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?

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.

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.

Haven't used a G5RV, but my experience
with the OCF has been fairly satisfactory. I would note that after it
broke, I elected to put up another doublet with ladder line and
tuner. I have been pleased with that.

In the dipole-category of HF antennas, I've used G5RVs, OCFs, dipoles
fed with ladder line and a Transmatch, coax-fed dipoles, fan dipoles
and coax fed trap dipoles. Plus inverted-V versions of most of those.

In my experience they are all comparable radiators of RF *if* they are
implemented in a way that keeps feedline/transmatch loss low and gets
the antenna up and in the clear. IOW, none of them are magic, and they
all have their applications.

They

[antenna software]

are also an excellent method of comparing the
efficiencies of the various antennas.

Not just the antenna but the feedline system as well.

One of the biggest reasons that I suggest the general purpose
doublet is
that the new Op gets an antenna up that doesn't have all of the
foibles
of a precise dipole, such as antenna height above ground,
interaction
with nearby objects, and can get multi-band operation in the deal.

That's true to a point, but there are other tradeoffs, such as the
absolute need for a Transmatch, the need to avoid certain lengths,
and the difficulty of handling balanced lines in some situations.

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.

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?

My first HF antenna was an inverted L - what some would call a "random
wire", even though there was nothing random about it.
It was end-fed and worked against a ground/counterpoise system
consisting of the radiator piping and a lone ground rod. I made many
QSOs with it and later versions.

The big problem with HF/MF antennas for the radio amateur is that
the best choice is so dependent on the site and what the amateur
intends to do. This is why it is impossible to give general advice
about HF antenna types that is any good, without knowledge of
the available resources and intended use.

73 de Jim, N2EY