Thanks Markus,

The shape of that curve seems almost what I am seeing. In one or two
days I will have a IR thermometer gun. Then if I can get some nice
weather I will have a look around the xfmr, coil and so on for any
hot spots. Something tells me I will not find any, but it is worth a
look.

73,
Paul


On 11/27/18 9:07 AM, Markus Vester wrote:
> Paul,
> 
> apparent asymmetrical thermal time constants would be plausible
> if you are just barely reaching a threshold temperature, above
> which the effect occurs.
> 
> Best 73, Markus
> 
> -----Ursprüngliche Mitteilung----- Von: N1BUG <[email protected]> 
> An: rsgb_lf_group <[email protected]> Verschickt: Di,
> 27. Nov. 2018 13:51 Betreff: Re: LF: No 2200m TX tonight
> 
> Hi Stefan,
> 
>> That is very strange! Thermal capacity and time constants
>> should be the same when heating up or down.
> 
> Well... yes, that seems to make sense. :-)
> 
>> How much antenna current do you get at 100 W RF power?
> 
> It depends on the time of year and temperature, since the loss 
> resistance is dependent on those factors. On the last day of
> testing it should have been about 1.1A.
> 
> I do not normally keep an RF ammeter in the antenna lead, but I 
> think I need to put one out there again to check whether antenna 
> current increases when I see current on the feed line
> increasing.
> 
>> If you say you have a scopematch, i assume you mean that you
>> are the curves of voltage and current on an oscilloscope,
>> right?
> 
> Yes, I always watch the curves of voltage and current on the
> feed line (measured at the transmitter end of the line).
> 
>> I have thoughts about EMI into the power supply or exciter but
>> this is unlikely since you have a decoupling xfmr at the
>> antenna feed point.
> 
> It's quite possible, but how could EMI into the power supply or 
> exciter result in feed line current increasing while at the same 
> time voltage is decreasing? I could understand a power change,
> but this isn't that, it seems to be a change in antenna / ground
> system resistance.
> 
>> Is your antenna earth separated from the mains earth? If not,
>> try to separate it and see if the effect is still there, apart
>> from the lower antenna current.
> 
> Actually it is not. I do have the isolating transformer at the 
> antenna base, but I do not think that accomplishes much at my
> station!
> 
> The 'T' antenna for LF is suspended between two metal towers.
> Each tower is grounded, and around one of the towers is a quite
> extensive system of wire radials because that tower is shunt fed
> on 1.8 MHz. The lightning ground for both towers plus the radial
> system are all tied together and this is the ground for the LF
> antenna.
> 
> The trouble is coax cables going to other antennas on both
> towers are connected to equipment in the shack, which of course
> is connected to the mains earth. So we cannot say the LF antenna
> earth and mains earth are separated. In fact, we can say it may
> be more like a big loop!!!
> 
> I can try to disconnect all of those cables temporarily but I am 
> quite certain this will shift resonance of the LF antenna. It
> will take some time to get it resonated for another test. It is
> quite a lot of work to change taps on the coil, as I must
> completely disconnect it and remove it from the large plastic
> drum to be able to reach the taps. Not the ideal setup at all,
> but it is what I could do.
> 
> I wonder if I should try to temporarily connect the coax outer
> to the antenna ground at the base of the LF antenna? At least
> that would provide a direct return path.
> 
>> And maybe it is worth to control the DC supply voltage of the
>> exciter and PA during the effect starts. If the PA runs at 12V,
>> use a battery and a floating signal generator. These are my
>> next thoughts.
> 
> This happens with two very different exciters. One is the U3S.
> The other is a HF transceiver through a home made down converter.
> The affect I am seeing is the same using either exciter.
> 
> Make that three exciters. Before it died, I could use my HP 3325B
> as an exciter. I saw exactly the same thing when testing with
> that.
> 
> I can easily control voltage to the PA but not so easily to any
> exciter.
> 
> The PA will run at 12V but does not produce enough output at
> that voltage to see the problem. Below a certain RF power
> threshold there is no problem!
> 
> 73, Paul