|Subject:||Re: LF: No 2200m TX tonight|
|From:||Markus Vester <[email protected]>|
|Date:||Sat, 24 Nov 2018 13:41:46 +0000 (UTC)|
|Dkim-signature:||v=1; a=rsa-sha256; c=relaxed/relaxed; d=aol.com; s=a2048; t=1543066909; bh=yy2pnxwr4lWJaILpIgfze/XIlnh+i8uLmIdkT2h4PYw=; h=Date:From:To:Subject:References:From:Subject; b=At1OCS2fdULexXDMUy4aYw2k+axdRXFVsFkWKx699iThmRxXkJMJHkks+IH67/yabl7eqfo9DXS3q0LCkJwxZ3XHUTsoehmygvoXP29t5UI7teQim+AEUOMCIwp3QS+gZlkGIZMjNC4tk03lP3amB7lXtPG+u87TqANiqaEZVtgGfRr3EwOKtAC/CiVoXC4X0JIlr4sdJzOD9j0JY4/deqB0THL0Ah0HfuhpAOoXuUGtZI/p0P+aiB79SDr6vxTmK2XepClCS0fnuLBzI12HJqhbKtPNKVbJjutOEf7DJ63RFopPBIKDS/U1WyaWnQ8qs7Zc5VVDN+beSTXkkSrmVQ==|
>> It's a physically small FET and maybe was not screwed down tight enough with the Sil-Pad interface,
Same thing happened here a few years ago during an attempt to melt off a thick layer of hoar frost from the antenna. It almost worked and I could see the resonance creeping up but then the PA died. At the time, the four BUZ111 devices in my 300 W class-D were bolted to sil-pads using M3 nylon screws. Due to increased dissipation with reactive load, the plastic became soft enough to release thermal contact. After FET replacement, I went to metal bolts and insulation washers, with no such problem since. This was actually the one and only FET that I lost in my LF career (knock on wood).
As capacitance is not a big issue at LF you could also solder your FET to a copper heatspreader plate and use a larger pad to a grounded heatsink. Or use a configuration with grounded drains and negative supply and take off the output from the source pin. You'll need to use an input transformer with a gate-source secondary but that's a good idea anyway.
Regarding dummy loads for LF: I've been using a coffee maker or a toaster, with 1 kW at 230 V being close to 50 ohms, and negligible reactance. Just make sure to use an older one containing no stupid and unnecessary microprocessor. Albeit American 120 V devices may have less favourable impedances.
Von: N1BUG <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Sa, 24. Nov. 2018 12:48
Betreff: Re: LF: No 2200m TX tonight
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.
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
> 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
|<Prev in Thread]||Current Thread||[Next in Thread>|
|Previous by Date:||Re: [rsgb_lf_group] Re: LF: overnight JT9 reception on 630m (23-24 november), Rik Strobbe|
|Next by Date:||Re: LF: No 2200m TX tonight, jrusgrove|
|Previous by Thread:||Re: LF: No 2200m TX tonight, Laurence KL7 L|
|Next by Thread:||Re: LF: No 2200m TX tonight, N1BUG|
|Indexes:||[Date] [Thread] [Top] [All Lists]|