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.

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.

A very useful tool is modeling software such as EZNEC or G4FGQ's
DIPOLE3. They will give useful predictions of shack-end impedance, so
you can judge if it's in matching range or not. So even if the antenna
is 'random' length, you can have a good idea if it will match and how
efficient it will be before you put it up.

With regard to transmatches, also called antenna tuners, for balanced
loads, the two typical amateur approaches are the unbalanced-tuner-
followed-by-a-balun method, and the link-coupled method. The
unbalanced-tuner-with-balun method assumes the balun does its job over
a wide range of impedances, which isn't always a good assumption,
while the link-coupled method can be complex to bandswitch.

A third method, described by AG6K, consists of a balun followed by an
L network - the balun is on the rig side of the transmatch rather than
the antenna side. Thus the balun only has to deal with 50 ohms
nonreactive once the L network is adjusted. Google AG6K to see a
description of his method. Although his tuner uses ganged roller
coils, fixed coils with taps could be used in a homebrew version for
simplicity and lower cost.

Random length, of course, is random. you COULD get lucky.
Or not ....

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

73 es GL de Jim, N2EY