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:
Band?PWR-W?I-mA
160?6.55?1150
80?5.35?928
60?4.99?925
40?5.73?975
30?5.68?1363
20?4.86?931
17?3.57?1258
15?5.73?1613
12?3.89?1553
10?5.05?1282
6?3.87?1080
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
--
Roelof Bakker, pa0rdt
