Message-ID: <ECF3692C6F5F4C30836990149F672B06@DELL4>
From: <jrusgrove@comcast.net>
To: <rsgb_lf_group@blacksheep.org>
References: <1050898420.5270936.1543008571489.ref@mail.yahoo.com> <1050898420.5270936.1543008571489@mail.yahoo.com> <a41bb5ff-0d58-a4f0-5764-3311bb78f28b@n1bug.com> <1UQe0OyxLZ.AUJjn6aniVR@optiplex980-pc> <c8bc00bd-8d92-95b0-df9a-16c96ee9a5e9@n1bug.com>
Date: Sat, 24 Nov 2018 08:48:41 -0500
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Subject: Re: LF: No 2200m TX tonight
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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" <paul@n1bug.com>
To: <rsgb_lf_group@blacksheep.org>
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, jrusgrove@comcast.net 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 <paul@n1bug.com>
>> Reply-To: <rsgb_lf_group@blacksheep.org> To:
>> <rsgb_lf_group@blacksheep.org> 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 <paul@n1bug.com>
>>>  An: rsgb_lf_group@blacksheep.org
>>> <rsgb_lf_group@blacksheep.org> 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
>