Thanks Jay,

Yes, I shamelessly copied your transformer design. it is a stack of
three 2.4" cores, 77 material. The cores are Fair-Rite parts
purchased from Mouser. Each core was wrapped individually, then they
were stacked and the whole thing wrapped to bind them together.

Good suggestion about moisture in the box. I just now went out and
pulled the cover off for a look. I see no evidence of frost or
moisture. I also checked inside the 50 gallon plastic drum
containing the coil with similar findings.

My present theory is that something common to both LF and MF changes
resistance as RF warms it. Assuming the change is happening at the
antenna end of the line, I think it is natural that on MF the
scopematch would show a reactance change even though it is really
resistance that is changing. I freely admit to being not too clever
with a Smith chart, but I believe such is expected with a 1/8
electrical length feed line.

But if that theory is correct, that leaves the coax itself
(doubtful), the antenna (not including coil), ground system or some
environmental factor.

73,
Paul


On 11/20/18 8:30 AM, [email protected] wrote:
> Paul
> 
> On your website the pix of the transformer looks to be a stack of 3) FT240 
> cores ... assume you're 
> using 77 material? If so, this same setup handled a kW of rf at WD2XNS with 
> no problem ... with one 
> exception.
> 
> Your pix also shows that you used the same sealed enclosure that I used. On 
> more than one ocassion 
> the core and inside of the box became heavily 'frosted' causing various 
> impedance matching problems. 
> Bringing the unit indoors and allowing it to dry out restored normal 
> operation when installed back 
> out at the antenna. Apparently, the sealed box allowed it to develop it's own 
> internal 'atmosphere' 
> and the problem occured during periods of wildly changing temperature and 
> humidity. The cure was to 
> drill a number of holes in the bottom of the enclosure to equalize the 
> 'atmosphere' inside and 
> outside the box. Alternatively, a small 'breather' could be fitted to the 
> case. Don't know that this 
> is what's going on in your case but it's easy enough to check.
> 
> One other thought ... did you wrap the 3 cores with tape individually before 
> assembling the stack? 
> Have seen unexpected problems at HF when not doing that so I now do it as a 
> matter of course.
> 
> Jay W1VD
> 
> 
> ----- Original Message ----- 
> From: "N1BUG" <[email protected]>
> To: <[email protected]>
> Sent: Tuesday, November 20, 2018 7:44 AM
> Subject: Re: LF: WSPR-15 last night
> 
> 
>> Hi Stefan,
>>
>> On 11/20/18 6:50 AM, DK7FC wrote:
>>> Don't waste to much time, it is now the best time in the season! Do you
>>> have a second PA, maybe a QRP version? Still worth to try!
>>
>> No, because I had an attack of stupidity. ;-) Instead of saving my
>> old PA, which would be 3 or 4 dB down from the new one, I took it
>> apart to experiment with the design. My idea was to upgrade and
>> modify it to create a spare PA that could be almost equal to the big
>> one. Seems the idea was OK. :-) But for many reasons I did not
>> continue that project. I will look around for the box which contains
>> that project, maybe I can easily put it back to its original form...
>> if the cat didn't find the box and take too many parts!
>>
>>> Maybe you can run a permanent carrier and reach negative R values then?
>>> The world's energy problems will be finally solved! ;-)
>>
>> Hmm, then I will be rich and famous or just famous?
>> Or neither and remain infamous? ;-)
>>
>>> Depends on how you measure the current. A saturating current transformer
>>> will become warm or hot. Should be easy to check.
>>> Or it is an iron dust core thermally drifting away. It is useful to
>>> measure the voltage behind the LPF.
>>> Just some spontan thoughts.
>>
>> OK, thanks for the ideas. I did notice last night something
>> interesting on MF. The phase was changing slightly during each TX
>> period. Something to note is that on MF, the coax to the antenna is
>> almost exactly 1/8 lambda electrical length. So any change in
>> antenna R shows up mainly as a change in phase at the transmitter.
>>
>> So it may be that something in the antenna is changing R on MF also
>> (with only ~75 watts RF). It's a completely different coil and
>> transformer so what could it be. I don't know.
>>
>> Maybe I am cooking trees? Just joking. It will be some engineering
>> defect in my RF systems. Good news is there is plenty of snow now,
>> so any fires around the antenna should be slow to spread. :-)
>>
>> 73,
>> Paul