Hi Paul,
I'm not in your shack, so i can only guess where the problem is coming
from...
The function of the transformer is not only to match the impedance, it
is also for decoupling the shack earth from the antenna earth. Many
problems have been cured by adding such a transformer, so just try it.
It takes just a few minutes to build one...
The C on the gate is not a good idea i think. Better try a 10...22 Ohm
resitor or a ferrite bead in front of the gate.
Is the bias voltage stable? It may be worth to spend 1000 uF parallel to
the Z diode. And a normal diode in front of the z diode, i.e. into the
yellow wire...
The blue core, maybe it saturates at higher power? Could be worth to try
2 more turns and better use abt 0.5 mm wire for higher power.
But the design is not good for > 30 W or so. Don't expect to much from
an IRF530 at 13.8 V.
73, Stefan
Am 22.12.2017 18:23, schrieb N1BUG:
Hi Stefan,
What ever the design is, i would use an isolating transformer. BTW how
do you match the antenna feed point impedance to 50 Ohm then?
Do I need two transformers? What I use now is three stacked cores
(FT-240-77) with 14 turn primary and 12 turn secondary. Both primary
and secondary have several taps to allow selectable impedance
transformation. Primary and secondary grounds are isolated. It seems
to work.
I remember many issues with stray currents affecting the input signal to
the PA. This could be a reason. If the PA works well on a dummy load,
without blowing FETs, it should work the same way on a well tuned and
isolated antenna!
The bias level is critical. It seems perfectly stable at 12 watts out
if I adjust bias to a point where my +24 dBm drive is just sufficient
to excite the amplifier. This is definitely not in the AB2 region, as
the bias is then well below the point of any static drain current. Any
more bias than that, it starts to do strange things. On the
oscilloscope I start to see what looks like HF superimposed on the LF
waveform, but how could that be? I am looking after the low pass
filter! I am very confused about what I am seeing and why. I
considered it might be my oscilloscope doing funny things, but when I
see this the signal also begins to sound "rough" in my receiver.
Earlier someone suggested adding a 10 ohm resistor and maybe a diode
at the amplifier input. I have to go back and find that email...
Can you show a link to the schematic?
Here:
http://qrp-labs.com/ultimate3/u3mods/lfamp.html
And a photo of the construction! :-)
Here:
http://n1bug.com/ASBamp1200.jpg
I added the choke (9 turns on FT-82-43) and 2200 uF capacitor to
improve isolation from the power supply. Before adding these, a lot of
RF was on the +13V lead. With the addition of these parts it seems I
have very little RF getting back to the power supply.
Hmm, does the supply voltage stay constant in all working points?
Yes, 13.2VDC. +5VDC for the exciter with an internal regulator.
73,
Paul N1BUG
|
