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
|
