[TenTec] TT and the rest of us...

[Dr. Gerald N. Johnson]

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