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Michael Coslo · September 27th 07, 03:53 PM posted to rec.radio.amateur.moderated

wrote:
On Sep 25, 11:03?pm, "xpyttl" wrote:
I would strongly suggest against a random length doublet.
A non-resonant
doublet will have impedances all over the place.

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

Spend a few
minutes looking up the right lengths for your doublet
and avoid potentially
a lot of grief.

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. 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.

Modeling software can be a big help in removing the randomness.

Good advice. They are also an excellent method of comparing the
efficiencies of the various antennas. Low VSWR is not always the mojr
indicator of antenna performance.

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.

The new guy or gal can then learn quite a bit by using the tuner to
match up the antenna to the rig, and can see which bands are interesting
to them.

Especially important, I believe is that they won't become confused and
give up. I know when I started in this hobby, I got enough confusing
advice that at one point I almost hung it up because there appeared to
be just no way that I could put up an antenna that would work. The
antennas that I could put up were going to be too low to the ground,
they were going to be too short, and on and on.

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.

Now I can cogitate on all the various antennas and their proponents
without keeping myself off the air in the meantime.

- 73 de Mike KB3EIA -