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Author Topic:   VHF Marine Band Antenna Testing
AtoZ posted 08-18-2008 10:40 AM ET (US)   Profile for AtoZ   Send Email to AtoZ  
This weekend at the Boston Whaler get together in Harbor Springs my VHF Marine Band radio was transmitting poorly. Since I had not given it a good test yet this season after replacing the connector a few months ago, that may be my problem. Is there a way to test this with a VOM or do I need a more sophisticated meter? (jimh, any ideas?)
jimh posted 08-18-2008 10:47 AM ET (US)     Profile for jimh  Send Email to jimh     
An antenna presents an AC impedance, not a DC resistance. You cannot measure the impedance of the antenna with a multi-meter. Some antennas have DC characteristics that are quite different from their AC characteristics (and here AC means radio frequency alternating current). Some antennas will show a very low DC resistance when checked, while others will show infinite DC resistance.

The best way to check the performance of the transmitter and antenna is to use a directional wattmeter. Insert a directional wattmeter into the transmission line as close to the antenna as possible. It will show the incident and reflected power on the transmission line. From those measurements you can compute the power output and the standing wave ratio on the tranmission line.

Chuck Tribolet posted 08-18-2008 03:09 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Did it sound bad? Did it not go very far? Or what?

How does it receive?


Buckda posted 08-18-2008 03:38 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Chuck -

We were receiving a static-filled and broken transmission from Terry's radio while about 2 miles away. Incidentally, we received a signal from JimH's handheld from the same location which was clear and strong.

Terry's transmission was heard with plenty volume, but was almost unintelligable, or difficult to discern. My observation as a crewmember aboard Home Aside was that most of the problem came from the broken signal, but there was a fair amount of static included in our reception of his signal.

David Pendleton posted 08-18-2008 10:36 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Define "transmitting poorly".

Could anyone hear you? Could they make out what you were saying?

Shakespeare makes an SWR meter for testing transmit power, reception, and SWR.

They aren't inexpensive, but I think owning one is a good idea. You'll be able to test your own radio whenever you need to and the radio of whomever you're cruising with (as we did on the most recent North Channel Rendezvous).

In my experience, poor VHF performance is almost always a connection/connector issue. Check them out.

jimh posted 08-19-2008 06:46 AM ET (US)     Profile for jimh  Send Email to jimh     
A method for testing a VHF Marine Band radio without any test equipment: three radios are required. The radios should all be located a mile apart from each other. Each radio transmits in turn while the other two radios receive. In this way, everyone can get a comparative signal report.

Terry--I heard your radio transmitting while we were close abeam. The modulation sounded OK. I did not hear your radio at any distance.

Pat reported that he was having trouble hearing your radio. At that same time he was copying my transmission on my handheld radio, and we were close abeam to you. My impression was that the handheld radio was being received with a stronger signal than your fixed radio.

AtoZ posted 08-19-2008 07:05 PM ET (US)     Profile for AtoZ  Send Email to AtoZ     
I believe I had a cold solder joint on the pin. I've replaced the connector and will test this weekend on Lake Erie.
jimh posted 08-19-2008 08:31 PM ET (US)     Profile for jimh  Send Email to jimh     
This might be a good example of the observation that most of the problems with VHF Marine Band radios are usually caused by the owner-installed connector on the transmission line. I think that factor accounts for most of the glowing reports given to one particular brand of antenna that comes with a pre-installed Mini-UHF connector. Those antennas tend to not have problems with the owner installing a connector and doing a poor job--the connector is already installed at the factory. As a result, that brand gets high marks from owners who probably threw away a perfectly good antenna because they couldn't solder a PL-259 connector.
Chuck Tribolet posted 08-20-2008 08:41 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
My experience is that most VHF problems are caused by bad
microphones (two of three problems, and maybe the third (I
replaced the radio rather than getting it fixed)).

I've had good luck with the CenterPin solderless VHF
connectors. I can solder, and I have a 300 watt soldering
gun, but the CenterPin stuff just works.


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