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-   -   VHF antenna question (http://www.sailnet.com/forums/electronics/56730-vhf-antenna-question.html)

pegasus1457 08-02-2009 05:56 PM

VHF antenna question
 
I noticed that my masthead VHF antenna is not getting any responses from radio check requests. I have a splitter which feeds the VHF and my FM radio.

The FM seems to work, but sensitivity is not what I would expect. Same for the VHF receiver. I can pull in a number of channels, but transmitting doesn't seem to get me anywhere. [I realize I could have a burnt-out transmitter, but I am trying to eliminate things one at a time.]

I disconnect the antenna cable at the base of the mast in the main cabin, and the FM reception seems to improve, while it is connected only to the cable running behind the cabin settees to the mast. I am surprised that co-ax cable can act as an antenna :confused:

I measured the DC resistance of the cable leading to the mast and it is quite low -- a few ohms.

Does anyone know what the resistance of the cable/VHF antenna should be? I think there is a balun coil at the base of the VHF whip, but I have no idea what the circuit looks like.

If this is normal behavior, I will be able to avoid a useless trip up the mast.


thanks ..

NautiG 08-02-2009 07:25 PM

That is normal behavior for a non functioning antenna. It takes very little energy and antenna to receive, but much more antenna and power to transmit.

I'd recommend getting a fiberglass deck level antenna as a backup antenna for your vhf, and fix your mast antenna. I'd bet you'd get a good response to radio checks when you connect the fiberglass deck antenna to the vhf. The problem with the mast antenna sounds like a corroded cable, connections, or antenna.

I don't know anything about splitting an antenna between an FM and vhf radio, but that may be an issue too.

Scott
Gemini Catamaran Split Decision

mitiempo 08-02-2009 07:53 PM

Pegasus
Try checking the masthead antenna for continuity between the shield and central wire. A friend had the same problem recently and if you get continuity it means it's shorted out somewhere and needs replacing or fixing. Also if you try to transmit with shorted antenna the outputs of the radio could be blown.
Brian

SEMIJim 08-02-2009 11:11 PM

Quote:

Originally Posted by mitiempo (Post 510505)
Pegasus
Try checking the masthead antenna for continuity between the shield and central wire. A friend had the same problem recently and if you get continuity it means it's shorted out somewhere and needs replacing or fixing.

That is not necessarily correct. It depends on the antenna design. Some antennas will measure close to zero ohms DC resistance normally.

Jim

pegasus1457 08-03-2009 12:01 AM

Quote:

Originally Posted by NautiG (Post 510501)
I'd recommend getting a fiberglass deck level antenna as a backup antenna for your vhf, and fix your mast antenna. I'd bet you'd get a good response to radio checks when you connect the fiberglass deck antenna to the vhf. The problem with the mast antenna sounds like a corroded cable, connections, or antenna.
Scott
Gemini Catamaran Split Decision

I already have a 6 ft whip and a 2nd VHF at the helm, as well as a handheld. So it is a question of getting the darned thing to function, not an immediate safety question.

The VHF cable down the mast was installed at the beginning of the year. The only thing remaining is the balun coil at the base of the Shakespeare whip on the masthead. The cable was purchased with the masthead connector installed, so if there is a problem it is likely to be with the soldered connector I installed at the bottom of the mast, or with the VHF antenna itself. Could also be the splitter for the FM that is faulty...

I can check most of these things out, but I really would like to know what the DC resistance should be of the cable+antenna. If someone could measure it on a working system that would be a big help.

scottyt 08-03-2009 12:09 AM

the coax and ant should show 40 to 50 ohms depending on a few things, but should be closer to 50

k1vsk 08-03-2009 09:01 AM

Quote:

Originally Posted by scottyt (Post 510567)
the coax and ant should show 40 to 50 ohms depending on a few things, but should be closer to 50

You are confusing impedance with resistance. An ohm meter is useless unless you want to measure continuity. You would need an impedance bridge to measure what you describe.

Also, there is no answer to the q what the typical resistance should be between the shield and inner conductor of the coax - it depends on the feed system to the antenna which can appear as a short with an ohm meter.

If you really want to determine what is happening at the antenna, you either need to remove and inspect everything or use an impedance bridge and SWR meter,

NautiG 08-03-2009 10:54 AM

My suggestion to get a second antenna was not just about safety, but also to problem solve your radio problem. Connect the new antenna to the radio with a short piece of cable. If the radio transmits properly, then you have determined that the problem is not the radio. Connect the antenna to the cable at the base of the mast. If the radio transmits properly, then you have determined that the problem is not the splitter. Connect the antenna at the top of the mast. If the radio transmits, then you have determined that the problem is not the in mast wiring.

I'm not sure what you are trying to accomplish by determining the impedance or resistance, but then again I'm not an electrician, and about the only thing I use my multimeter for is to check voltage. I'm just describing the problem solving process I would use on my boat, if I had your problem.

Scott
Gemini Catamaran Split Decision

SEMIJim 08-03-2009 11:33 AM

Quote:

Originally Posted by scottyt (Post 510567)
the coax and ant should show 40 to 50 ohms depending on a few things, but should be closer to 50

As k1vsk notes, and I noted earlier: That would be incorrect. In fact: It is unlikely that any properly operating antenna system will measure 50 ohms DC resistance. Most antenna systems will show one of two extremes in measuring DC resistance: An open (infinite) or something darn close to a short (close to zero ohms resistance), if not a dead short.

Jim


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