Pindiode experiments.

I bought a couple of pindiode to make an ALC for the transverter.  It must be a voltage controlled attenuator.

The attenuator worked but I am not happy with the performance. The performance is probably Ok for weak signals but my signal levels are much higher. I see too much distortion of the signals.

Another problem is that the 2 MAR's and the PA sometimes oscillate. I think it can be solved by adding an attenuator between the 2 MAR's. Between the MAR's I will add a fixed attenuator. Before the MAR's I will add some switchable attenuator..

High IF results.

I did some test with a high IF to see if this is a good idea.

I used a 40 MHZ xtal oscillator block, 2 mixers and 3 tuned circuits at 67 MHz.
The idea is to make a transverter from cb to 67 MHz. That transverter/ CB radio combination would be the IF for the HF transverter.

Results were not as good as I had hoped. Id did work but the intermodulation was way too high. Especially at 7 MHz. Beside that The noise was very high even without antenna. That noise was generated by the 40 MHZ xtal oscillator. I replaced it with a signal out of a HP signal generator. That reduced the noise quite a lot. Unfortunately the intermodulation problem could not be solved easily.

I could have exepected this result. After all modern radio's use a roofing filter.
Those filters have bandwidths of 3 to 22 KHz. That solves some of the intermodulation problems.

I could try to get one of those. However it will increase the cost of this project. The filter is not cheap and I also need an xtal oscillator at an non standard frequency.

So I Think I stick to the 27 MHz IF. The transverter will be usable at 21 MHz and below. I already check the 21 MHz signal on the spectrum analyser. The spurious is -50 dbc. It may even be possible to use it at 24 MHz. Some aditional filtering is needed though

Power amplifier

The 5 power amplfier kit form box73.de was build and mounted on top of the enclosure.
It has a big heatsing so it will stay cool during wspr/qrss operation.

I have also mounted a switchable lowpass filter near the PA. I still have to wire it to the bandswitch.

To drive this amplifier I made a two stage amplifier using mar-4 amplifiers.
A single mar-4 did no provide enough power gain to drive the amplifier. Output power was 1 watt on 80 meter and 0.5 on 12 meter. So I have added another one. Now the gain is more than enough. Actually attenuation is needed to prevent overload.
The plan is to make an ALC circuit using a voltage controlled attenuator. Pin diodes will be used in the attenuator. I have just ordered some.