I get asked this all the time, "What test and support gear should I have or might I need to maintain my repeater?"

If you are the major technician type, then you can dig further into your repeater's electronics than someone less technologically inclined might feel comfortable with. For the average GMRS repeater owner that does not have years of electronics knowledge, there are still a lot of things you can do to troubleshoot your system with some simple and inexpensive test gear.

First and foremost, to check for output power from a transmitter, you need to use a wattmeter. It just so happens that most wattmeters are also capable of checking the reflected power also. This is the portion of the power that comes out of the transmitter intended for the antenna to be radiated, but the antenna system does not accept or radiate due to one problem or another, and reflects back to the transmitter.

The ratio of forward power to reflected power is also known as the standing wave ratio. Most often, this is referred to as 'SWR'. A wattmeter and an SWR meter are essentially the same thing. Both methods of reading reflected power, either direct or SWR, are accurate. The method depends on the meter itself.

What makes one wattmeter better suited to GMRS than another is determined by the internal design. Accuracy of the meter can be affected by many factors including line impedance, system reactance, internal frequency response, and others. Some of the more expensive meters available are very accurate and reliable over a wide frequency range. One could spend over $600.00 for a Bird Thruline meter that is considered the industry standard, for instance.

It was the intent of this installment to find the best GMRS frequency compatible wattmeter for under $100.00 we could. Given that low price, that narrowed the choices down to two! The first is the Para Dynamics PDC9, available from Radio Shack (model number RSU 10508018) for $39.99. The second meter for under $100.00 is the MFJ model MFJ-862 available from your local ham radio store or mail order for an average price of $69.95.

Initially, I tried each meter using a fixed power level of 4 watts from a Motorola Maxtrac radio. The reason for using this power level is that many of the GMRS licensees requesting information about these meters use portable radios connected into base antennas and low power repeater radios. Below is the result of checking each meter under identical circumstances. Included is the result of the high dollar Bird meter as a reference.

Here's the result of the test using 4 watts into a dummy load:

• Para Dynamics reading: 5.5 watts
• MFJ reading: 2.5 watts Bird reading: 4.0 watts
• Bird reading: 4.0 watts

Next, using the same non-reactive dummy load, here's what the meters read for reflected power:

• Para Dynamics: 0.01 watts, indicated an SWR of 1.2:1
• MFJ: 0.2 watts, indicated an SWR of 2.0:1
• Bird: 0.0 watts, translates into an SWR of 1:1

Since I got such different readings, I did a frequency sweep of the meters. This will show the frequency range that each meter will pass without changing the resistance or reactance of the lines connected to it. In english, that means that I checked to see how transparent each meter would be if inserted in your antenna system.

Well, I was not surprised to see that the high dollar wattmeter was essentially flat across several MHz, including 450-470 MHz. However, the two low cost meters were not.

(See the sweep graphs at the bottom of the article.)

This test was performed by connecting the transmitter sides of the meters to the tracking generator's bridge, and the antenna connections to the non-reactive dummy load. The ideal result would have been a flat frequency response 30 db down.

As you can see, the impedance through these devices was not constant. The effect of inserting these meters inline would be the same as inserting a piece of coax that was damaged in some way. This is unimportant if the meter is not left inline all the time, but could pose a real problem if your intention is to keep it inline permanently. Also, this does make the readings somewhat untrue, although to which way the inaccuracy would go is unpredictable.

Of these two meters, the Para Dynamics would be the best choice. However, if at all possible, for the serious repeater owner, investment in a higher quality meter would be advisable. In comparison to your dollar investment in the repeater equipment and antenna itself, the cost of a high quality wattmeter is negligible.

The short conclusion is that an inexpensive wattmeter is better than none. The long conclusion is that although an inexpensive wattmeter can give you an idea of what is happening with your antenna system, it is not to be taken as gospel. The best advice is to check your system using a high quality wattmeter upon installation, then check it with the inexpensive one you intend to use in the future. Make a note of the readings you get from the inexpensive meter and use them as a reference. If the measurement changes, you will know that there may be a problem. Then, find a high quality meter to troubleshoot.

Next month, some more test equipment you should keep handy. We'll also discuss some very basic trouble shooting for your repeater.

Gummers^(TM) is a trade mark of Paul Shinn