Comments on "2xIRF520 amplifier evaluation on 40m"

Hi Scott KB0KFX,

Usually one inserts about 3dB attenuator before the input transformer to tame the input SWR. The 3dB attenuator assures, that the transceiver sees at worse 1:2 SWR. I am not an RF engineer, so please take my comments with a grain of salt. I am a software engineer with the specialization on computer graphics.

My understanding is, the input SWR is not much a problem for the PA, but more for the driving transceiver. The FET gate is strongly reactive and the only resistive load there is the gate dumping resistor, in the case of the WA2EBY amplifier it is the the bias resistor, which also dumps the input RF energy into the decoupling capacitor at the bias potentiometer.

I will measure the amplifier on higher bands once I make the low pass filters. I did measure the amplifier on higher bands without the low pass filter, but the results are difficult to interpret. Without the low pass filter, the amplifier generates a lot of harmonics. With the low pass filter, the harmonics are reflected back to the amplifier and this has a positive effect on DC efficiency and on the maximum output power at the base frequency. The drain curves on the higher bands were not that square as on 40m or 80m. On 80m I bet the amplifier will work as well as on 40m. The square wave at the 50Ohm load has the same voltage on 80m as on 40m and the output transformer does not overheat, therefore the single turn primary of the output transformer has sufficient inductance and coupling. On higher bands the efficiency will likely suffer a bit. I did not test the amplifier on 160m and I do not plan to. The output transformer may not provide enough impedance on 160m. My main goal are the usual SOTA bands - 40m to 15m and I do not shoot at higher frequencies at this time of the solar cycle.

Regarding the input transformer, I built the transformer of the NM0S 50W linear amp with 2xIRF520 push pull FETs. If it is good for NM0S, then it will be good for me. The HF Packer amp input transformer transforms from 50 Ohm to 2x 100 Ohm (2x the same voltage, but half the power at each secondary winding). The transformer of NM0S amp transforms from 50 Ohm to 2x 25 Ohm (half the voltage, half the power at each secondary winding). It is logical to drive the 2x higher input capacitance of the IRF520 with lower impedance.

A very efficient yet simple way to build the output transformer is 1:4 or 1:9 impedance ratio (1:2 or 1:3 turns ratio) with primary made of copper or brass tubing, secondary going through the tubing. Secondary wound inside the tubing increases coupling. Increasing the impedance ratio increases PA power, but also increases heat strain on the FETs and on the output transformer. There is also chance, that the transformer will not have sufficient inductance on low bands therefore producing insufficient coupling. The 1 turn primary may have insufficient inductance on 160m. Loading the FETs with a too high impedance ratio will magnify the effects of the FETs on resistance, therefore increasing thermal losses. The 1:4 impedance ratio seems to be a good compromise for the 2xIRF520 on 13.8V. It may be interesting to use 2x two IRF510s in parallel instead of the IRF510 to improve thermal coupling, as electrically 2xIRF510 is roughly equivalent to 1xIRF520. This setup would likely be all right at 100W.

WA2EBY wrote, that he experimented a lot before achieving reasonably flat gain on all HF bands. I did not do that (yet). There is really not much one could do. One has two means to control the self oscillations. Loading the gates with resistors and inserting resistors between RF source transformer and gate. Those two methods may be frequency controlled by a series inductance at the gate load resistor and parallel capacitance at the gate series resistor. WA2EBY reduced gain losses at higher bands by the gate bias inductances. I will likely try the same.

I am curious how long it will take for me to burn the 2nd set of FETs. The first set died of insufficient thermal coupling to the heat sink. I found following article enlightening:
http://sound.westhost.com/heatsinks.htm

At another place, I read that the silicon thermal pads are not as good as they are supposed to be if the heat sink surface is not sufficiently flat and polished, where the greased mica is supposed to work better.

Rod Elliot warns of the low quality of mica pads. Indeed, I measured the thickness of the mica pad I bought in a local "drug store" in Prague, conveniently placed on my commute way to work. The thickness varied from 0.1 to 0.15. Advised by Rod Elliot, I chipped one mica pad to two or three, with the thickness of 0.04mm. With the fan on the heat sink running, I was surprised how cool the FETs stayed at >100W RF output with 40W disspated.

73, Vojtech OK1IAK