Measuring the impedance of an antenna, a component, or a circuit such as a matching network, can often help us optimize our equipment and improve our understanding of its operation.
Commercial network analyzers, RF impedance meters, or antenna analyzers can do the job, but are out of the realm for many amateurs, especially since the equipment is expensive and would be sitting idly on the shelf most of the time. If we only need a few measurements, an old technique — the three voltmeter method — that uses a signal generator (or a QRP rig) and the ubiquitous digital voltmeter may fill the bill.
These items together with a few dollars worth of parts, a bit of software, and some manual work on your part can produce the data you need in just a few minutes. It also provides an easy, low cost way of acquainting yourself with the art of impedance measuring. While not as convenient as some commercial equipment, there is a satisfaction that comes with building and understanding a device that you have built yourself.
Discussed in this article are the details of this unusual RF impedance analyzer which uses a simple circuit with diode RF voltage samplers, as well as software for your PC that processes the data, compensates for the diode nonlinearities, and calculates the impedance, SWR, and other parameters of interest at frequencies up to 21 MHz or more.