Curing transmitter instabilities with the”High Performance TRX” for 14MHz

I have used a simple dipole for 14MHz temporarily. This antenna was not much as quiet as the Deltaloop but much simpler to install and fix, compared to the loop that has been my standard antenna for 14MHz for years. The noise in the receiver when using the dipole was beyond words.

Thus I changed the dipole for a quad loop again the last weekend. When connecting the transceiver I found that in spite of good SWR (1:1.2) the transmitter was self-oscillating. With the spectrum analyzer I could recognize that there were oscillations in the medium wave range as well as around 14 MHz. This made me examining the transmitter more closely.

After estimated a dozen of checks and failed attempts I changed to improve the performance I decided to alter the input transformer of the final stage. This item,  which by origin had a winding ratio of 4 to 2+2, then was changed by a transformer having 3 to 3+3. This even deteriorated the situation.

Next I concluded that the second winding might have too high impedance. So I went to a 4 to 1+1 transformer (pig nose core BN43-202).

This finally cured the problem to a 100% and gave a very stable transmitter. Output power increased to 28 watts when fully modulated. This might be the direct outcome of better impedance matching.

Here is the current transmitter circuit with all the improvements made so far:

DK7IH - High performance Transceiver - Transmitter section
DK7IH – High performance Transceiver – Transmitter section (Full size image)

73 de Peter and “Thanks for watching!” 😉

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