In article .com,
wrote:

Maybe you should ask them why they want customers to do that. It sounds
like witchcraft to me.

Well, I'd probably call it "lore", as in "Everybody says that it seems
to work better that way."

I don't think it has anything to do with the actual RF radiation
behavior of the dipole antenna, once a match is achieved... it
shouldn't matter for this purpose which side of the dipole is longer.
I think it has everything to do with the tuner's ability to find a
match.

As I understand it (and this is all second- and third-hand knowledge):
the SGC tuners (like the SEA tuners from which they're said to have
been copied and like a lot of similar tuners from Motorola and others)
were designed primarily to match short verticals (whips and wires)
working against a much larger ground system. They're designed for
unbalanced applicatios. Their original application was marine and
vehicle use, with the tuner very robustly "bonded" to the metal
chassis of the vehicle. The manuals still make it clear that a large,
well-bonded ground/counterpoise system connected directly to the tuner
chassis is what these tuners really "want to see". The acceptable-SWR
range for a dipole to be matched is rather limited... IIRC the SGC-230
manual says that it's limited to 5:1 or so.

When used to center-feed a dipole, making the ground-side leg longer
than the hot-side leg increases the capacitance to ground on that side
and may tend to mimic the environment for which the tuners were really
designed.

As to _why_ they want to see it - my guess is that it has to do with
the details of the tuner's internal circuitry (it's an L or PI tuner),
the tuning-setting search algorithm in the microprocessor, and perhaps
the electrical details of the SWR-and-impedance measuring/bridge
circuit.

The manual comments that if the counterpoise system isn't
significantly bigger than the radiator, the tuner's microcontroller
may "become confused" and try to feed power to the ground system
rather than to the radiator. Yes, I know, this doesn't really makes
sense electrically... I suspect that it really means that the
matching-component search algorithm starts making decisions which
actually drive the system further away from a good match rather than
towards it. It's also possible that such installations are more prone
to high levels of RF current flow on the feedline from the
transmitter, and also to high common-mode RF flow on the power and
control lines, and that this RF might tend to confuse the tuner's
SWR-and-impedance sensors and thus disrupt the match-search process.

My own experiences with an SGC tuner seem to confirm the limitations
and warnings that SGC publishes about the hookup required to establish
a match. A couple of years ago I picked up a first-generation SGC 230
(apparently never used, as it was still in the original shipping box
and bag) at a hamfest for all of $30. I've tried it, and I _can_ get
it to work, but in my installation it's finicky in the extreme about
its ability to match a wire.

I've never had any success in getting it to match a dipole. It
doesn't do well at all when trying to match a longwire (even one of
what the manual says is an optimal length) when mounted on my house
wall... I suspect that the 8' of wire between the tuner case, and the
point at which I've bonded my counterpoise/radial/ground system, has
too much inductance.

It also seems to be sensitive to which radio it's used with. It
"likes" my old Ten-Tec Scout better than my new Kenwood TS-2000 -
it'll achieve a match for the former that it won't achieve for the
latter. I suspect that the problem is that the TS-2000's transmitter
has SWR-sensitive power foldback - the TS-2000 keeps changing its
transmit power as the SGC switches between different match-component
settings, and I believe that this is completely confusing the SGC's
search logic. Using the same tuner and antenna, on the same
frequency, with the Ten-Tec (which doesn't use an SWR-sensing power
reduction circuit), a match can often be achieved in a few seconds.

I've given up trying to use it for my home station, as I haven't been
able to put together a reliable installation with it. I'll keep it
around for field use - grounded to a vehicle or a big chain-link fence
or something like that, it may work well enough.

My conclusion is that these L/Pi long-wire tuners are really best
suited for the uses for which they were designed... matching a whip or
wire, fed against a Big Yellow Taxi or some other large mass of
sheetmetal or a really good radial system. For feeding a balanced
antenna, I think you may have better results with a tuner which is
explicitly designed for that sort of job.

Possibly SGC has improved the matching/searching behavior of their
tuners/couplers when used in balanced-antenna applications, in the
newer versions. The one I have is at least ten years old, I think -
so old that it's in a non-weatherproof aluminum chassis rather than a
weatherproof plastic one, and it doesn't have the "tune lock" feature
of the newer ones. Dunno for sure.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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