Hi Stefan,

I understand and agree about Class-E, but this same PA was operating
fine last winter with some very poor antenna conditions such as snow
and ice changing the phase and resistance greatly. There were no
problems. Also I have the exact same problem with the Class-D PA,
only a bit worse because more power.

Today I did some tests.

First test:
PA > LPF > ScomeMatch sensor > very short coax > dummy load. 15
minute RF carrier at 150W. At start, voltage and current waveforms
on the scope were equal amplitude and in phase. After 15 minutes no
change.

Second test:
Same setup but moved dummy load to antenna end of long coax, so
PA > LPF > ScopeMatch sensor > long coax > dummy load. 15 minute RF
carrier at 150W. At start, voltage and current on the scope equal
amplitude and in phase. After 15 minutes no change.

Third test:
Moved dummy load to output of ferrite matching transformer with
ratio set for 1:1 impedance, so
PA > LPF > ScopeMatch sensor > long coax > 1:1 xfmr > dummy load. 15
minute RF carrier at 150W. At start, voltage and current on the
scope equal amplitude and in phase. After 15 minutes no change.

Fourth test:
With coil and antenna, so
PA > LPF > ScopeMatch sensor > long coax > xfmr set to match initial
antenna R to 50 ohms > Loading coil > antenna. Started RF carrier at
150W, voltage and current on the scope equal amplitude and in phase.
After 45 seconds, voltage started to decrease, current started to
increase, and phase began shifting with voltage leading. After 3
minutes the phase stopped changing and remained with voltage leading
by a few degrees. After 6 minutes, voltage and current reached a
steady state with current much higher than initial. RF power had
increased significantly due to the lower Z load on Class-E PA. It
remained steady for the remaining 9 minutes of the test.

Fifth test:
Same as fourth test, but after phase became stable (3 minutes), I
retuned the coil (variometer) to bring phase back to zero. Voltage
continued to decrease and current continued to increase until the 6
minute point at which it reached steady state and did not change for
the remaining 9 minutes.

Sorry for the long text, but I wanted to describe exactly what I did
and the results.

I think:

PA OK
LPF OK
ScomeMatch OK
Coax OK
Transformer OK
Loading coil ???
Antenna ???
Ground system ???
Other nearby antennas, structures ???

73,
Paul






On 11/24/18 11:08 AM, DK7FC wrote:
> 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