VHF antenna issues? - Page 5 - SailNet Community
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post #41 of 48 Old 10-06-2013
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Re: VHF antenna issues?

In over 50 years of working on radios, the biggest problem by far is that the connectors on the coaxial cable are badly installed. The gold-plated twist-on connectors are worthless in a salt-water environment. Even if the connector doesn't corrode, the wire will. Using an SWR meter on a mast-head mounted antenna is NOT a reliable way to check the antenna/cable. Measuring SWR assumes that there is no loss in the cable. In fact, RG58 cable (the 1/4" diameter stuff that is cheap and easy to run) loses about 80% of the power in 100', thus the SWR will be low even if the antenna is shorted or missing.
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post #42 of 48 Old 10-07-2013
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Re: VHF antenna issues?

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In over 50 years of working on radios, the biggest problem by far is that the connectors on the coaxial cable are badly installed. The gold-plated twist-on connectors are worthless in a salt-water environment. Even if the connector doesn't corrode, the wire will. Using an SWR meter on a mast-head mounted antenna is NOT a reliable way to check the antenna/cable. Measuring SWR assumes that there is no loss in the cable. In fact, RG58 cable (the 1/4" diameter stuff that is cheap and easy to run) loses about 80% of the power in 100', thus the SWR will be low even if the antenna is shorted or missing.
You are right a about that! Another less common failure is corrosion of the cable shield due to a small cut or gouge in the jacket allowing water into the shield. The cable looks great to the SWR bridge but nothing much gets to or from the antenna.

About the easiest way to diagnose this problem is to measure the radio output power at the radio and compare that reading to a measurement taken at the antenna. You will always see some drop. But if nearly nothing is getting through the feed line, Check the connections first, if they are good, change the feed line.
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post #43 of 48 Old 10-11-2013
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Re: VHF antenna issues?

You're right on about the corroded shield. I've only seen it once, but I wasted more than enough time actually finding the problem to have bought a Fluke TDR meter about 3 times.
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post #44 of 48 Old 10-15-2013
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Re: VHF antenna issues?

A geophysical boat had a problem with their Inmarsat terminal. Repeated service calls by various Rep company techs and the factory techs didn't find the problem. I DID!

A little cut in the cable jacket let in saltwater spray. The shield coroded and turned the coax into a very effective dummy load. The only external sign of the damaged cable was a slight increase in diameter, well, that and a complete lack of conductivity to RF.

A time domain reflectometer would have been a welcome addition to my bag of tricks!

Have FUN!
O'

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post #45 of 48 Old 10-18-2013
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VHF antenna issues?

The TDR is not likely to identify the corroded shield either. Tdr's use a dc pulse that isn't effective at isolating rf cable faults like corrosion, kinks, or badly crimped connectors unless they're open or shorted.
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post #46 of 48 Old 10-18-2013
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Re: VHF antenna issues?

I was guessing, without any evidence, that a shield that had corroded open would show up via TDR. As for corrosion that hadn't completely removed the shield I would expect a less accurate diagnosis with TDR. WRT bad connectors, an antenna bridge or SWR meter is your best choice as long as you correct the SWR reading for cable losses.
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post #47 of 48 Old 10-22-2013
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Re: VHF antenna issues?

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The TDR is not likely to identify the corroded shield either. Tdr's use a dc pulse that isn't effective at isolating rf cable faults like corrosion, kinks, or badly crimped connectors unless they're open or shorted.
The TDR may not show any reflection at all from a cable that is acting as a dummy load.

What it will show is the absence of the expected impedance bump for the antenna connector.

You should have an idea how long the antenna cable is and if you see bumps in places where they should not be or you don't see bumps where they should be, these are indications of a problem.

The common UHF connector makes an impedance bump that should be easily seen even at the end of 100+ feet of RG58. Also, the TDR should spot the connectors at the base of the mast and something from the antenna.

If the cable is properly terminated using impedance matched BNC, N or some other impedance matched connector, there may not be enough of an impedance bump to see them.

Sometimes what you don't see is every bit as important as what you do see.

Antennas and feed lines! I can see why some people think it is all magic.

Sheesh!
O'
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post #48 of 48 Old 10-23-2013
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VHF antenna issues?

I should have added that an FDR is the ideal tool for measuring performance and accurately isolating problems at rf frequencies. TDR's have trouble with all but the worst corroded shielding, poorly crimped connectors, kinked or damaged cables short of open or shorted. The FDR gives accurate return loss measurements at operating frequency and can actually show the bump that exposed shield exhibits before it actually fails, something that would otherwise pass the TDR sweep. My only point was the TDR wasn't a good tool in the case presented.

I don't mean to come across as a nit picker correcting anyone. I forget this is a sailing page and such technical minutia is not very useful to non RF Enginners. I read my earlier post and it sounded a bit like that to me. I hope i didn't offend.
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