Measuring SWR

Measuring SWR

The SWR measurement is the most important part of your setup. The term VSWR or SWR stands for Voltage Standing Wave Ratio. In basic terms this is the reflected voltage or currents that travel from your transmitter to the antenna but due to impedance mismatch (not 50 ohms) you will have a reflected wave. The ratio of forward vs. reflected power is measured in a ratio, which is SWR.

Typical SWR values range from 1:1 to 2:1. Anything 2:1 and under is very satisfactory. There are several myths about having an SWR range over 1:1 and they are just that a myth. Now that we have an idea of what SWR means let’s hook up our meter and see what SWR readings are obtained. You will need a small coax jumper to connect your radio to the SWR meter. Make sure you connections are correct for input and output. Output is the antenna and input being your radio.

There should be a FWD setting this is the forward power sampling. You will want to key your transmitter, set the to the FWD setting and adjust (using the SWR cal knob) for a full-scale reading or to the set mark on your meters scale. Next un-key the transmitter and set the switch to REF this will measure the reflected power. Your meter should read this reflection as an SWR reading. The circuit components will calculate the reflected power and display this as an SWR reading on the scale.

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Resistance vs. SWR

Resistance values for a dummy load or antenna impedance value will determine the SWR. Looking at the chart represented above we can see how the SWR changes with the increasing resistance values on the “Y” axis. We can also have different values of lower resistance's for SWR levels. For example we can have a 1.5 SWR with the resistance values of 75 ohms or 35 ohms. A rough estimate would be that for every ohm of resistance +/- over/below 50 ohms you would see a drop in SWR of .02 See the chart below.