Dennis Frie

- a couple of projects and pictures


(2012) Automatic antenna VSWR measurements

If you have ever worked a bit with RF, you know how difficult it is to measure anything at all with hobby-equipment. Tuning antennas can be done from formulas etc. but it just never beats a good tuned antenna with proper test equipment.

Using a directional coupler to measure forward power and reflected power, is a great tool to tune an antenna. But while it does give a good indication if the performance with a RF source connected, it doesn't tell much about the bandwidth, perfect tuned frequency etc. As I would like to perform sweeps, I decided to give try with an adjustable RF-chip controlled with a microcontroller, and another microcontroller to read the reflected power.

To accomplish this I have used an OpenLRS/RFM22B to sweep in the 430 mhz area, and an Arduino/Atmega 328 to read the forward and reflected power.

The directional couplers, RF-meters and 50 ohm dummy loads (2 x 2.4 ghz directional couplers and 1 x 433 mhz):

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Quick and dirty test-setup with OpenLRS as RF-source and an Arduino/Atmega328 to take measurements.

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First, let's test the forward-power with 50 ohm load. The filter in the RFM22B should limit the frequency to somehting like 400-450 mhz. But for some reason the forward-power measured is the same. Can't explain why.

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- next, let's look at the reflected power with 50 ohm load.

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Now let's look at an actual antenna. In this case a Nagoya 771. First, the forward-power:

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Next, the forward-voltage and the reflected voltage.

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This give the following transmitted power (voltage):

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- Converting to VSWR this gives:

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Same measurements, but with standard 430 mhz coil-antenna from Flytron.

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