A Complete Do-It-Yourself Kit .
Although by their very nature, QRP transmitters radiate less power, the output from such a transmitter does require adequate filtering. Usually to keep the circuit compact, these transmitters have a final stage run in Class C and being driven hard with RF power. Of itself, this is a recipe for high harmonic output and a well designed low pass filter is essential. For many years I have used low pass filters calculated from a series of figures produced by Ed Wetherhold, W3NQN, (a G QRP Club member) and published in two articles in the UK Short Wave Magazine in December 1983 and January 1984.
Ed Wetherhold has been the ARRL adviser on passive filters for several years and published many fine articles on audio and radio frequency passive filtering. I believe that the two articles in Short Wave Magazine still represent the best source of information for the design of good low pass filters for RF amplifiers.
The articles are comprehensive but here I just want to share enough of the information to enable readers to build useful filters to add to their home made transmitters. There is very little mathematics - about 4 pushes of a calculator is the most required to produce information for a buildable filter. I will also give a chart for “off the shelf” low pass filters, which can handle up to 10 watts or RF power, suitable for every HF amateur band.
Here is the datasheet in PDF format.