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

why is that 'a good idea' what happens that makes this sorta advisable


b) they recomend the longer leg to be the rf 'ground'' side , really
confused as to why, my gut would tell me to make the hot side the
longest leg so i am confused



can anyone help me understand the above, i kinda understand how the wave
looks on a regular horizontal ctr fed dipole is


any help appreciated

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

why is that 'a good idea' what happens that makes this sorta advisable


b) they recomend the longer leg to be the rf 'ground'' side , really
confused as to why, my gut would tell me to make the hot side the
longest leg so i am confused



can anyone help me understand the above, i kinda understand how the wave
looks on a regular horizontal ctr fed dipole is


any help appreciated



It is the same reason that when you build a standard 2 meter ground
plane antenna, that the ground plane is constructed with wires or rods
about 20.5 inches each. That is close to a quarter wave without
compensating for end effect. The radiating element is about 19.5 inches
which does take the end effect into account.

It will result in less RF on the outside of the coax.

In article ,
BKR wrote:

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

why is that 'a good idea' what happens that makes this sorta advisable


b) they recomend the longer leg to be the rf 'ground'' side , really
confused as to why, my gut would tell me to make the hot side the
longest leg so i am confused



can anyone help me understand the above, i kinda understand how the wave
looks on a regular horizontal ctr fed dipole is


any help appreciated



It is the same reason that when you build a standard 2 meter ground
plane antenna, that the ground plane is constructed with wires or rods
about 20.5 inches each. That is close to a quarter wave without
compensating for end effect. The radiating element is about 19.5 inches
which does take the end effect into account.

It will result in less RF on the outside of the coax.

ahh less rf on outer coax that i get glad i followed the directions!

thanks very much

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

why is that 'a good idea' what happens that makes this sorta advisable


b) they recomend the longer leg to be the rf 'ground'' side , really
confused as to why, my gut would tell me to make the hot side the
longest leg so i am confused



can anyone help me understand the above, i kinda understand how the wave
looks on a regular horizontal ctr fed dipole is


That suggestion does not make very much sense.

First, a ground isn't a "better ground" just because it is longer. The
lowest impedance and "best" ground or counterpoise occurs when the
ground is exactly resonant.

Second, the least common mode current is influenced heavily by the
length of the feeder, and is often the least when a dipole antenna is
evenly balanced.

The only possible reason for that is to moderate the impedance
presented by the antenna, but again that would also take special
consideration of the feedline length.

Even a 1/4 wl groundplane does not improve from radials longer than 1/4
wl. As a matter of fact it often gets worse.

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

73 Tom

Tom,

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.


It is witchcraft, but if you're selling antenna tuners to people who
don't know anything about the impedance issues with an all band wire,
it's better to give them a rule of thumb to avoid cooking the tuner,
especially when they don't read the bit in the manual where it's a
"100W" tuner on SSB and they fire up on RTTY.

BKR, where did you get that explanation?

I think you should measure the RF on the outside of your coax when it's
an odd multiple of a quarter wave long including the loading effect of
the case of the radio and the ground half of your dipole is made 5%
longer than the other half.

Listen to Tom. He knows common-mode current on the feedline!

Dan
N3OX

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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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.


It is probably because those tuners do not do well with a resonant
antenna. With one legg longer than the other there is more chance
that the antenna will be reactive and make the tuner happier.

73
Gary K4FMX

Gary Schafer wrote:

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.


It is probably because those tuners do not do well with a resonant
antenna. With one legg longer than the other there is more chance
that the antenna will be reactive and make the tuner happier.

73
Gary K4FMX

Maybe with dipoles, but my SGC has no problems with resonance on my
vertical.

FWIW, the vertical is resonant by itself on 40M and has a relay
switched loading coil for 80M I put in before I got the SGC.

With the loading coil out, the SGC has no problems 80M to 10M.

With the loading coil in, 160M to 20M.

Normal operation is loading coil in for 160M and 80M.

--
Jim Pennino

Remove .spam.sux to reply.

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?

It is the selction of length combinations of antennas and of course
length and impedance of feedline that has to be considered.

Since everything considered here so far hasn't been logical, it would
be interesting to find out what SGC actually intends.

73 Tom

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?


Assuming you're speaking of a resonant dipole or verticle, it would
seem to me that by doing that you are moving the feedpoint away from the 50
ohm (70 ohm?) feedpoint at the center.... and probably towards a higher
impedance feedpoint. Not sure what effect that would have on those
autotuners, but perhaps they work better with a bit higher impedance than
50 ohms?




Ed K7AAT