On 12 May 2007 21:25:33 -0700, dykesc wrote:

My recent experience and your post has convinced me that a direct
termination of my balanced transmission line (300 ohm twin lead) to
the 259b is going to be problematic.

The way you described it (presuming an efficient choking BalUn) with
battery operation and you remote from it, suggests any issue of
"unbalance" is strictly academic. You can force it to become a real
problem if the case of the 259 is close to ground where the chassis
adds a capacitance to ground, but that is a rapidly diminishing value
as you raise it (couldn't be more than 1 or 2 pF at 6 feet up).

I am now measuring through the
4:1 current balun in my MFJ tuner.

This is extremely unlikely (being a current BalUn) unless it is
specifically specified as one (and even then, many professed 4:1
current BalUns are in fact no such thing).

You have the means to test the assertion, use your 259 to measure the
isolation of the BalUn. This was the subject of a recent thread.

Wish I had a 1:1. At 7.185 Mhz
through the 4:1 balun (tuner bypassed) I get 19 -j48. Assuming an
ideal balun I believe your previous post stated this would be 76 -j192
on the high side. At most even harmonic frequencies I've measured, it
appears the 4:1 balun in the tuner is actually resulting in too low a
resistive term impedance.

Fixation on BalUns has clouded a simpler solution: wind a choke in
the line and dump the ferrites of suspect quality.

As I write this I recall some text in the
antenna book about calculating the proper 1/4 wave Zo transmission
line impedance needed to transform to a desired impedance. Will this
work for any odd multiple of a 1/4 wave transmission line?

Yes, but discrepancies mount up dramatically as you multiply them
(tolerances at 1/4 demand greater precision at 3/4, and even greater
at 5/4). Besides, this doesn't address the odd readings you
experience.

On second
thought this wouldn't work on the harmonics would it? If I set it up
for 20 meters it wouldn't work on 40.

Sub Harmonics wouldn't suffer terribly. You do have a tuner after
all.

The whole deal with the off
center feed is to be able to use it on even harmonics (80, 40, 20
meters).

Off center feeds merely give you different Zs for the same resonances
- something of a shell game where you get to move your problems to
another band (guess what? This is what may be happening.).

Guess I'll just work on figuring out the best compromise
transmission line, but I'm fairly convinced I can do better than the
300 ohm twin lead.

It would be simpler to hang a second, half-length dipole beneath a
full size dipole and forget the off center feed.

This is all just for the challenge of understanding the theory and
making it work in application. The tuner is doing fine for all 3 bands
in my current configuration.

Many antennas work just fine until the operator discovers a new tool
that proves it doesn't (in spite of a wall full of QSL cards).

Thanks for helping out a Stuggling Crippled Newbie Street Urchin.

Wait until you face the sewer rats of Rio.

73's
Richard Clark, KB7QHC