The tentec 538 tuner is a reversible L. That means it has 2 configurations
for the L. One for high impedance and one for low impedance.
Carl Moreschi N4PY
121 Little Bell Drive
Bell Mountain
Hays, NC 28635
----- Original Message -----
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
To: <tentec@contesting.com>
Sent: Thursday, May 21, 2009 12:45 PM
Subject: Re: [TenTec] TT and the rest of us...
> On Thu, 2009-05-21 at 01:19 -0400, Richards wrote:
>> Thank you for the further gloss on tuner performance... I must
>> read it a dozen more times before I will get half of it. I am afraid
>> that you have now jumped way over my level of comprehension.
>>
>> I have a doctorate degree, but in something other then electronics,
>> and as new ham, I am teaching myself the requisite electronics.
>>
>> I do gather, however, that I need to learn the best radios of
>> capacitance, reactance, and impedance.
>>
>> In my case, I have one of those big stick 43 foot vertical antennas,
>> and I believe (from playing with my antenna analyzer) that the
>> impedance is nearly ALWAYS higher than 50 ohms. In fact I just
>> replaced the original 4:1 current balun with a 4:1 Un-Un, at the
>> suggestion of DX-?Engineering, which said it will usually end up
>> having a higher impedance, but that it is (they say) easier to bring
>> impedance down to 50 ohms, than it is to raise it to that level.
>>
>> I believe your comments are consistent with that claim. Thus, I
>> might be OK to run the input capacitor as far down as possible,
>> and work the output capacitor higher - rather than try the other
>> way around.
>>
>> I do find, in practice, that the antenna with the newer Un-Un is
>> somewhat more difficult to tune, meaning it is more difficult to
>> find the sweet spot, and that smaller variations in the input and
>> output capacitor settings will cause larger changes in SWR than
>> I saw on the former current balun - but that it does seem to give
>> better resulting SWR when I am finished. Oddly, the Inductor
>> settings are substantially lower with this newer transformer at
>> the base of the big stick.
>
> The capacitors being more sensitive is a hint at higher Q and so higher
> loss. There are frequencies where the un-un would work better turned to
> raise the impedance. That 4:1 impedance also means the capacitance
> required to tune out the antenna reactance is 4 times as large. Running
> out of capacitance is one of the limits of feeding short antennas.
>>
>> I am also getting from your commentary, that other considerations
>> apply, and perhaps a different pattern would be best if the subject
>> antenna has a lower impedance than 50 ohms. Somehow that
>> makes sense, as an odd sort of "inverse" rule to apply.
>
> Its not odd. Its that it takes a different L to feed a low impedance.
>>
>> I truly appreciate your comments and the time you took to spell
>> it out. They will become part of my permanent files and I hope
>> to someday read them with total comprehension.
>>
>> Unfortunately, your final comment eludes me - other than to
>> realize you would rather build your own from scrap parts than
>> spend the big money to end up no farther ahead. I, unfortunately,
>> must spend the penny at this time, as I lack sufficient knowledge
>> to follow your lead.
>
> There has been much good tuner information in ARRL handbooks, at least
> since the late 1930s. And still is.
>>
>> Happy trails.
>>
>
> One other thing. Loosing 50% of the transmitter power in the tuner is
> 1/2 S-unit at the other end of the path. Yes, it warms up the tuner, but
> 1/2 S-unit rarely wrecks an HF contact, egad most propagation paths fade
> several S units in a minute. Most of the time unless the load Z is
> extremely low or high, the least optimum tuner settings that match won't
> cause that much loss and it can be better to make contacts than to sit
> fretting about an excess 7.3% tuner loss.
>
> 73, Jerry, K0CQ
>
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