Make Club Meetings Interesting AND Useful

One of the things that has helped me in finding speakers for club meetings is my experience as an editor with Test&Measurement World magazine. My job as an editor was to first figure out what topics the readers were interested in (or should be interested in) and then find someone to write the article. My job as club president is to come up with ideas for club meetings and then find guys willing to giva a presentation.

Generating ideas is not all that hard. All you really have to do is think a little about what interests you or what would be useful to you and before long you’ll have more ideas than you could ever use. For example, the other day I was preparing a lesson for my General Class license course. The lesson was to cover antennas and feedlines, and I was cutting hunks of different feedlines to pass around at class.

At the same time, I was talking to our club’s technical coordinator on the repeater. I asked him if he knew how to determine the characteristic impedance of a feedline. For a second there, I had him stumped, but in the end, he came up with a reasonable answer.

The answer is that the characteristic impedance of a feedline is the impedance that a source would see if the line were infinitely long. Think about this. If you connected a power supply to a piece of coax or ladder line, a current will begin to flow. It will, however, flow for only a short period of time because sooner or later that current is going to reach the end of the line and have nowhere to go.

If, however, the feedline were infinitely long, the current would continue to flow. Determining the impedance of the line is then very simple. Z = V/I.

From a practical point of view, you can measure the characteristic impedance with a time-domain reflectometer or an oscilloscope and a pulse generator. For the test method, take a look at this article from Test&Measurement World. Putting together such a test setup, shouldn’t be difficult to do, and I bet that it would make a great presentation at a club meeting. I’d be willing to be that one or more of your members have an oscilloscope that you can use. Finding a signal generator with enough drive might be a problem, but not on insurmountable one.

Not only would the presentation be interesting, but it would be useful as well. You could invite members to bring in their coax and test them to see how close the feedlines are to their nominal impedances. Taking it a step further, you could then measure the feedlines with antenna analyzers and see how close the measurements turn out to be.

So, there you have it, a club meeting that’s both educational and useful.

A couple of other ideas that occur me in this same vein include:

• Checking the calibration of members’ DMMs. To do this, you could build a voltage reference with a part like the MAX6126. The MAX6126 has an accuracy of + 0.02%, which should make it more than accurate enough to check most DMMs. Another option would be to purchase a voltage standard from Geller Labs. Their least expensive model is only $25.
• Check the spurious emissions of members’ transceivers. To do this, you’ll need access to a spectrum analyzer. Fortunately our club does have one available. If yours doesn’t, you could perhaps borrow one from a local business or test equipment rental company.