VK1SV 136 kHz page

IRF540 amplifier for LF (VE7SL design)

I am working on a MOSFET (IRF540) amplifier for use in the new "top band", 136 kHz. The basis of my design is the VE7SL transmitter.

This is a picture of the drain voltage (trace 1) and the voltage on the 50 Ohm dummy load, after the low pass filter (trace 2). I can get about 40 W RMS without any serious heat being generated by the FET.

IRF640 amplifier for LF (DF3LP design)

I made yesterday a copy of the DF3LP simple low power transmitter (details here). It works very well! As you can see in the oscilloscope display below, the drain voltage looks very clean (top trace) and the dummy load voltage (bottom trace) indicates a very healty almost 70 W RMS (or so). The FET was powered with 1bout 18 V when this picture was taken.

The prototype of the DF3LP copy can be seen below. I made some changes to the actual design, specifically to the capacitors and the coil at the drain circuit of the power FET. The values suggested by DF3LP were the starting point and only small changes were made in order to fine tune the circuit. The efficiency measured at 12 V is 75% and I believe it is slightly better at 18 V. I also used an IRF720 at the driver stage instead of the suggested IRF630/IRF640 because that's all I had in my junk box.

I am quite happy with the results so far, so I am looking at the possibility of using more than one FET (parallel) in the output stage, for higher output.

136 kHz signal source

My main signal source for the 136 kHz transmitter is based on a divide by 200 circuit and a 27 MHz crystal. In fact I have a number of 27 MHz ex CB crystals which allow me to change my frequency as needed.
I have also added a second input (selectable by a switch at the front of the transmitter panel) which accepts a 13.6 MHz ~10 dBm sine wave and divides it by 100 to produce a signal in the band. This is handy when a very accurate frequency is needed, for example when we have an "LF activity day", which is normally done on 135.920 kHz. I don't have a crystal that produces that frequency so I use my external signal generator tuned on 13.6 MHz. This setup has proven very stable and suitable for slow QRSS.