Kevin Normoyle <knormoyle@surfnetusa.com> said recently on Towertalk:
> I got really confused looking at the TLW results. For my test case, they
> suggested a result, which was very bad for power loss?
You didn't change the series output capacitor in TLW to see its effects on
tuner loss. Please look at the Help file (TLW.pdf), accessed by clicking on
the Help button on the upper right of the main TLW window. On page 15 it
says:
"Now, for fun, increase the size of the output capacitor in the
Tee-network
and/or increase the unloaded Q of the inductor to help unstress the
beleaguered antenna tuner in the example above -- or change to a lower-loss
configuration than the Tee-network.
Now, let's try something really dramatic. At a frequency of 3.5 MHZ for
a
transmission-line length of 0 feet, enter a value of 0.001 ohms for the
resistive part of the load, with a reactance of 0 ohms. Then select the
Tee-network, with an output capacitor of 100 pF. The tuner will absorb all
1500 W of input power -- in other words it tunes up wholly into itself,
given a short at the output!
In general, L-networks will exhibit the least loss among the various
network configurations, but they often require awkward values for inductance
and capacitance. The Tee-network configuration is often used because it can
accommodate a wider range of impedances with practical values of variable
capacitors and inductors, albeit with sometimes-disastrous internal losses.
The Pi-network configuration is flexible, but it too will often require very
large values for capacitors."
I would also urge you to read Chap 26 in "The ARRL Antenna Book," 21st
edition, starting at page 25-8 and going to 25-13. This section discusses
the high-pass Tee tuner, and then goes on to describe the operation of the
AAT.exe program, which together with TLW is on the CD-ROM accompanying the
book.
<snip>
> At first I thought "This seems like a bug in TLW". I found the
> problem below, which I am calling a program design flaw..i.e. you have to
have
> some knowledge to realize why it's a bad result. It's easy to believe
there is only one
> possible result, and not tune things to get something good.
Yes, I guess it could be called a "program design flaw," although I'd be
somewhat more inclined to call it a failure to read the manual. But I'm
biased. ;>)
<snip>
> So what TLW is really doing is giving you a good result for what
> you specified,
> but doesn't give you enough info to realize you need to tweak the
> input you gave
> it, to see different possible results.
Yes, that's true. TLW gives you the correct result for the series output
capacitor you choose -- this is just exactly like what happens when you
manually tune a high-pass Tee tuner in the real world. The recommended
starting point involves setting the output series capacitor at maximum,
especially on the lower frequencies, where efficiency is frequently an
issue.
73, Dean Straw, N6BV
Author of TLW
Senior Assistant Technical Editor, ARRL (Retired)
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