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Saturday, August 7, 2021

Mythbuster Video #13 -- RF Power Amplifier, and Relay Switching Plan

In this episode we enter into the most fraught part of the construction project: the production of RF power. This is where amplifiers stubbornly turn into oscillators, and where components release magic smoke, or at least burn the fingers of hopeful builders.

I kind of ran out of room when I built the low-pass filters. But, thinking ahead, I wanted to have them on a separate board. And it is good that they ended up in the far corner of the rig.

Just going from one band to two bands adds to the complexity of the rig. I had to add two relays, one to switch the low pass filters, the other to switch the bandpass filters. I ended up with 5 DPDT relays in this transceiver. It was very helpful to have a plan and a diagram for the relays and all the switching.

It looks like each of the three RF amplifier stages provides about 15 db of gain -- about what I need to get to the 5 watt level.


  1. That looks good Bill and should work FB. Tell us the power difference between 80m and 20m, it's sometimes a bit of a struggle to get anything like 5w on 20. The flow looks good, I don't think youll have instability problems. Have you seen how tightly packed some of the 5w IRF510 PAs are on some kit PCBs? Beats me how they don't howl like a Banshee! IRF510s do get hot, you might need that bigger heatsink. Good luck with it, 73 Paul VK3HN.

  2. Thanks Paul. Yes, there is a noticeable power difference between 75 and 20. I think I get about 9 watts PEP on 75 and about 4 watts PEP on 20. But that's only about 3.5 db. No big deal, and about what you'd expect from a simple IRF510 amplifier. I might swap in a Mitsubishi RD006HHF1. I do have a larger heat sink that I might use, especially if I go with the Mitsubishi, or if I try to put 24 volts on the IRF510. I was pleased to NOT have any feedback problems with this amp. I think (as you indicate) layout is very important. 73 Bill


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