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Michael Coslo · October 2nd 07, 08:20 PM posted to rec.radio.amateur.moderated

wrote:
On Sep 28, 9:29?am, Michael Coslo wrote:
wrote:
On Sep 27, 10:53?am, Michael Coslo wrote:

Why not,

[use a 1/4 wave dipole]

other than the fact that such a short dipole will present a
feedpoint impedance that has a low resistive part and a high reactive
part?
That's a pretty good reason, though. 1/4 wavelength dipoles - make sure
to read that as total antenna length, not 1/4 wavelength per leg, are
just a troublesome combination that a lot of tuners can't tune very well.

I'm not convinced. Depending on the transmission line impedance and
length, the shack-end Z could be within the matching range
of the Transmatch.

A few years back I did a modeling of an antenna that was 1/4 wavelength
long at I think 40 meters. The SWR of the antenna was approaching
infinite. If I get the chance, I'll model it again tonight - I'm doing
the mass mailing for the PAQSO party tonight, and if all goes well, I
should have a little time.

I don't consider the G5RV and OCF to be 'trick' antennas.
They're
simply intelligent combinations of dipole and feed systems that
have been worked out to present reasonable impedances so that
line losses and Transmatch requirements are reasonable.
They are very clever. The OCF especially is a joyful
playground to work
on with antenna design programs.
It is a great way to learn both the
programs and antenna theory.

Agreed. But so are other implementations such as trap dipoles and
the classic dipole-with-ladder-line feed, where you try different
dipole and transmission-line lengths.

Often the idea of "low SWR" is put out as if it is the sole criteria.

Too often.

In
defense of SWR lovers, modern Rigs really hate High SWR,
especially
reactance of the capacitive kind.

Well, that depends on how you define 'modern'...

Non-tube? Don't get me wrong, I LOVE tube rigs.

But a 50 ohm resistor has 1.1:1 VSWR,
and some manufacturers have taken advantage
of that sort of thing in the past.

Not just in the past. Google "Maxx-comm matcher" (try different
spellings). Those folks are still in business.

That's the one I was referring to. Hard to imagine they are still doing
business.

A lot of Western and Eastern EU, Great Britain, Iceland,
Norway, most of
South/Central America, about half of Africa, Israel, Australia,
Antarctica, and a couple others.

On which bands?

Mostly 20 and 40. Just a few of the near out of country neighbors on 80.

The technical details are that it is a 96 foot total length dipole, up
around 55 feet, the center support is a short length of pvc tubing.

I suspect that the 96 foot length was decided because that's what
would fit in the available space.

Yup, I tried a few dipoles that were longer, and made a Z shape. They
worked okay, but were a lot more maintenance. Lots of tree whipping in
storms here, and I almost made plastic pully's into the end insulators
to combat the extra movement. But I went back to the 96 foot length

The ladder line is soldered to the respective dipole wire. Ladder
line makes
an almost straight drop to the Shack window. Definitely not the
best
thing going, but not too bad.

Actually, what you describe is pretty close to optimum for a simple
multiband antenna system in limited space, which I suspect is the
main issue.

I agree. What really surprised me was that the performance on 75 meters
was pretty acceptable. I mostly get down there during contests, and have
had nice results. Worked enough people to make it worthwhile.

Are you using true ladder line, or "window line" (Twin Lead with
holes punched in the insulation)? True ladder line (heavy wire, wide
spacing, mostly air insulation) has lower loss and less weather
effects. If the line is short there's not much difference, but as
frequency and line length increase it can be worth changing out.

I use the window line. I have heard of the advantages of the true ladder
line, and certainly the higher impedance is one of them. I've heard of
some of the drawbacks of window line, such as it's performance when wet.

I did take issue with the test method cited by many, in which the window
line was dunked in water that included a wetting agent. My contention is
that the experiment showed the effects on window line with wetted line.

My experience has been that window line does not wet in this manner.
When the experimenter has to add a chemical to coat the line with water,
it is altering the conditions and producing results germane to only
those conditions.

I wonder what would happen to open wire ladder line under those conditions?

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