dykesc wrote in
oups.com:

....
Owen,

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

In my experience, the balun integrated into MFJ tuners is far from ideal,
and it is usually a voltage balun.

appears the 4:1 balun in the tuner is actually resulting in too low a
resistive term impedance. As I write this I recall some text in the

I don't really understand what you are hoping for.

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

You seem to be trying to operate a dipole over multiple bands.

The issues that this brings include:
1. is the pattern acceptable;
2. can you get power from the equipment room to the antenna reasonably
efficiently;
3. can you deliver the rated load impedance to the transmitter.

Impedance transformation for 3. can conveniently be done and is often
done in the equipment room, eg an ATU.

In that case, the feedline will operate at high VSWR at some frequencies
if you use an unloaded dipole.

Such a dipole with say 20m of feedline is difficult to feed efficiently
if it is shorter than about 35% of a wavelength.

A dipole of more than 120% of a wavelength has a pattern with multiple
lobes and intervening nulls, which may or may not be an issue.

Low loss open wire feedline can operate with very high VSWR (as would be
encountered with a multi-band dipole) without great loss. It is most
unlikely that coax is acceptable for this application, and possibly even
lossy forms of open wire line like TV ribbon or ladder line... depending
on your acceptability criteria.

Balun loss may be an issue if the balun is operated at very high
impedance (ie high flux for moderate power). If the balun gets very hot,
you have your warning.

Have a look at my article on feeding a G5RV, in particular Fig 10, the
classic tuned feeder configuration. a 30m long dipole with low loss open
wire line and an efficient balanced tuner is a frequency agile antenna
with good efficiency and good pattern up to 12MHz. Above 12MHz, the
pattern breaks into many lobes and nulls.


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.

That is what real ham radio is about!

Owen