On the other hand, many inductors and capacitors are lossy (as seen in Figure 1) and have both a resistive and reactive part with the reactive part being positive for inductors and negative for capacitors. For many resistors, the reactive part is very small, except for wire wound types. The reactive part is usually frequency dependent and is preceded by the math operator “j” to indicate it is the imaginary part. Impedance is the more complete expression defining current flow than resistance alone and is typically expressed as a complex quantity such as R + jX where R is the resistive part (real) and X is the reactive part (imaginary). To get started, let’s review some concepts of impedance. I suggest that you build the circuit inside of a metal enclosure (not a solderless breadboard like I did) since that would limit your frequency range. The circuitry is very low cost (less than $5) and uses readily available parts.
If you decide to build the project, you will need a PC running Windows®, a signal generator to cover the frequencies of interest with at least 13 dBm output into 50 ohms (about 1.414 peak volts or 20 mW), and a high input impedance digital voltmeter (DVM) with a DC accuracy of at least 0.5%. This approach to impedance measurement has its limitations, but on the plus side, it is an easy project to build, is low cost, requires no power supply, and has the capability of producing results comparable to equipment costing much more.
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.ĭiscussed 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. It also provides an easy, low cost way of acquainting yourself with the art of impedance measuring.
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. 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. 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.Ĭommercial 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. » Skip to the Extras A voltmeter, signal generator, and a few parts combine to make a powerful impedance measuring tool.