Hello Paul,
Ooh it is a class-E PA? That explains much! They need a very accurate
SWR to reach the high efficiency. Initially i've been a friend of
class-E but what you see now is exactly the reason why i'm preferring
class-D now!
Generally: Avoid TO-220 cases for a real PA. Prefer TO-247 instead,
especially below 500 kHz.
Regarding the effect you're observing: I would guess it is a thermal
problem in a ferrite core, a capacitor or even the away-drifting
on-resistance / working point of the class-E PA.
73, Stefan
Am 24.11.2018 12:47, schrieb N1BUG:
Hi Markus,
That is very interesting. It changes about 25 % during the first 2-3
minutes, then it seems to settle down and not change any more. It
could be moisture somewhere, but any moisture here is solid ice or
frost now. I did not find any ice or frost in the transformer box or
the variometer. Could be insulators or something with the antenna
itself. I don't see any "fuzz" on the scomematch voltage trace so I
think (hope) nothing is arcing.
What worried me is this did not happen last winter so something has
changed. Everything accumulates some dirt here because of blowing
dust, smoke, etc. I wonder if a small amount of dirt on insulators
plus moisture can combine to make funny things happen.
Anyway I examined the little PA and it seems to have died due to
poor thermal interface between FET and heatsink. It's a physically
small FET and maybe was not screwed down tight enough with the
Sil-Pad interface, which was also a previously used one.
Normally I do not like to put drain voltage on the heatsink but as
an experiment for this little PA (which is totally an experiment
itself, but served me very well last winter) I will isolate the heat
sink from the chassis/PCB and mount the little FET directly to it.
The thermal resistance would be much lower! I think this is fine so
long as nothing shorts the heat sink to ground. In that case some
fuses die. ;-) This would add some pf of capacitance between drain
and ground but it would be in parallel with the quite large C of the
Class E tank, probably not much difference at 137 kHz!
Parts to repair the big PA should arrive Wednesday.
73,
Paul
On 11/23/18 4:29 PM, Markus Vester wrote:
Hi Paul,
sorry to read that. Have pity on the poor FETs!
You mentioned that the antenna resistance is gradually decreasing
(i.e. improving) during longer transmissions. I often see that
effect here, with the current rising by say 20 % during the first
few minutes. I've put it down to moisture or dew around the coil
and insulators (tiny little polycarbomnnate pencil tubes), which
evaporates as things warm up. It is more prominent during cold
damp weather, and much more so with the very high antenna
impedance at VLF than at LF.
Good luck, Markus
-----Ursprüngliche Mitteilung----- Von: N1BUG<[email protected]>
An: [email protected]<[email protected]>
Verschickt: Fr, 23. Nov. 2018 22:12 Betreff: LF: No 2200m TX
tonight
No transmissions from me this night. The little amplifier has
died. I think it may be related to this resistance change in the
antenna which is getting worse and worse.
I'm going back to MF for this night, sorry!
73, Paul
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