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

[Bob Tellefsen]

Hi John
I'll swing at your questions.
First, you have to look at a nonresonant dipole along with its feedline
as a system.  You can't take each part separately.

Let's talk about 40m for an example.  With different combinations of
antenna length and feedline length, you can have a current maximum
(lowest impedance), a voltage maximum (maximum impedance), or
something in between at the shack end of the feeder.
I try to adjust such an antenna system for the current maximum at the
feedpoint.  That makes a resistive load for the ATU, and allows use
of a simple 1:1 balun.

The rub is when you go multiband.  Let's talk 20m now.  Suddenly that
nice, tame low Z feedpoint is now a high Z feedpoint with lots of rf
voltage.
Very tough for an ATU to match.  Also, not a good place for a balun.

On 15m, the third harmonic of 7 MHz, the feedpoint will again be low,
but it may not be exactly at a resonant point, meaning lowest reactance.
Ten meters again will be a high Z feedpoint.  And so on.  The WARC bands
fall in between these points.

Here I use a 60 ft fat non-resonant dipole on all bands, 10m down to 80m.
Since I'm out in the garage, it is easy for me to loop some additional
feedline
through the rafters to bring a middle-of-the-road impedance on all bands
to the feedpoint at the rig.  This is not a perfect solution, as almost
every band
has some reactance along with the resistance.  However, if it is not
extreme,
a good antenna tuner can cancel it during the matching process.

For contests, where I'm jumping from band to band, I find my 50 ohm:12.5 ohm
balun gives the best overall results.  However, if I expect to spend a lot
of
time on one band, say 40m for ragchewing, I will pick my 1:1 balun, as it
works
best with this antenna/line combination in my situation.  If 20m, I might
wind up
with my 50 ohm:200 ohm balun.

Since these nonresonant systems are all different, you have to figure out
what
antenna length you will have, how long a feeder you can run, and then in the
shack adjust for the best match under these conditions.

One way to do this,  rather than run extra line through the rafters, is to
use extension sectons connected with banana plugs and jacks.  This way, you
can have the shortest possible feedline coming to the shack, and then insert
correcting lengths as needed to
match any given band.  This is an easy fix, especially if you have a band
decoder to control relays which can do the switching for you.

I hope this gives you some ideas, John.
Good luck and 73
Bob N6WG