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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