On Mon, 22 May 2006 23:19:25 GMT, ml wrote:

hi

I was wondering a few things i read about in my sgc tuner manual about
dipoles


a) they say or sorta recomend that if your making a simple horiz ctr fed
dipole, it's 'best' to have one leg longer

This might be related to the sometimes heard myth / nonsense that a
half wave dipole does not need a 1:1 balun at the feedpoint, but that
one leg should be made a little longer than the formula length to
compensate for the feedline common mode current.

I don't know the source of this concept, but I hear it from time to
time from avid readers, usually introduced with "of course, as
everyone knows..." .

Owen
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writes:

wrote:
I expect there's not supposed to be any feedline between the antenna
and the tuner in this installation, so I think that it probably is to
moderate the impedance for a ham band antenna cut for the lowest band.

If I have a different length on the case of the tuner compared to the
length on the hot terminal, how does that moderate impedance?

May be they want the impedance on the second harmonic to be less
*exciting*.

If there is a length of lossy feedline between my resonant 80 m
antenna and the tuner, and I try tuning up on 40 m, I won't fry my
tuner. But if the tuner is at the feedpoint, the voltage gets extremely
high.

73
LA4RT Jon

I was thinking along the same lines as Jon, LA4RT when I posted my
comment... using an 80m dipole on 40m, maybe you want to move the
feedpoint away from the current node.

How about this. If you make one leg 5% longer than the other: The
length of leg 1 is L, the length of leg 2 is 1.05L. What do you need
to have an integer number of half wavelengths on each side of the
feedpoint? (This is the only time you can get a mid element current
node, right?)

L=m*lambda/2
1.05L=n*lambda/2

= 1.05(m*lambda/2)=n*lambda/2

The lowest integers that satisfy this: n/m = 105/100 = 21/20.

So until you hit 21 halfwaves on one side and 20 halfwaves on the
other, I think you don't have a current node at the feedpoint.

Is this desirable? I think it probably is, but I am not a tuner
expert.

73,
Dan

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just wanted to thank every for the help

In article ,
(Dave Platt) wrote:

snipped

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.


a note here, the SGC, SEA, and Motorola tuners were all deisgned to
feed unbalanced antennas. There were a few hardy souls who took two
SEA 1612B's and made one slave to the CPU of the other, so as to
make a balanced type tuner, with a bit of success, but the firmware
was never designed to do this and it was a hack to be sure.


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.

I have a few 1612B's that drive dipole antennas with vairous feed systems
scattered around Alaska, and they seem to funcntion at least as good as
an unbalanced Marconi type antenna in the same fixed location. The
whole point of trying this was to be able to ignore the RF Ground at
these fixed sites, which mostly were on Glacal Till, which is just
about the same as a sheet of Glass, as far as RF is concerened. When
I was first entertaining this idea, I spent a lot of time with Bill
Forgey and Mark Johnson @ SEA talking about the possibilites, and we
came to the conclusion that I would work, if one took the 50Ohm Feedline,
DC Power Lines, and Tuned Anunciator Lines and wound the Trifiler on
a big Torriod to isolate the tuner from the the radio.

There are a few that tried to use a 1:1 Balun across the output of the
tuner to ground, and then feeding a balanced feedline/antenna system,
with some sucess, but some of the better MF/HF Antenna Designers that
I consulted, were dubious if that approch was a real valid system.


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.


All of the autotuners of this type have serious limitations, especially
as they approch the 1/2Wave point of the physical antenna, where
impedances ramp up to infinite. All of them use the Phase, Frequency,
and Forward/Reverse Power, type sensers to let the cpu know what is
going on as it ripples thru the Fixed L and C in Binary Increments.
Some of the firmware actually has some smarts built in, but most
use the same basic algorythem the Sillbe wrote for the original Motorola
Micom Tuners.

Bruce in alaska who was around the BIZ, when all this was going on....
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add a 2 before @