Tuesday, August 4, 2015

Tuning HF antenna with SWR Bridge and Rigol Oscilloscope

After viewing this Youtube video from W2AEW on #112: Use an Oscilloscope and Signal Generator help tune an HF Antenna, measure complex impedance, I decided to do a similar SWR bridge with a AD9850 as a signal generator.

Front of SWR bridge

Back of SWR bridge

SWR bridge with 3W 50ohm and 2x1K resistors
The actual circuit was on the above mention Youtube video.

SWR bridge mounted on Dremel Vise

Arduino + AD9850 + 8digit 7segment with VFO
I had the AD9850 on top of the Protoshield with a large 8 digit 7-segment display as the signal generator frequency with a rotary encoder as a VFO.

Sinewave of 50ohm compared to 50ohm terminator
Almost match impedance
The connection from the SWR bridge are BNC connector with a T-joint to the Rigol DS1052E with a 50ohm terminator. This ensure both ends are 50ohm terminated.

 To check the HF antenna impedance visually, turn VFO to the desired frequency and visually check the yellow (50ohm reference) and blue ( antenna ) sinewave. The above image is an almost match impedance. Once the two sinewave are close enough, turn the coax switcher to the IC-7200 and the SWR reading will be quite low.

Unmatch impedance
The above is an example of an unmatch impedance, with such big differences in the sinewave, it is almost certain the SWR reading on the IC-7200 will be very high.

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