Fake SD2942

[nrgkits.nz]

I thought I would try a SD2942 - this device is capable of 350W. Spent the best part of a Saturday trying to figure out why I couldn't get a good match into the gates. Tried different methods, using a transformer with different turns ratio's, using coax instead as had been used in a Nautel design. Even the Nautel design didn't work and I always ended up with a really rotten SWR at the input - almost all power being reflected back to the exciter.

As it turns out the capacitance of each gate to ground is in the order of 1.7nF, so no wonder it wouldn't match - that's basically a short at RF.

Using a simple RC circuit (series resistor, parallel capacitor after resistor) fed with a square wave, unknown capacitance can be calculated. After confirming my test setup with a known capacitor, I connected one gate of the SD2942 as the unknown capacitance. I measured the rising edge on scope, calculated out the unknown capacitance at 1.7nF.

The SD2942 came from a Chinese vendor on taobao.com - they all sell on AliExpress also. So just be careful buying these out of China. This is actually the first dual LDMOS I've had issues with matching - all the other types I've tried have actually put out their rated power and had an input SWR better than 1.3 after adjusting the input match. I suspect they have put something else into this "SD2942".