[Elecraft] OT: Vertical doublets (was: Home made Sigma-GT5 & KRC2 or SGC?)

[George, W5YR]

Interesting finding, Ron . . .

Conventional wisdom holds that the loss in a tuner is predominately in the
inductor since even modest quality variable caps have very high Q and low
loss. So, the general idea when tuning a T-network or a PI-network - either
of which will have multiple combinations of L and C that appear to "match" -
is to find the tuning that uses the least possible amount of inductance.
I believe you will find that Witt has made that statement also.

I have verified this experimentally with my MFJ 989C tuners by using r-f
ammeters in the balanced line output and finding the tuning that gives
maximum line current. Invariably that tuning is the one that has minimum
inductance.

Another verification is the W9CF simulator  you mentioned. Try different
values of Q for the inductance and then different values of L and C and the
optimum value will have the least inductance.

I have also investigated this using Reg Edward's tuner design program and
find the same result: least loss in the tuner with least inductance.

So, I find your conclusion and result that you seem to measure minimum loss
with maximum inductance difficult to understand or explain. Perhaps you can
shed some light for me.

Interesting . . .

73, George W5YR
Fairview, TX
w5yr@att.net
http://www.w5yr.com


----- Original Message ----- 
From: "Ron D'Eau Claire" <rondec@easystreet.com>
To: <elecraft@mailman.qth.net>
Sent: Tuesday, March 23, 2004 3:22 PM
Subject: RE: [Elecraft] OT: Vertical doublets (was: Home made Sigma-GT5 &
KRC2 or SGC?)


I found that article useful too, George.

Having a several tuners handy, I took a slightly more "nuts and bolts"
approach to evaluating losses and hooked up two identical T-Match tuners
back-to-back. Measure the power in at the 50 ohm input and measure the power
out at the 50 ohm "output" with the second tuner running "backwards". I knew
what settings my antenna used on each band, so I set the ATU on the xmtr
accordingly and adjusted the second ATU for a 1:1 match at the rig with a 50
ohm dummy load and wattmeter attached.

I found attenuations of less than 2 dB across the band PROVIDED the T-match
ATU's were adjusted for the highest inductance at which a match could be
found. It is very easy to make it 'match' at much smaller inductance
settings in many cases, but sometimes the losses of the two tuners was in
excess of 8 dB in that state!

I suspect that's why some ops find that the inductor melts in their T-match
circuits.

One of the very nice T-network simulators that I like is W9CF's version at:
http://fermi.la.asu.edu/w9cf/tuner/tuner.html

I had another relevant experience using a commercial ATU that had nylon
"banana plug" connectors for wire antennas. I was loading a voltage-fed wire
and it worked just fine. But shortly after commencing a transmission I
smelled something and the SWR jumped off the scale. I was running 15 watts
output at the time.

Following the hot smell I discovered that the banana connector had melted
until the hot lead for the antenna shorted to the case. Inexpensive plastic
connectors are NOT good high-voltage RF insulators. There was a very good
reason why the "ancient ones" always used good-quality ceramic insulators
wherever high RF voltages might be present. Keep in mind that I melted that
connector in short order running ONLY 15 WATTS output!

Ron AC7AC