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Old 25 August 2009, 11:32   #1
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testing aerials

is there anyway of finding out whether a vhf aerial and its cable and connections are not faulty? i have just bought a new aerial because the previous one didn't have enough signal strength. Now i cannot receive or transmit or listen to anything being transmitted. I thought it may have been because the original cable which was trunked through the boat had deteriorated because the outer metal cable had become powdery. But even when conected up with fresh cable i still get nothing.

Is there anway of testing an aerial and its connections with a multimeter?

the setup i have is a Simrad RS86 if that makes any difference.

cheers fellas for any advice you can offer
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Old 25 August 2009, 11:55   #2
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Check the connections either end. could there be a tiny bit of screen touching the inner?

Use multimeter to check the resistance between the centre and scree on the plug. it shouldn't be zero, but I don't know what it should be.

Could you have damaged the cable pulling it through?

Is the connector on the radio corroded? shorted?

Try connecting the aerial directly to the radio using a short lead. if the problem still exists, matbe the new aerial is faulty.

Can you measure the current taken by the radio? Does it draw current when you key the mike? You might be able to see, say, nav lights dim slightly when you transmit on battery only. That would indicate that the radio is drawing current and hence probably transmitting.
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Old 25 August 2009, 15:16   #3
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re testing your aerial

unfortunatly you cant test it fully with a multimeter, what you need is a SWR meter, this will tell you how healty your cable and aerial are. The meters arn't that expensive and I use one when I service the boat just to check if everything is still ok.

here is a cheap one on ebay for example http://cgi.ebay.co.uk/ZETAGI-COMPACT...QQcmdZViewItem

Regards

Mark
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Old 25 August 2009, 18:53   #4
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Originally Posted by JABS View Post
Use multimeter to check the resistance between the centre and scree on the plug. it shouldn't be zero, but I don't know what it should be.
It may well read zero or, at most, the resistance of the cable. Many VHF antenna are of the 'J' aerial type and these will have both conductors connected at the bottom of the J so will appear as a short circuit to a resistance meter.

Have you mounted the antenna to the boat? If not, simply hold it and connect it directly to the radio so you know there is nothing untoward about the connection. I suppose it is possible the new antenna is faulty but it's not likely.

It is very easy to make a working J-antenna for test purposes. A bit of wood (garden cane or something similar) a length of wire and a bit of 50ohm co-ax with a connector for the radio. A couple of terminal blocks can be used so you don't need to make solder connections.
I'll post details if it's any use to you.
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Old 26 August 2009, 04:31   #5
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Or the radio has sh1t the bed?
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Old 26 August 2009, 12:37   #6
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It may well read zero or, at most, the resistance of the cable.
Resistance of the cable? Are you referring to the resistance of the center conductor length, plus the shield length return run? That implies the antenna reads as a dead short (which I am not arguing - I think my Shakespeare does exactly that.)

You may also get a complete open: Digital Antennae read exactly the opposite from Shakespeare (though I don't recall 100% which reads which.)

The Coax cable itself, I believe, should read open with an ohmmeter. It will have the 50 ohm impedance at the rated frequency, not at DC (which meters use.)

jky
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Old 26 August 2009, 16:58   #7
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Resistance of the cable? Are you referring to the resistance of the center conductor length, plus the shield length return run? That implies the antenna reads as a dead short (which I am not arguing - I think my Shakespeare does exactly that.)
Yep, exactly that. A J antenna is kinda that J shape and can be made from one conductor. The bottom of the J folds up to be parallel with the long leg - this is the impedance matching section - the long leg is lengthened to form the antenna. Connections are made at the bottom, one to each leg so the bit between the legs produces the full short to a resistance meter.
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