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 VHF Antenna - Resistance
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JanS48
Navigator

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USA
127 Posts

Initially Posted - 05/21/2021 :  06:26:13  Show Profile  Reply with Quote
Greetings everyone.
As I'm getting my boat ready for launch I decided to test the VHF only to find I could not raise a radio check. I receive the weather channel ok but an attempt to radio check does not yield anything.
I'm by no means a VHF seasoned user, what channel is it ok to do a radio check on? In RI there was an automated radio check on 24 but I could not raise that either. In checking the cable I get no resistance between the center and ground. Is that normal? - I'm expecting it should have some load to force the signal to the antenna.
Any input is much appreciated.
Thanks
Jan

82 C25 SR FK
Sailing out of Newport Harbor.

Voyager
Master Marine Consultant

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USA
4821 Posts

Response Posted - 05/21/2021 :  07:18:37  Show Profile  Reply with Quote
Jan, VHF radio, like anything else on your boat, has its ins and outs and troubleshooting details.

Radio checks are usually conducted on channel 9 however you may have to try several times to get someone who is monitoring ch 9.

Some people do radio checks on channel 16, but the USCG takes a dim view of people doing this. Ch16 is generally reserved for emergency use and hailing other boats, so I donít use it for radio checks. You can find more details about US VHF usage here.

https://www.navcen.uscg.gov/?pageName=mtBoater and
https://www.discoverboating.com/resources/how-to-use-a-vhf-radio

Testing a VHF radio is different than most other equipment. You can check the radio by testing the current draw from the RED power supply lead during transmission. 5W transmitter power should draw ~ 0.7A while 25W transmission should draw ~ 2.5A DC.

Testing the antenna cable and antenna is a different matter. An antenna, by its nature, is an open circuit for DC when using an ohmmeter. Measuring cable resistance with the antenna attached will result in infinite Ohms, or at least you hope so. A short circuit is a bad situation.

To measure the DC resistance of the antenna cable, you must remove te antenna and short out the far end between the center conductor and the shield wire. A piece of tin foil will do the trick. Then you can check the DC resistance of the cable, connectors, etc. It should be a few ohms at most.

To measure the antenna actively thereís a gizmo called a VSWR meter that measures the voltage standing wave ratio of the antenna. Interesting information, but for most of us, not needed. The best bet is to check the connectors on both ends of the cable for corrosion, good contact with the shield wire, no breaks in the center conductor, and no shorts in the cable. Clean or replace the end connectors and give it another try.

Word of caution, donít transmit without an antenna cable attached to the radio or if thereís a dead short in the cable.

If youíre unable to access the antenna at the top of the mast, you can always buy another antenna and mount it on the stern pulpit and wire it to your radio. Itís a good idea to have a backup rigged just in case.

While you wonít have as large a reach of radio transmissions and reception with the lower antenna, it will still work fine for hailing nearby boats and the USCG in case of an issue.

My 2 cents

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain ó Milford, CT

Edited by - Voyager on 05/21/2021 07:30:14
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JanS48
Navigator

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USA
127 Posts

Response Posted - 05/22/2021 :  07:17:02  Show Profile  Reply with Quote
Thanks a lot for the input. I'll double check a radio check on ch9. I do like the idea of a backup antenna so I may rig one for 'just in case'. I do have a good ammeter so I'll check the current when x-mitting.
Jan

82 C25 SR FK
Sailing out of Newport Harbor.
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Voyager
Master Marine Consultant

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USA
4821 Posts

Response Posted - 05/22/2021 :  18:53:32  Show Profile  Reply with Quote
Most ammeters max out around 200 mA however some have a special connection for 2-10A measurements. Take care when measuring the current.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain ó Milford, CT
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JanS48
Navigator

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USA
127 Posts

Response Posted - 05/23/2021 :  18:35:46  Show Profile  Reply with Quote
Thanks again for the input but I have a high-end clamp on ammeter I use for solar panels etc. It will read milli-amps to 400 amps DC.
Jan

82 C25 SR FK
Sailing out of Newport Harbor.
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