VK5FQ Other Stuff

Building a ScopeMatch for LF/MF antenna tuning.

Simply, this bit of kit is used in the matching of the PA output to transmitting antenna. It requires a CRO or DSO as the display and utilises a separate current and voltage transformer to obtain the required signals. The original article (PDF) can be found in the link below.

The basic principle is simple. The voltage transformer is wired across the PA's output and gives a RF voltage waveform that is proportional to the available RF signal. The current transformer is wired in series with the RF output and gives a RF voltage waveform that is proportional to the RF current. If we make the ratio of the detected voltage waveform fifty times greater than the detected current waveform, then we have an indication of output impedance. When both detected waveforms are equal, then we have a match to 50 ohms. This is a quick way to see if the tapping point on the matching network needs adjustment.

If we want to know if the antenna needs to be made shorter or longer, we can use the phase relationship between the two detected signals to tell us if we need to shorten or lengthen the antenna wire. If the detected voltage leads the detected current waveform, the antenna is inductive and needs to be shortened. The converse is also true. Adjustment is made of the variometer to suit. When the waveforms are in phase, then we have an impedance match with minimal reactance.

Reference should be made to the article for any clarifications and for other possible configurations. Particular attention is drawn to the voltage transformer being substituted with a capacitor voltage divider circuit, when using high power. GL. YMMV.

 

Useful Links

• Original article by Jim Moritz, M0BMU:
  • Tuning Aids for the 136kHz and 500kHz Bands