40m CW Transmitter

This homebrew transmitter is based on the “A First Transmitter” design in the “Experimental Methods in RF Design” (EMRFD) book by Wes W7ZOI, Rick KK7B and Bob W7PUA which I recommend – it’s a really good read. The errata at w7oi.net contains extra information about additional measurements for the “First Transmitter” which is very useful.

The transmitter consists of a VXO, buffer amp, class C driver, PA and LPF. It is the same as in EMRFD, with these differences:

  • The class C driver is coupled to the PA using a capacitor and choke, rather than the transformer recommended by EMRFD. I couldn’t work out how to wind the transformer correctly, and always got a huge loss between the driver and the PA.
  • I couldn’t source the recommended PA transistor, so I used a BD133 instead.
  • The keyer fall time seemed too short (340ns) using the values in EMRFD, so I changed the the ‘fall time’ resistor from 510R to 2.2k. I also increased the capacitor from 100nF to 470nF, and the ‘rise time’ resistor from 3.9k to 4.7k. This gives rise and fall times of around 6.8ms.