Buck wrote:
I see you use 450 ohm ladder line (or window-line as some call it.) I
am wondering if the concept will also work with other impedance
feedline such as 600, 300 or 75 ohm twin-line or even possibly with
coax. ****

Might as well go into some detail here. The feedpoint impedances
encountered in a 130 foot dipole may range from ~50 ohms to
~5000 ohms. In order to limit the maximum SWR the feedline Z0
should be ~SQRT(50)(5000) = ~500 ohms. Thus the choice of 450
ohm line. Given that the feedpoint impedances may range from
~50 to ~5000 ohms, here are the maximum SWRs that may be
expected for the different Z0s. Since the antenna system
is fed at a current maximum point, I have included the
impedance at the current maximum point which needs to be
between 25 ohms and 100 ohms in order to avoid foldback.

Z0 SWRmax Imax Impedance
600 12:1 50 ohms
450 11:1 41 ohms
300 17:1 18 ohms
75 67:1 1 ohm
50 100:1 0.5 ohms

It should be readily apparent why coax is a no-no for this
antenna system. Even 300 ohm twinlead will result in a
50 ohm SWR of 50/18 = 2.8:1, high enough to cause foldback.

I am interested in making one, but I only have 300 ohm.

Let's take 40m as an example. The dipole is one wavelength
and will have a 300 ohm SWR of 16:1 on 7.2 MHz according
to EZNEC. 300/16 = 18.75 ohms at the current maximum
point resulting in a 50 ohm SWR of 2.7:1 enough to cause
foldback without an antenna tuner. Such is life.

I do use 300 ohm line on my 20m rotatable dipole and it
does work 20m-10m but there are probably 10m frequencies
where the 50 SWR is too high.

Does the feedline act as part of the antenna? I am sure it will, at
least up to the 'no-tuner' if it acts like a G5RV, but do you know if
the feedline radiates?

I monitor the current balance in my feedlines. They are
so well balanced that there is no hint of common-mode
current on the coax side of my current/choke/balun.
Balanced currents radiate a negligible amount.

Have you measured the power at both sides of the 'no-tuner' to see
what loss there might be? I doubt there would be much considering
that you are using window-line.

No I haven't measured the losses. I have trusted Owen's
transmission line calculator for that data.

I see a couple of the frequencies are above 1.5:1, which I am not
comfortable going over with solid state rigs. Do you think that could
be fine-tuned with the addition of a 1/2 foot section or maybe with
that and a 1/4 foot section?

The disadvantage of this method is that it lacks one dimension
of tuning necessary to achieve 50 ohms. Unless you do one more
impedance transformation, no amount of fine-tuning the ladder-
line length will get any closer.

Would it be safe to assume that I can create a mono-band dipole, maybe
even multi-band - if I am lucky, by fixing the length of the dipole
and the feedline such that the increment gives me the imax at the
balun for the desired frequency(s)?

One such example is at: http://www.w5dxp.com/HEDZ.htm
This antenna works on 75m and 40m with a fixed length of ladder-
line.

Finally, you have a 1:1 choke at the feeder. I see that the better
quality coax, the more toroids are needed. Would there be a problem
with using a foot of RG-58 with the fewer toroids and then connect
that to the high-quality line?

Just use RG-58 entirely unless you are running high power.

Does the 'no-tuner' feedline need to be spread out. I see from the
photo that your 16 foot section is one large loop, I figure it must be
close to a four-foot diameter loop.

I have my 16 foot loop coiled in a 4 turn spiral around a piece
of fiberglas fishing pole.

Is there a similar system that would work with a vertical?

Verticals are not usually fed with ladder-line so probably
more trouble than it is worth.
--
73, Cecil, w5dxp.com