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Michael Coslo · September 28th 07, 03:29 PM posted to rec.radio.amateur.moderated

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

it's not a reason to reject the 'random length'
doublet.

There are indeed certain lengths that are best avoided.
Certainly you
don't want the doublet to be near 1/4 wavelength in total
length on a
band you intend operating on.

Why not, 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.

In the MFJ tuner manuals, there is some
text on lengths you would want to avoid for our purposes.

Never had an MFJ - my transmatches are all homebrew. I've read the MFJ
manuals, and it seems to me that they were trying to avoid
lengths of antenna-plus-feedline that would present very low or very
high impedances at the Transmatch end of the line.

Pretty much the case. I would surmise that the more you have to tune
out that way off impedance, the more loss you might have. It's the old
thing about the tuner being able to match a coat hanger. I suppose so,
but it probably isn't the best way to go.

It's not just the doublet length that matters but also the length,
impedance and loss of the transmission line. Antennas like the
G5RV choose a combination of dipole and transmission line
length that present reasonable impedances on several bands.

Trick antennas such as the G5RV and OCF dipoles
utilize some clever
techniques to match impedance.

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.

IMHO, it is better to have a station that works well on a few bands
than to have one that works poorly on all bands. Many multiband
antennas, such as many commercially-manufactured "all band" HF trap
verticals, are so full of compromises that their performance on some
bands is highly compromised.

Often the idea of "low SWR" is put out as if it is the sole criteria. In
defense of SWR lovers, modern Rigs really hate High SWR, especially
reactance of the capacitive kind. But a 50 ohm resistor has 1.1:1 VSWR,
and some manufacturers have taken advantage of that sort of thing in the
past.

On the subject of antennas that work well on a band or two, it is sound
technical advice. The problem as I see it is that most new folks these
days start out with an "all band" radio, and are inclined to want a
antenna that is likewise all band.

Fortunately my Elmer pulled me aside, and said "try this". Within a
week, I had my doublet up and running, and I've worked the world
with it.

That's the ultimate test of any antenna system: what have you worked
with it?

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. No JA or far eastern countries, but I
haven't tried really hard, I just work 'em if I happen to hear them.

I once accidentally worked a fair part of a contest once on 75 meters on
probably around 3 watts, mostly into California - I had tuned the
antenna, and forgot to turn the power back up. I had works around 25
QSO's before catching that one.

Of course, that isn't quantified data, it's just anecdotal. But running
at QRP levels does make for a more stern test of an antenna's abilities,
especially if there isn't obvious signs of it, such as not getting calls
answered. Sold me on the thing.

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. 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.

- 73 de Mike KB3EIA -