Antenna won't work with extension

[PlankWalker]

My GPS antenna died of corrosion so I bought a cheap Chinese one on ebay (the big mushroom type).
This works fine on my old Garmin, but the cable of 5m is too short,
when I extend it to 11m using BNC connectors my Garmin can't find itself.
I have checked the continuity of the extension, the inner core is fine, though the screening is showing 3.5V from a 12v battery,
Does anyone know what I'm doing wrong here, are these antenna's calibrated for a certain length of cable?

Plank

 

[lw395]

The GPS passes something between 3 and 6 volts up the antenna to power the preamp that is built into the antenna.
Both the core and screen must have continuity.
The cable is generally 50hm, 75ohm as used for tv aerials may or may not work.

 

[William_H]

I have checked the continuity of the extension, the inner core is fine, though the screening is showing 3.5V from a 12v battery,
Does anyone know what I'm doing wrong here, are these antenna's calibrated for a certain length of cable?

Plank

I don't know what "the screening showing 3.5 volts froma 12v battery " means. Taken separately the extension cable should show zero ohms from centre to centre of the cable and likewise from body to body of each connector. It should show very high resistance from centre to outer (body) of either connector. Thus the extension should be just that an extension. There will always be more loss especially at GPS frequencies but if the antenna has an amplifier (hence needing some volts of DC up the cable) then losses after the amplifier should not matter too much. If all else fails make an extension of 5 metres and see how that goes. If it is iffy but does work a bit then the cable has too much loss.
You could try a lower loss cable or very heavy low loss cable like RG 8. good luck olewill

 

[PlankWalker]

Many thanks for the input,
I'm using the cable from the old antenna as an extension, so maybe theres some corrosion of the screening in the middle somewhere.
I'll get some new cable and try again, its good to know that it should work if I do this.

Plank