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Wednesday, July 22, 2020

A 1929 Transmitter on 14MHz

This year the Bruce Kelley Memorial 1929 CW QSO Party will include 20 meters. I decided to see how my Push-Pull Colpitts does on 14MHz. It has a fairly good note and is very usable in the BK.

I wanted to stay with a high capacity tank circuit. This minimizes RF current through the tubes and should improve tone and performance. In addition, I wanted a number of turns in the tank coil so that I would have reasonable transformer coupling between the tank coil and the antenna link. I ended up with two 4 turn, 3" long coils wound on a 1.25" form. This 1.25" form resulted in more tank coil turns on 20 compared to the 2.5" form used for the tank coils on 40 and 80 and the antenna link coils. Tank capacitance is about 100 pF on 14 MHz. My Colpitts now runs at about 20% efficiency giving 2-3 watts output on 20 meters.

My 14 MHz CW Signal as recorded on July 22, 2020 using the Northern Utah WebSDR:

For comparison listen to this same transmitter on 80 meters: