Hello all,

Last week I finished the build of a rev1 QMX+.

No problems were encountered and it worked fine on powering up.

During the build, I have measured the frequency response of the low pass filters.These appeared to agree very well with a simulation carried out with help of RFsim99.

One anomaly was encountered: the series resonance of L507, a RFC feeding DC to the centre of the 60M-80M low pass filter caused an additional loss of about 0.5 dB just in the 60M band.

To get rid of it the Q of the RFC was lowered by mounting a 2K2 SMD resistor in parallel of L507.

I have been reading that many builders experienced a drop in output power on the higher bands and my build was no exception.

As I like tinkering a lot, this appeared to be a nice challenge to see if this 'issue' could be improved upon. My build is for a supply voltage of 9V.

Reading up on this issue on this list, it appears that the output transformer L501 is critical. Muhsin, TA1MHS, wrote that he had replaced the BN43-202 binocular core with a BN-61-202 core, which improved performance on the higher bands.

Apparently a lower inductance for L501 helps. I did not have stock of a BN-61-202 core, but I? had a BN-43-302 at hand and wound a WTST on it for 9V operation. The change in output power was negligible, though.

The next thing I tried was using 2 x 3 BS170 instead of 2 x 2. Powered with 10V, gave these results:

...

This more or less solves this issue for me, though I don't think this is recommended in actual operating.

I wonder what the reason is for the much higher current consumption on e.g. 15M compared to 80M, whilst delivering about the same power? A matter of lower impedance?

Anyway, this is great fun.

To be continued.

Test gear:

Sensor Monitor Power Meter

100W dummy-load

Rigol DSA-815-TG spectrum analyzer

Lab power supply 0-32V 2A

73,

Roelof, pa0rdt