The existing EFHW antenna matcher has worked really well on 40m and 20m on many SOTA activations. It uses a small T50-2 core and is limited to 5 Watts.
Thought it would be nice to have a matcher that could handle at least 50 Watts for my portable amplifier, so out came the soldering iron again.
This time I used a T106-6 yellow core from minikits and made the primary only 2 turns and the secondary 16 turns. This is the same 1:8 windings ratio as before which is an impedance ratio of 1:64. The reason for the lower number of turns was to reduce the capacitance back to the core so I could use it at higher frequencies. There might be a trade off at 7 MHz (need to test this). The turns on this one were also a little loosely wound. The plan was to use it between 7 MHz and 25 MHz. The problem with the higher power requirement is the high voltage produced on the output. The original poly variable capacitor would have arced over.
I found that a 3-55 pF ARCO mica trimmer cap could stand the high voltage and this alone allowed the matcher to work from 14 MHz to 28 MHz. For 7 MHz, I switched in two 220 pF 2kV capacitors in series. So far it matches nicely on those bands into a 3200 Ohm dummy load.
So what is the voltage across the output? Assuming 3200 Ohm impedance and 50 Watts, , sqrt (2 x 3200 Ohm x 50 W) = 565.7 V Peak. Peak to Peak volts will be 1131.4 V.
Next to measure the losses through it. At first it looked like 7 MHz losses were bad but it turned out to be the two capacitors used. They were replaced and the losses decreased.
Losses through the matcher were less than 0.3dB from 7 MHz to 21 MHz. I couldn’t measure frequencies above this as the scope probe capacitance was causing problems. The old Tektronix 10x probe has a capacitance of 13 pF and this was detuning things.
Update: 2014-06-22. The toroid was changed to a T106-2 core with 17 turns tapped at 2 turns. The 2 turn tap is the 50 Ohm input and the end of the 17 turns connects to the antenna. This change was made to allow better 10 metre band operation.
The trimmer also had a knob added as in the field it was too hard to find an insulated screwdriver and tune it.
The knob extension is simply a screw of the same thread as the trimmer screw that is screwed into a 6mm long metal hex standoff. The head of the screw was sawn off. The adjuster knob is an RC servo arm that is a perfect press fit onto the standoff. Keep in mind there are high voltages present on the trimmer, so fingers should be kept away! Even 5 Watts gives a surprising shock.