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Antenna cable question.

Antenna cable question.

Antenna cable question.

(OP)
Have a Sony Radio who's antenna is going to be mounted on a rail car roof.  Got antenna... Cut cable near the radio connector because we have to feed the cable thru a small hole in the car roof.  The connector is the typical point with a body crimped around the cable body.  You know! Like the typical car radio antenna.

We measure the resistance from the center conductor to the antenna and have continuity.  We measure from the center conductor to the plug center pin.  Nada!  We did the same with another antenna and found the same result.

Is this normal?  Is there a capacitor in the connector or something?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Antenna cable question.

Yes, I have seen series pF capacitors inside the Motorola plug end of the antenna cable for car radio antennas. Open it up and see. Or take it on faith and just functionally test the antenna to see if it works (or not).

PS: I happened to obtain a few of the short bendy Fuba antennas as found on the trailing edge of some 90's VWs. They need power. I was pleasantly surprised that the embedded RF preamp (hidden in the small base) worked over the SW bands as well as AM/FM.

RE: Antenna cable question.

(OP)
Interesting on those VW antennas.

What about those GPS antennas?  Are they active?  The little domey ones you see out on a boat's railing.  Does the antenna have to match the specific GPS?

Thanks on that other point.  I was starting to suspect that was the case.  I will see if we can do a necropsy on it.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Antenna cable question.

Most GPS antennas are active. Many (most?) use +5Vdc.

The professional market can be mix-and-match where the receivers and antennas have their own specifications that need to be compared to make sure that they're compatible. There are even GPS 'splitters' for the aircraft market where several systems might share the two (for redundancy) GPS antennas. The integrator needs to (carefully) decide which system provides the antenna power and which systems are DC-blocked.

For the consumer market, you normally get everything in one box and just plug them in. The average consumer doesn't know what a 'volt' is.

Back to car antennas, the normal car whip antenna in the AM band is probably a poor match for low-Z cable. I've seen some car antennas where the cable was of quite abnormal design (not RG-58). In other words, keep an open mind for car radio antennas. There are some strange system designs out there.

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