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.
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.
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.
Happy trails.
============== James Richards- K8JHR ============
Dr. Gerald N. Johnson wrote:
On Wed, 2009-05-20 at 17:22 -0400, Richards wrote:
Well... shoot, Jerry. Now I AM confused. I spoke with Paul
Hrivnak and company at Dayton last Friday and that is what they
told me to do... but now you are giving me somewhat different
instructions. So... now I all conflicted...
Is this reason to get a beer?
Is there some way to reconcile the two methods?
If so... is THAT reason to get a beer?
Is it reason enough to get the TT tuner instead of the Palstar? or to
justify getting the Palstar auto tuner (which I think I want) because it
may work more like the TT tuner, and also be automatic for "just" a few
hundred more?
========== Richards - K8JHR ===============
Well, I suppose one could run the input capacitor to minimum and adjust
only the output capacitor and the coil to get a match, nearly an L
match, But that's only for the case when the load Z is higher than the
input desired 50 ohms. That's just backwards when the load Z is lower,
like feeding an 8' whip on 75 meters. If you crank in minimum input C
but not the absolute minimum you deviate from that almost L. A pi is
sometimes analyzed as two L networks, but it works out for the
conventional PI, that the loaded Q of the network is closely
approximated by the ratio of source R and input C reactance (R/Xc) and
that ratio of the load R and output C reactance (R/Xc) is very close to
the same. You have to adjust the load C to compensate for the reactance
of the load. I assert that the network will have the lowest losses when
the reactance is largest, e.g. the capacitance smallest because that
corresponds to the lowest Q and so the least resonant circulating
current in the PI while still preserving the greater versatility of the
PI for matching Z both higher and lower than the feed Z without
reversing the network.
However having the lowest loaded Q means having the least harmonic
rejection, and its conceivable that using lowest C means largest L and
more loss from the larger coil. But it seems to me that minimizing the
multiplication of the input current by keeping the Q low minimizes the
circuit losses.
I'm one of those been around so long that a "few" hundred bucks was my
beginning salary before taxes and a Collins receiver only cost 2/3 that
with the employee discount. Makes it harder for me to pay a few hundred
bucks more for a tuner that may actually contain parts comparable to
those in my stash of good parts, and not be as capable as that tuner I
built out of a huge broadcast transmitter coil back in 1964 that runs
cold at all the power I've ever fed it, even with the output open or
shorted. In the last year, I've built two tuners out of parts on hand.
Neither will handle a KW but that 811 amp I bought 8 years ago, still
sets on the floor unused. I'm glad I didn't spend a few hundred bucks on
it.
73, Jerry, K0CQ
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