confused. I've always believed that the core of a co-axial cable must always be insulated from the braid, from the antenna to the pcb terminals.

Am I wrong about gps signals?

Is the instrument faulty if the core and ground terminals make a circuit?

In the antenna cable if the core and the braid are not insulated from each other, could this be caused by a faulty antenna?

Any info on gps antennae and cables would be most appreciated, thanks

Kev Nurse
 
Last edited:
kevnurse said:
I am trying to fix my GPS (old Garmin 120), which stopped receiving satellite signals. The power/data cable connection is fine and the GPS starts up as expected. However, it cannot find any satellites.

The antenna BNC connector on the back was corroded and the central core had perished. So, I bought a new BNC connector, which I fitted to the cable correctly. However, still no satellites. Now the interesting thing. I checked the core of the cable to ensure that it is insulated from the copper braid (ground). It was not insulated. I cut the cable back a bit to get away from any end of cable damage, but it still wasn't insulated. Then, with the cable disconnected, I checked the inside of the instrument to see if the core is supposd to be insulated from the outer braid. Inside the instrument, at the point where the core and the braid are soldered on to the pcb terminals they make a circuit (through my tester). I've no reason to suspect that the gps instrument is faulty. It was working fine until the antenna connection corroded away.

Now I'm confused. I've always believed that the core of a co-axial cable must always be insulated from the braid, from the antenna to the pcb terminals.

Am I wrong about gps signals?

Is the instrumant faulty if the core and ground terminals make a circuit?

In the antenna cable if the core and the braid are not insulated from each other, could this be caused by a faulty antenna?

Any info on gps antennae and cables would be most appreciated, thanks

Kev Nurse
Click to expand...
It depends - which isn't much help to you.

It is entirely feasible for the coax centre core to be showing a DC short circuit to the braid and the thing to be working perfectly. I am not familiar with the Garmin circuitry, so can't comment on your specific set up. Do you know if Garmin uses an active or passive antenna for the GPS?

Please be careful about checking continuity with your digital meter across the antenna terminal of the GPS! The GPS should be ok - but it might not like the DC volts from the tester on the antenna connection...
 
Try it with the braid disconnected at the GPS end and see what happens.
I don't know about the 120 but there is no connection between the core and braid on my 128.
 
kevnurse said:
I am trying to fix my GPS (old Garmin 120), which stopped receiving satellite signals. The power/data cable connection is fine and the GPS starts up as expected. However, it cannot find any satellites.

The antenna BNC connector on the back was corroded and the central core had perished. So, I bought a new BNC connector, which I fitted to the cable correctly. However, still no satellites. Now the interesting thing. I checked the core of the cable to ensure that it is insulated from the copper braid (ground). It was not insulated. I cut the cable back a bit to get away from any end of cable damage, but it still wasn't insulated. Then, with the cable disconnected, I checked the inside of the instrument to see if the core is supposd to be insulated from the outer braid. Inside the instrument, at the point where the core and the braid are soldered on to the pcb terminals they make a circuit (through my tester). I've no reason to suspect that the gps instrument is faulty. It was working fine until the antenna connection corroded away.

Now I'm confused. I've always believed that the core of a co-axial cable must always be insulated from the braid, from the antenna to the pcb terminals.

Am I wrong about gps signals?

Is the instrument faulty if the core and ground terminals make a circuit?

In the antenna cable if the core and the braid are not insulated from each other, could this be caused by a faulty antenna?

Any info on gps antennae and cables would be most appreciated, thanks

Kev Nurse
Click to expand...




Every thing could have been fine except for a bad connection untill you went testing. :rolleyes::rolleyes:


You could have an active antenna in the mushroom and if this is the case you will not get an infinity reading on a test meter and there might be cicuitry that can be affected by a reverse voltage. The normal voltage for these things is 5 volts DC fed up the coax.

The same goes for the GPS though it might be protected by a capacitor to keep the 5 volts in the right place.

Are you absolutely sure you have fitted the connector correctly with the right amount of insulation between core and shield.

Maybe you can find someone with another coax fed antenna and see if the GPS works.

How long have you left the GPS running before deciding it is goosed. SOmetimes the older ones if not used for a while take hours to re initialise.
 
As far as I know, all this Garmin series use active antennas. The GPS sends 5V power up the coax to the antenna, and receives the signal back down the coax.

Shorting the cable will not damage the GPS, but mine gives you a warning that the antenna is shorted.

I just bought a new antenna for one of these for £7.80 including free postage from an ebay seller in Hong Kong. It took about 2 weeks to get here and it works fine.

I take it the lack of message to warn you of a short means your problem was probably an open circuit.
 
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