During the initial use and on air testing I had a failure due to mounting the voltage sense toroid too close the case and after several transmitting sessions the high voltage present there jumped the gap and destroyed the coil. You can see the melted wire in the picture on the right. NB. You must be aware of high RF voltages on the coil as in the ScopeMatch it is directly connected to the coax inner as you will see on the circuit diagram (link below).
After repairing, modifying and testing the new coils and with the new layout I made an additional feature which I liked in the LF Tuning Meter, that was the antenna current meter. I fitted a double pole switch to allow me to quickly check the antenna current without the need for messy calculations. This feature has operated very well and the unit has been able to work with the high RF voltages present, see the new stand-off PTFE insulator below.
I had previously read about the ScopeMatch in the “Low Frequency experimenters Handbook” and as I owned a double beam oscilloscope I decided I would build and try that method of transmitter to antenna tuning. This method very accurately indicates the above mentioned three critical measurements on the oscilloscope screen without the need for two meters or switching the mode of one of the meters. So the state of the tuning was easily seen.
View or Save “Low Frequency Tuning Aids” PDF By James Moritz, M0BMU
LF Tuning meter and ScopeMatch The LF tuning meter which I built some while ago (December 2009) is one of the finest aids to help tuning the transmitter to the antenna. This used two moving coil meters to indicate the antenna feed current, voltage and phase (link below). Some time later I was looking for a similar device to construct which would give greater accuracy and other features to aid the adjustment of the transmitter to antenna matching and tuning.