End Fed Half Wave Matcher Version 3

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.

Matcher Mk 3

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.

T106-2 Toroid 17 turns and knob added to trimmer

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.