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Old 24-09-2010, 05:27   #1
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VHF Antenna Resistance Value

Trying to trouble shoot a transmission problem with my Furuno VHF (Tx range is very limited).
No problem with reception. As a staring point I want to check the antenna and antenna cable.
The antenna is a standard glomex metal whip.
Can anyone advise the resitance value I should get if I disconnect the plug from the radio and measure the resistance value across the plug pin and the outer part of the plug (ground).
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Old 24-09-2010, 06:00   #2
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An SWR meter is better than an ohmmeter continuity tester for antennas.

Some antenna manufacturers will indicate whether or not their antennas should ring "open" or "short" (DC Continuity between center conductor and outer shield).

Although many VHF antennas will read “shorted” on DC; they are NOT shorted at VHF frequencies. A resistance of about 50 ohms appears when applying the RF frequency at which it was designed to operate.
See ➥ antenna basics
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Old 24-09-2010, 06:01   #3
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Good morning,
1 or 2 ohms, like a short circuit.....internally this is a J pole antenna.
J-pole antenna - Wikipedia, the free encyclopedia
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Old 24-09-2010, 06:43   #4
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If you disconnect the antenna wire at the back of the radio, you should get only an ohm or two between the center and the shield wires. If you don't, disconnect the other end of the cable and measure the antenna itself--it should also be about 1-2 ohms. If it is, then short across one end of the cable and read resistance across the other end--if its more than an ohm or so, the cable/connectors are bad.

Another important test is to disconnect the antenna wire at both ends to make sure you get infinite resistance between the center and shield wires.
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Old 24-09-2010, 06:45   #5
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Receiving OK Transmitting poor range. If it is really receiving OK Then the problem maybe you have selected the low power function (1Watt). Check this function in the owners handbook before going too far.
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Old 24-09-2010, 06:48   #6
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Glomax catalog says the elements are all DC grounded to eliminate static charges.

I was just up the mast here. That poor antenna has the hard life up there. It's rusty, the coil is leaking rain water, and the tip of the wire is melted into a blob. And it's only a few years old.
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Old 24-09-2010, 06:56   #7
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OK the good news is the lifetime warranty!!

Nigel1? daddle?
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Old 24-09-2010, 07:42   #8
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Thanks for the replies. The VHF radio is set to High Power, first thing I checked. I'm at the boat tomorrow, will check the cable and antenna as suggested, plus will try radio with the emergency antenna and see what I get. Good news about the lifetime warranty.
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Old 24-09-2010, 07:52   #9
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Check the DC voltage & current at the set when transmitting. current approx 4Amps on a 12V system
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Old 27-09-2010, 01:36   #10
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Had a chance to measure the resistance over the weekend. At the antenna itself, resistance was 0.6 ohms. Then with the antenna cable connected (about 15 meters), the resistance measured at the plug connection was 1.1 ohm. Does this sound about right for a DC grounded antenna?
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Old 27-09-2010, 02:52   #11
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Yes; but any DC measurements really only give very very basic information when trouble shooting a RF problem.
You really need a SWR (or RF power) meter (as suggested above) and 50 ohm dummy RF load; these allow you to measure the power out of your transmitter, the power delivered into the antenna (& therefore loss of coax feeder) and the efficiently of the antenna (i.e. its VSWR).

Buy, beg, borrow or steal these items and you will know exactly what is going on with your setup.
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Old 27-09-2010, 03:02   #12
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There is a guy at the marina who has one, just waiting for the opportunity when we'll both be there at the same time.
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Old 27-09-2010, 04:02   #13
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Quote:
Originally Posted by nigel1 View Post
Does this sound about right for a DC grounded antenna?
"Yes."
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Old 27-09-2010, 17:54   #14
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Receives OK & DC check of aerial is normal. Prob. not aerial. (However you were going to try putting one on the back rail.)

Leaves to check the power & modulation (deviation) Very min.SWR meter. might have a power indication on the SWR meter.

The DC current check of the power to the unit if way out of the 4-6Amp range would indicate a failure within the unit. Outside this range indicates failure of components in the transmitting section. Replace or repair the radio.If the current is within the 4-6amp range then you might have a modulation fault & not a power output fault.
Modulation. monitor on handheld or another radio. If it sounds poor quality audio all you could do is replace or repair mic.

Without the correct test equipment & radio training there is not much more you can expect to achieve. VHF radios are not too costly these days.
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Old 27-09-2010, 19:10   #15
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You have good advice, a standing wave ratio measurement can indicate a problem with the coax or antenna. Now the one thing I did not read in earlier suggestions is what type of wire is between the tranceiver and the antenna. Wire such as RG-58 is junk for this application.Something like RG-8 or one of the newer coaxial cable is a must.

Finally, there are antennae and then ... I used to have crappy gain with a 3dB antenna on top of my mast. After upgrading to a 6DB I never had a problem.

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