Steve Bonine posted on Wed, 26 Sep 2007 10:12:27 EDT

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

I guess I'm lucky. I've never had a problem matching a random-length
dipole on any frequency I've tried. That does not imply, of course,
that any random length can be successfully used on any arbitrary frequency.

And of course, "random length" should have read "as long as possible,
given the placement of the trees."

Thanks to everyone who chimed in on using an external balum and an
automatic tuner. My gut told me that the consensus ("it won't work")
was correct, but it was nice to get some more factual backup for that.

Steve, the factual backup on antenna matching has abounded
in texts, mostly the 'pro' kind, for years. It's been used for years
to impedance-match all kinds of things within a radio box...as well
as outside it. As to amateur equipment, the subject gets colored
(and both glamorized and defamed) by the lack of comparisons
to other matching equipment and the affinity that some have for
certain brands from certain manufacturers.

If you wish, I can send you a copy of the L-section matching math
that I've previously sent to Mike Coslo in e-mail. It isn't "formal"
but it is accurate, but it does involve simple algebra. The L-
sections are used in most of the automatic antenna tuners
because it is simple (and therefore low-cost) and adapts to the
measure-and-change L- or C- component algorithms that fit into
small microprocessor programs.

The heart of all of them is the Bruene RF voltage and current
detector that senses the phases of each at the load end. [or
variations on that 1955-beginning detector] The micro then
determines which parts of the L- or C-components are to be
switched in or out to get close to the ideal in-phase E & I of
RF for most power transfer.

Now the designer-manufacturers don't make auto-tuners that
will match ANYTHING...even though it CAN be done. To reduce
manufacturing costs they limit the number of internal inductors
and capacitors and THAT will reduce the ability to auto-match
anything. They are trying to be competitive on price. The newer
transceivers have SOME internal auto-tuning capability but they
clearly state the limits of their equipment. Not all separate auto-
tuners specify that. [I have both just as a backup]

"Baluns" aren't all perfect, either. They are good but just not
perfect. Some are better than others but it would take ALL of
them and some good lab test equipment to do a good
comparison. However, MOST work well enough for amateur
radio purposes and do allow for balanced-to-unbalanced line
conversion at HF.

Now ANY impedance-matching tuner will let one load up just
about anything. All that serves is to transfer the most RF power
into a load. What is NOT known is WHERE all that RF is going.
Unless some ham has a balloon-borne sensor and data transfer
gizmo, NOBODY can know just where the pattern is going to be.
Big trees WILL affect the pattern, especially changing it between
dry and wet climate times and between different kinds of trees.
So will structures and assorted conductive things (aluminum
patio covers, small garden sheds, power, phone, and TV cables)
all within the near-field (within five or so wavelengths). Even
some houses which have had aluminum siding added on
compared to similar houses with just wood or stucco or brick
siding.

One can take an example of the U.S. Army's little AN/PRC-104
backpack transceiver. It covers the whole of HF using a whip
antenna. It has had an auto-tuner built-in since it went
operational in 1986. A human bean is a poor counterpoise for
any antenna with HF wavelengths and soldiers aren't all "built
to spec" for that purpose. Further, the transceiver and whip must
operate from unknown field environments, in trees or well away
from them, in swampy soil or dry desert. The transceiver can't
get any higher than the soldier carrying it. But, the little built-
in antenna tuner assures him that the whip antenna is going to
get as much RF power into/out-of it as possible. The rest of it
is trying to keep the whip as vertical as possible while in-use.

Now a PRC-104 won't win any DX awards or enable contacts
with Antarctica or Yurp, but it is a case-in-point where an auto-
tuner certainly helps maximize signals in a 1:10 frequency
range with a practical-minimal fixed antenna, allowing for a
highly-variaable counterpoise/ground-plane environment. The
vertical whip will probably maximize its pattern between 10
and 40 degrees above horizontal, give-or-take. It works in
practice (for the equivalent of QRP amateur-style). Works
well enough, that is. The auto-tuner built-in certainly helps
it.

Everyone's residential location varies greatly and only a very
few are "perfect" (as to the antenna analyzer programs). One
can load up practically anything with a tuner but only the shape
and arrangement of conductive elements is going to determine
where most of the RF goes to (or comes from). No tuner can
help that.

73, Len AF6AY