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  Using Citizen Band SWR Meter to Check VHF Marine Band Transmitters

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Author Topic:   Using Citizen Band SWR Meter to Check VHF Marine Band Transmitters
meridian posted 08-14-2011 02:52 PM ET (US)   Profile for meridian   Send Email to meridian  
A buddy gave me an old Olson CB-391 SWR/Watt meter along with some hand written instructions. Can this be used to check my VHF? I understand these meters have to cover certain frequencies.
jimh posted 08-14-2011 03:16 PM ET (US)     Profile for jimh  Send Email to jimh     
Measurement of the voltage standing wave ratio (VSWR) on a transmission line can be useful in designing and building antenna systems. In the case of a VHF Marine Band radio, there typically are no adjustments possible to either the transmitter or the antenna. By FCC regulations, users are prohibited from making adjustments to VHF Marine Band radio transmitters. Most VHF Marine Band radio antennas are of a design in which adjustment of the antenna is also impossible. In these circumstances, an in-line directional coupler that can measure VSWR is only useful for two purposes:

--verifying that the antenna is still attached to the transmission line;

--verifying that the transmitter is producing some power output.

The accuracy of VSWR measurement depends entirely on the directional power measurement precision of the in-line directional coupler. Most inexpensive units, such as those sold for use with Citizen Band radios, are of little value in making any sort of accurate measurement. Their poor design prevents them from being able to actually measure VSWR below about 2:1. Thus they can essentially only tell you if the antenna is connected or not.

The directional coupler element in most devices is often frequency sensitive, and will only function with reasonable accuracy in the frequency range it was designed for, which is often limited to a narrow band of frequencies. More sophisticated devices have much wider bandwidth and employ plug-in coupler units which are compensated for each particular band. For example, the famous BIRD MODEL 43 directional watt meter can be used from 0.1-MHz to 2,000-MHz with reasonable accuracy by selection of the proper plug-in element or coupler.

Directional couplers can be calibrated to read directly in Watts, measuring the Incident Power and the Reflected Power, from which the transmitted power to the antenna can be deduced.

David Pendleton posted 08-14-2011 03:34 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
The short answer is probably not. CB is in the 10-meter band, marine VHF is in the 2-meter band.
jimh posted 08-14-2011 11:19 PM ET (US)     Profile for jimh  Send Email to jimh     
Checking the VSWR on your transmission line on a boat's VHF Marine Band radio can lead to only three outcomes:

--the VSWR is high, causing you to buy a new antenna;

--the power indicated is low, causing you to buy a new radio; or,

--everything looks reasonably good, causing you to do nothing.

This is somewhat akin to the forward pass in football. There are only three possible outcomes and two of them are bad.

meridian posted 08-15-2011 02:17 PM ET (US)     Profile for meridian  Send Email to meridian     
so, how do I know if my radio or antenna and associated connectors are bad?
jimh posted 08-15-2011 03:47 PM ET (US)     Profile for jimh  Send Email to jimh     
I have been thinking about bringing a professional quality directional Wattmeter along with me to a few rendezvous and checking various radio installations. What I would check for is;

--forward power output from the radio on HIGH POWER setting with battery voltage at 13.2-volts or more (i.e., engine running) on Channel-16;

--VSWR on transmission line on Channel 16 (156.8-MHz)

I was thinking of noting the results of the survey to establish what the general condition of VHF Marine Band radio installations on small boats might be.

A fast way to make a radio check is to establish contact with another station who is about five miles distant. Reduce to 1-Watt power, and see if you can still communicate.

A directional wattmeter designed for CB is going to give you some sort of indication, but it won't be particularly accurate. If it shows a VSWR of more than 2:1, the antenna is probably not even connected to the transmission line. Assessing power output using a CB meter will be difficult. The calibration is not going to be particularly accurate. If it says you are putting out 10-watts, would you buy a new radio? I'd check with something that can give accurate data.

jimh posted 08-20-2011 08:56 AM ET (US)     Profile for jimh  Send Email to jimh     
I think some readers have found my comments too technical. Let me use an analogy. If your child were ill and you wanted to measure their temperature, you wouldn't use a meat thermometer.

If you want to make meaningful measurement of transmitter power output and standing waves on a transmission line for a VHF Marine Band radio, I am suggesting that you get proper measuring equipment. If you use equipment that is not suitable for the frequency range of VHF Marine Band, and use equipment of low accuracy or poor calibration, the measurement you will get is likely to not be very accurate. You are only going to get coarse information.

Coarse information can be useful in detecting problems of extreme variance from normal. In some cases such coarse information may prove to be better than no information.

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