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

[Ron D'Eau Claire]

I found that article useful too, George.=20

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.=20

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!=20

I suspect that's why some ops find that the inductor melts in their =
T-match
circuits.=20

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!=20

Ron AC7AC

-----Original Message-----
...Frank Witt, AI1H, has written in QEX an excellent article on =
measuring
tuner loss using an impedance meter such as the MFJ 259B. I and several
others have performed numerous tests on various types of tuners and have =
yet
to find one with an inherent loss exceeding one dB. Most come in the =
range
of 0.5 to 0.75 dB on 20 meters. One KW tuner was measured at 0.11 dB!

As a rule, and according to theory, L networks tend to show less loss =
than T
or PI networks.

Any tuner will exhibit larger losses when "mistuned" or when one =
attempts to
tune a load that is outside the envelope for the tuner.

There are excellent computer programs available for tuner analysis that
include calculation of losses in real components with finite Q values. =
They
confirm the minimal power loss in a well designed and adjusted tuner.

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