Use an ATU between the TX and heater element to match the 50ohm Z
G3KEV
-----Original Message-----
From: N1BUG
Sent: Saturday, November 24, 2018 2:39 PM
To: [email protected]
Subject: Re: LF: No 2200m TX tonight
Hi Jay,
I haven't been able to attend a flea market in years. I'm sure I can
find a Cantenna on the auction site, but it won't be for little money.
I've been thinking about an 800W flange resistor on a big heat sink,
but would need to find a big heatsink. Maybe a medium heat sink
packed in ice... or sitting in snow as the case may be!
I did look at water heater elements after you mentioned that a year
ago, but I couldn't find any with a voltage / wattage rating that
looked close to 50 ohms. Also, I am not sure what the resistance vs.
temperature curve looks like on them. I'm torn. Being on a budget it
is always tempting to try the inexpensive route. But on those
occasions when the inexpensive solution fails to work out, one ends
up spending more in the long run.
If I don't find anything with the dummy load tests, I will modify
the coil to eliminate the shorted turn and see if anything changes.
That's an interesting task as one must lay in the snow to remove the
coil from the housing to get at the connections. It's all part of
the continuing adventure... ;-)
I'm going to stay on MF one more night as the trans-Atlantic QSOs
are fun. But I will get the small LF PA fixed up today and be ready
to come back very soon.
73,
Paul
On 11/24/18 8:48 AM, [email protected] wrote:
Paul
An old Heath Cantenna makes a good load. They're often seen at flea
markets for short money.
In a pinch, an electric hot water heater element (of appropriate voltage /
current rating for 50
ohms) submerged in a kettle of water ... or better yet ... still in a
small hot water heater tank
will dissipate lots o' watts. Reactance can be tuned out with series
capacitor at HF ... probably
not necessary to do so at LF / MF. Works very well.
I didn't short turns in my setup ... just connected to the desired tap ...
all other turns float.
Jay W1VD
----- Original Message -----
From: "N1BUG" <[email protected]>
To: <[email protected]>
Sent: Saturday, November 24, 2018 8:06 AM
Subject: Re: LF: No 2200m TX tonight
Hi Jay,
I wouldn't blame them for cracking in Maine. Sometimes I feel on the
verge of cracking. ;-)
I need to get an amp repaired and do some dummy load tests outside
as you suggest. First just the feedline, then feedline and
transformer. My biggest dummy load is a 250W flange resistor on a
heat sink that usually won't take 200W for long periods. But sitting
outside now, it will be supercooled. :-) I need to get a 1 kW load
if I'm going to play with LF stuff.
I'm trying to remember exactly when I added the new taps very near
the bottom of the loading coil. It may have been after most of my
transmitting in the spring. I've got the very bottom of the coil
connected to the transformer secondary, then a tap from the very
bottom to one turn up. Could this shorted turn be a problem?
73,
Paul N1BUG
On 11/24/18 7:21 AM, [email protected] wrote:
Paul
Any chance one (or more) of the 3 high stack of cores in the
transformer might have cracked? Might be interesting to hook a 50
ohm dummy load to the secondary of transformer, adjust turns
ratio to 1:1, and have a look. At least this test would absolve
anything up to and including transformer from being the problem.
Jay W1VD
----- Original Message ----- From: N1BUG <[email protected]>
Reply-To: <[email protected]> To:
<[email protected]> Sent: 11/24/2018 6:47:45 AM
Subject: Re: LF: No 2200m TX tonight
________________________________________________________________________________
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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