The VK1SV classy Class-E 160 m AM transmitter

Dimitris Tsifakis, VK1SV/SV1DET

Version 1

I will document here my adventures in the Class-E land. For those not familiar with this excellent and easy technique for producing high power extremely efficiently, there is no better introduction than the Class-E RF Power Amplifiers paper, by Nathan O. Sokal, WA1HQC.

This is what v1 of the amplifier looks like. I am using an IRF520.

...and this is the schematic. The two caps in the Class-E amplifier should be selected 20% components or combination of fixed caps to give a capacitance as close as possible to the design value.

V = 13 V, I = 1.5 A, producing 16 W of RF.

I cranked up the voltage to 20 V and I got about 36 W from the poor IRF520. It was still quite comfortable to touch and the only signs of distress came from my cheap lab power supply...

Version 2

This one is designed for 24 V and an IRF640 MOSFET (200V max drain-source voltage).

Things to notice are the use of silver-mica capacitors and the much larger L1, which is now wound on a FT114-43 core.

The oscillogram above shows the drain waveform (ch2) and the output waveform (ch1) of the transmitter, operated at the design voltage which is 24 V.

In this oscillogram, the voltage provided to the transmitter has been cranked up to 30 V (the max my PSU will give). As you can see, the power is now around 70 W and there is plenty of safety margin for the drain voltage.

For AM operation, I would suggest running this at a slightly lower than 24 V to ensure that the drain voltage remains less than 200 V at the peaks of the modulation. If you want to use this for CW (Morse code) only, then I think you could increase the voltage to say ~34 V or so for an output power of 100 W, or you can go even further up for more power, but keep an eye on the drain voltage to ensure it doesn't go higher than 200 V. But, of course, 1.8432 MHz is not normally used for CW however it is permitted according to the band plan.

Modulation transformer

A modulation transformer is trully a thing of the past. I believe it will be very hard to find a transformer that will work well for modulating this 160m transmitter, so the choices are two: either make a transformer yourself or use a transformer that was designed for some other task. Being lazy, I decided to try a number of different power transformers that had the appropriate turns ratio. It turns out that a big (300 VA) toroidal power transformer produced excellent results. I used the two primary windings (110 V + 110 V) as the primary and secondary circuits. The impedance presented to the audio amplifier was close enough to the design impedance but of course, one could easily add a few more turns on the toroid and make further adjustments. With this setup, I had a very enjoyable QSO on 160 m AM with VK1DSH Dale and VK1BEK John.