SGC coupler to Dipole feedling question
 

Our ARES group plans on installing an Inverted V antenna on the
second story flat roof edge of a local building. The antenna mast is 13
feet tall above the roof edge. The Inverted V will run parallel the edge
of the roof and be approximately 35 - 40 feet per leg. Our primary
operations will be 80/75/40M with a desired ability on 60M. The
building custodian/owner will not tolerate open wire feedline with its
associated standoffs due to aesthetic considerations, so we must feed
this antenna with coax fastened to the mast. At the base of the mast,
on the roof, we will be using an SGC-237 antenna coupler.

The above setup is a given, with no room for compromise.

My questions for this group are as follows:

Would we be better feeding the above antenna feedpoint with twin coax
runs, using the center conductors as a 'balanced' feedline, or would we
be better of using a single coax to the feedline? In either case, the
coax runs will not exceed 20 feet and we must accept the losses in them.
Email response from SGC seems to indicate we would be better off with a
single feedline, but I am dubious about the SGC Tech Rep's response
since he/she does not seem concerned about feedline radiation.

Also, what recomendations do you guys have for use of a balun? I
believe, at the least, we would need a 1:1 balun at the Input of the SGC
coupler so as to keep RF from getting back down the shield and into the
building. SGC response seems to indiate they don't think a balun is
necessary anywhere, which is another reason I am not thrilled with
their response.

Comments?


Ed K7AAT

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in message
. 192.196...

Our ARES group plans on installing an Inverted V antenna on the
second story flat roof edge of a local building. The antenna mast is 13
feet tall above the roof edge. The Inverted V will run parallel the edge
of the roof and be approximately 35 - 40 feet per leg. Our primary
operations will be 80/75/40M with a desired ability on 60M. The

I think you are going to have a difficult time operating on 80 meters if you
can not get around 120 feet of overall length. Then it may be difficult on
40 meters. If limiated to one single wire , coax fed, look at the off
center fed antenna.

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in message
. 192.196...

Our ARES group plans on installing an Inverted V antenna on the
second story flat roof edge of a local building. The antenna mast is 13
feet tall above the roof edge. The Inverted V will run parallel the edge
of the roof and be approximately 35 - 40 feet per leg. Our primary
operations will be 80/75/40M with a desired ability on 60M. The
building custodian/owner will not tolerate open wire feedline with its
associated standoffs due to aesthetic considerations, so we must feed
this antenna with coax fastened to the mast. At the base of the mast,
on the roof, we will be using an SGC-237 antenna coupler.

The above setup is a given, with no room for compromise.

My questions for this group are as follows:

Would we be better feeding the above antenna feedpoint with twin coax
runs, using the center conductors as a 'balanced' feedline, or would we
be better of using a single coax to the feedline? In either case, the
coax runs will not exceed 20 feet and we must accept the losses in them.
Email response from SGC seems to indicate we would be better off with a
single feedline, but I am dubious about the SGC Tech Rep's response
since he/she does not seem concerned about feedline radiation.

Also, what recomendations do you guys have for use of a balun? I
believe, at the least, we would need a 1:1 balun at the Input of the SGC
coupler so as to keep RF from getting back down the shield and into the
building. SGC response seems to indiate they don't think a balun is
necessary anywhere, which is another reason I am not thrilled with
their response.

Comments?


Ed K7AAT

If the SGC-237 coupler is on the roof and properly grounded, try it the way
the SGC rep suggested. Any feedline radiation should occur from the antenna
side of the coupler and might help with getting a bit more signal out. Go
with the simplest solution first and run a single coax to the coupler from
the transceiver. Maybe it will work just fine.

If there are problems with RF on the coax outer, add some ferrite beads over
the coax to form an RF choke.

Look at more complex solutions when you have actually identified a problem
rather than worrying in advance over something that probably won't happen.

Mike G0ULI

SGC coupler to Dipole feedling question
 

On 29 Feb 2008 00:51:16 GMT, "Ed_G"
wrote:


Our ARES group plans on installing an Inverted V antenna on the
second story flat roof edge of a local building. The antenna mast is 13
feet tall above the roof edge. The Inverted V will run parallel the edge
of the roof and be approximately 35 - 40 feet per leg. Our primary
operations will be 80/75/40M with a desired ability on 60M. The
building custodian/owner will not tolerate open wire feedline with its
associated standoffs due to aesthetic considerations, so we must feed
this antenna with coax fastened to the mast. At the base of the mast,
on the roof, we will be using an SGC-237 antenna coupler.

The above setup is a given, with no room for compromise.

My questions for this group are as follows:

Would we be better feeding the above antenna feedpoint with twin coax
runs, using the center conductors as a 'balanced' feedline, or would we
be better of using a single coax to the feedline? In either case, the
coax runs will not exceed 20 feet and we must accept the losses in them.
Email response from SGC seems to indicate we would be better off with a
single feedline, but I am dubious about the SGC Tech Rep's response
since he/she does not seem concerned about feedline radiation.

Also, what recomendations do you guys have for use of a balun? I
believe, at the least, we would need a 1:1 balun at the Input of the SGC
coupler so as to keep RF from getting back down the shield and into the
building. SGC response seems to indiate they don't think a balun is
necessary anywhere, which is another reason I am not thrilled with
their response.

Comments?


Ed K7AAT
My SGC-237 is hard to mess up. Where ever you attach the wire to the
tuner is the beginning of the antenna. The coax attach will simply be
a matched line to the transceiver. I would be inclined to simply
attach the coax to the inverted V as you outlined and use it.

The antenna is in an environment that will not model well. The
radiation from the coax will have an effect on the aggregate
performance but nothing you can really measure.

Although I doubt anyone can explain just how it works, the SGC-237 and
the wire you have described will work fine.

Modern antenna tuners perform a lot like Magic in my estimation.

My own feeble experiments have led me to believe that it is worthwhile
to put an antenna analyzer on the configuration and make sure that the
array is NOT resonant on any frequency of interest. The tuner seems to
like that best.

Power supply: I have a very old telephone power supply tweaked down to
12 volts. I leave it on 24/7.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

SGC coupler to Dipole feedling question
 

My SGC-237 is hard to mess up. Where ever you attach the wire to the
tuner is the beginning of the antenna. The coax attach will simply be
a matched line to the transceiver. I would be inclined to simply
attach the coax to the inverted V as you outlined and use it.

The antenna is in an environment that will not model well. The
radiation from the coax will have an effect on the aggregate
performance but nothing you can really measure.

Although I doubt anyone can explain just how it works, the SGC-237 and
the wire you have described will work fine.

Modern antenna tuners perform a lot like Magic in my estimation.

My own feeble experiments have led me to believe that it is worthwhile
to put an antenna analyzer on the configuration and make sure that the
array is NOT resonant on any frequency of interest. The tuner seems to
like that best.

Power supply: I have a very old telephone power supply tweaked down to
12 volts. I leave it on 24/7.


Thanks for the feedback, John. I imagine that we will end up doing
as you suggested. We WILL test the antenna on the ground with a
temporary mast to see if there are any issues, but we also realize that
things can, and probably will, change when it is permanently mounted on
the building. One of the reasons I am posing these questions here now
is that once the antenna is up, it will be difficult to get the
building personal ( its a firehouse ) to lower the mast for changes.
Its a rigid one piece aluminum mast that will be bolted at its base to
the building.

73

Ed K7AAT

SGC coupler to Dipole feedling question
 

Ed_G wrote:
The above setup is a given, with no room for compromise.

Your antenna is already quite compromised with a
~490:1 SWR on 75m at the antenna feedpoint. The line
loss with RG-213 is about 4.5 dB and the tuner is
required to match 0.55-j48 ohms, another lossy
situation. Your antenna system efficiency may be ~10%.

How about turning your 13 foot pole into a radiator
and using the dipole wires for a top hat?
--
73, Cecil http://www.w5dxp.com

SGC coupler to Dipole feedling question
 

Your antenna is already quite compromised with a
~490:1 SWR on 75m at the antenna feedpoint. The line
loss with RG-213 is about 4.5 dB and the tuner is
required to match 0.55-j48 ohms, another lossy
situation. Your antenna system efficiency may be ~10%.

How about turning your 13 foot pole into a radiator
and using the dipole wires for a top hat?


The aluminum 2" mast will be side mounted on the wood building...
about 5 feet below the edge of the roof. The remaining 14 or so feet
will be above the roof. With two 35 foot wires off the top, do you
really think that would work to our needs better? There is one
problem with this setup, though.... I do not think there is a suitable
ground on the building roof. I suppose it is possible to run a simple
wire straight down from the area near the bottom of the mast to the
ground..... the SGC-237 would still have to sit on the roof and be
connected to this now lowered feedpoint.....

I think it would be easier to extend the length of our inverted V
legs. I had posted their length between 35 and 40 feet, but if we run
out of building to extend the ends to, I suppose we could go on
straight down from the end mounts, toward the ground a bit. Even an
additional 10 feet could be had on each leg with the new ends being
still 10 feet above the ground. It IS a wooden structure, for the most
part.

Ed

SGC coupler to Dipole feedling question
 

John Ferrell wrote in
:

My SGC-237 is hard to mess up. Where ever you attach the wire to the
tuner is the beginning of the antenna. The coax attach will simply be
a matched line to the transceiver. I would be inclined to simply
attach the coax to the inverted V as you outlined and use it.


John,

Any conductors or transmission lines carrying a net current (or common
mode current) will contribute to radiation. If your meaning of "antenna"
is something that contributes to radiation, then the feedline on both
sides of the ATU might be part of the "antenna" no matter what you might
declare.

If Ed connects parallel line from the centre of the dipole to the hot and
common terminals of the ATU, there is likely to be common mode current on
the parallel line adjacent to the ATU. If the only connection on the tx
side of the ATU is the coax, then it will also have a common mode current
adjacent to the ATU and near enough to equal to the common mode current
on the other side of the ATU.

That is not to say it won't work. People regularly build antennas with
radiating feedlines, some (mainly commercial interests) even call that
out as a significant advantage.

Nevertheless, there are downsides and some measures to minimise the
common mode current on the feedline may be warranted... or even
necessitated down the track.

Ed has been deliberating over a solution to this problem for a long time.
It has been discussed to some depth!

Owen

SGC coupler to Dipole feedling question
 

Ed_G wrote:
There is one
problem with this setup, though.... I do not think there is a suitable
ground on the building roof.

I was thinking you could lay some radials on the roof
to obtain your ground plane. You could also insulate
the pole from the mounts using sections of PVC pipe.

I think it would be easier to extend the length of our inverted V
legs. I had posted their length between 35 and 40 feet, but if we run
out of building to extend the ends to, I suppose we could go on
straight down from the end mounts, toward the ground a bit.

I had assumed that your "no compromise" statement included
antenna length. A full-sized dipole would certainly solve
your 75m problem but then the full-wave dipole would be
mostly non-functional on 40m when fed with coax.

Here's another idea. *Change the pole support to fiberglass*,
use heavy duty 300 ohm balanced line, and run the 300 ohm
feedline inside the fiberglass pole to the dipole. You could
use the G5RV length of 51 feet per dipole element. With such
a configuration, the feedpoint impedance on 3.8 MHz would be
about 20-j130 ohms and on 7.2 MHz would be about 100-j450 ohms
which the SGC would have no trouble matching. This could
multiply your system efficiency by maybe 6-8.
--
73, Cecil http://www.w5dxp.com

SGC coupler to Dipole feedling question
 

Antenna tuners 'like' antennas that are 'too long' better than
antennas that are 'too short'. That doesn't say anything about how
well that 'too long'/'too short' antenna will perform, just that the
average tuner will find it easier to deal with one that's 'too long'.
(Easier to 'cram' more capacitance into a tuner than inductance.) If
this antenna is a doublet, 'balanced', why would you need a
'groundplane'? And while I'd guess that it'll never be 'ideal', the
building is acting as a 'groundplane' anyway, sort of.
- 'Doc

(What am I not understanding about the situation?)

SGC coupler to Dipole feedling question
 

Antenna tuners 'like' antennas that are 'too long' better than
antennas that are 'too short'. That doesn't say anything about how
well that 'too long'/'too short' antenna will perform, just that the
average tuner will find it easier to deal with one that's 'too long'.
(Easier to 'cram' more capacitance into a tuner than inductance.) If
this antenna is a doublet, 'balanced', why would you need a
'groundplane'? And while I'd guess that it'll never be 'ideal', the
building is acting as a 'groundplane' anyway, sort of.
- 'Doc

(What am I not understanding about the situation?)


I don't think you are missing anything; your comments sound
reasonable to me. The building is the nearly new local firehouse HQ.
The Chief and staff have outlined very strict parameters for our antenna
location and setup. They fabricated the aluminum mast and will install
it on the side of the building per their own design. Even at that, they
gritted their teeth at the visual impact it has. No stand-off
supported open ladder line for us! We must work with this and I don't
see any feed alternative than coax. The only thing we 'may' have some
flexibility with is the length of the antenna. ( Sorry Cecil, I guess
I should have mentioned that up front ). We are limited to the length
of the side of the building, which I estimated would give us up to 40
feet of element from the top of the mast. However, in hindsignt, I
believe we could at that end point drop some additional length down off
the end for a longer antenna.... would probably give us better operation
on 75 & 80.


Ed

SGC coupler to Dipole feedling question
 

In article . 196,
"Ed_G" wrote:

we will be using an SGC-237 antenna coupler.

Comments?
Ed K7AAT

Ok, first thing, NONE of the Lumped Constant Binary Switch Tuners like
SGC's Knockoff of the SEA Design, will tune ANYWHERE within a few
percent of the Natural Halfwave Point of the antenna. So you must design
the Antenna, so as to move that Halfwave Point to a section of the
HF Spectrum that you NEVER plan on using. Second thing, none of the SGC
Employees around today, were around when this Tuner and it's Operating
software was designed, and most don't have much experience with actual
operational considerations.
Thirdly, After a lot of experimentation with the SEA1612b Series Tuners,
from which the SGC's were plagiarized, when using them to drive a
dipole, there are two schools of thought. One school says that you
should add a 1:1 Balun on the output of the tuner between the RF Ground
Stud, and RF Output Connection to make your Balanced Feed. Second school
says to take the Feedline Coax, Dc Power Lines, and Tuning Feedback
Wire, and wind them, in a Bifilar fashion on an appropriate Torriod
to decouple the Tuner from Radio Feed and connect the dipole to the
RF Ground Stud, and the RF Output Connection.
I have used both Systems on Maritime Mobile Limited Coast Stations,
around Alaska, and find that they both work about Equally Poor. It
should be noted here, however that Alaska is notorious for not having
any kind of decent RF Grounding Soil, so usually this type of Antenna
System works much better than an type of Longwire antenna that needs a
good RF Ground, to work against. Where a GOOD RF Ground is available,
(Salt Water) the Longwire Antenna outperforms the Tuner Driven Dipole,
ever time, but without that GOOD RF Ground, the Tuner Driven Dipole
works better than just about anything else, especially over a wide range
of Frequency Bands available, in both the Maritime and Amateur Radio HF
spectrums.

Bruce in alaska

--
Bruce in alaska
add path after fast to reply

SGC coupler to Dipole feedling question
 

Ed_G wrote:
They fabricated the aluminum mast and will install
it on the side of the building per their own design.

Get them to fabricate a piece of fiberglass tubing
and all your problems will disappear.
--
73, Cecil http://www.w5dxp.com

SGC coupler to Dipole feedling question
 

Get them to fabricate a piece of fiberglass tubing
and all your problems will disappear.



I fully agree, but we have absolutely no input in regards to the mast
they have provided, and to attempt to persuade them otherwise would only
jeopardize the situaion, so we must accept what they have provided and work
with it.


Ed

SGC coupler to Dipole feedling question
 

Ed_G wrote:
Get them to fabricate a piece of fiberglass tubing
and all your problems will disappear.

I fully agree, but we have absolutely no input in regards to the mast
they have provided, and to attempt to persuade them otherwise would only
jeopardize the situaion, so we must accept what they have provided and work
with it.

There is someone, somewhere, who has the authority to
change the mast from aluminum to fiberglass for the
purpose of multiplying your radiation efficiency by
a factor of 7. I would seek that person out and bend
his/her ear.
--
73, Cecil http://www.w5dxp.com

SGC coupler to Dipole feedling question
 

There is someone, somewhere, who has the authority to
change the mast from aluminum to fiberglass for the
purpose of multiplying your radiation efficiency by
a factor of 7. I would seek that person out and bend
his/her ear.

Believe me, Cecil, that is not the case here. This is a very
small community. The Fire Chief and his Deputy have ultimate authority
over this building. Besides, to attempt to go over his head would
merely jeopardize the good relations we already have which allowed us to
get this far anyway. We'll leave well enough alone and work with what
we have. If it weren't for the fact that we need operation on 3589 &
3980, ~7090 & 7248, and hopefully 60M, we'd probably have just gone with
a trapped wire antenna off this mast instead of using the SGC-237 (
which we already possess).



Thanks.

Ed

SGC coupler to Dipole feedling question
 

Ok, first thing, NONE of the Lumped Constant Binary Switch Tuners like
SGC's Knockoff of the SEA Design, will tune ANYWHERE within a few
percent of the Natural Halfwave Point of the antenna. So you must
design
the Antenna, so as to move that Halfwave Point to a section of the
HF Spectrum that you NEVER plan on using. Second thing, none of the
SGC
Employees around today, were around when this Tuner and it's Operating
software was designed, and most don't have much experience with actual
operational considerations.
Thirdly, After a lot of experimentation with the SEA1612b Series
Tuners,
from which the SGC's were plagiarized, when using them to drive a
dipole, there are two schools of thought. One school says that you
should add a 1:1 Balun on the output of the tuner between the RF
Ground
Stud, and RF Output Connection to make your Balanced Feed. Second
school
says to take the Feedline Coax, Dc Power Lines, and Tuning Feedback
Wire, and wind them, in a Bifilar fashion on an appropriate Torriod
to decouple the Tuner from Radio Feed and connect the dipole to the
RF Ground Stud, and the RF Output Connection.
I have used both Systems on Maritime Mobile Limited Coast Stations,
around Alaska, and find that they both work about Equally Poor. It
should be noted here, however that Alaska is notorious for not having
any kind of decent RF Grounding Soil, so usually this type of Antenna
System works much better than an type of Longwire antenna that needs a
good RF Ground, to work against. Where a GOOD RF Ground is available,
(Salt Water) the Longwire Antenna outperforms the Tuner Driven Dipole,
ever time, but without that GOOD RF Ground, the Tuner Driven Dipole
works better than just about anything else, especially over a wide
range
of Frequency Bands available, in both the Maritime and Amateur Radio
HF
spectrums.

Bruce in alaska


Bruce,

Thanks for the response to my query. I have a couple comments to
these. One, we already own a new SGC-237 coupler, so that is what we
will use. Two, thanks for the info on the SGC personnel.... that
might explain why some of their responses to my emails didn't seem to
make sense... especially their comments about NOT needing any balun.
Three, and more direct to my initial questions, it appears we will be
using coax between the coupler output and the antenna feedpoint.... up
to 20 feet of low loss... we'll just eat the loss. But more
importantly, I had intended, and will proceed along the lines of your
comments on baluns, by installing a 1:1 balun at the input to the
tuner. This will, at least, keep the RF out of the building and our
station location.

thanks.

Ed

SGC coupler to Dipole feedling question
 

Ed_G wrote:
There is someone, somewhere, who has the authority to
change the mast from aluminum to fiberglass for the
purpose of multiplying your radiation efficiency by
a factor of 7. I would seek that person out and bend
his/her ear.

Believe me, Cecil, that is not the case here. This is a very
small community. The Fire Chief and his Deputy have ultimate authority
over this building.

If the goal of those guys is to sabotage amateur
radio communications, they have probably succeeded.
It's like them going to fight a fire with big holes
in the hoses.
--
73, Cecil http://www.w5dxp.com

SGC coupler to Dipole feedling question
 

Thanks for the feedback, John. I imagine that we will end up doing
as you suggested. We WILL test the antenna on the ground with a
temporary mast to see if there are any issues, but we also realize that
things can, and probably will, change when it is permanently mounted on
the building. One of the reasons I am posing these questions here now
is that once the antenna is up, it will be difficult to get the
building personnel ( its a firehouse ) to lower the mast for changes.
Its a rigid one piece aluminum mast that will be bolted at its base to
the building.
===============================
Suggest you fit a pulley at the top of the mast such you can lower and
subsequently change the antenna using a halyard.

Frank GM0CSZ / KN6WH

SGC coupler to Dipole feedling question
 

In article . 196,
"Ed_G" wrote:


Bruce,

Thanks for the response to my query. I have a couple comments to
these. One, we already own a new SGC-237 coupler, so that is what we
will use. Two, thanks for the info on the SGC personnel.... that
might explain why some of their responses to my emails didn't seem to
make sense... especially their comments about NOT needing any balun.
Three, and more direct to my initial questions, it appears we will be
using coax between the coupler output and the antenna feedpoint.... up
to 20 feet of low loss... we'll just eat the loss. But more
importantly, I had intended, and will proceed along the lines of your
comments on baluns, by installing a 1:1 balun at the input to the
tuner. This will, at least, keep the RF out of the building and our
station location.

thanks.

Ed


Ed, you would be Much Better Off, if they would allow you to use (2)
runs of Coax, side by side up the mast, and connect only the Center
Conductor of each, to the tuner, with the shield left open on each end
and sealed against water intrusion. Even if taped to the aluminum mast.
This would provide a much better situation than a single coax feed.
Also if you are stuck with a single coax, then make SURE, that the
shield side of the coax is connected to the Ground Stud of the tuner.

--
Bruce in alaska
add path after fast to reply

SGC coupler to Dipole feedling question
 

Ed, you would be Much Better Off, if they would allow you to use (2)
runs of Coax, side by side up the mast, and connect only the Center
Conductor of each, to the tuner, with the shield left open on each end
and sealed against water intrusion. Even if taped to the aluminum
mast.
This would provide a much better situation than a single coax feed.
Also if you are stuck with a single coax, then make SURE, that the
shield side of the coax is connected to the Ground Stud of the tuner.



Well! That was part of my original question in this thread! I had
intended to do just that, but some here seemed to steer me toward a
single coax, including SGC personnel. I do NOT think we have any
decent ground at all available anywhere near the roof location where
this antenna mast will be located, so I was unsure if the twin coax
feed would be OK without grounding the shields. Left floating, I
don't know what effect, if any, we will have for incidental radiation.
One thing for sure, it won't be any worse than using a single coax with
no balun at the feedpoint.


Ed

SGC coupler to Dipole feedling question
 

If the goal of those guys is to sabotage amateur
radio communications, they have probably succeeded.
It's like them going to fight a fire with big holes
in the hoses.


It the goal was to sabotage our communications, they would have denied
permission to use their facilities totally, let alone, not have purchased
and fabricated the nice aluminum mast which they did for us. Admittedly,
they seem overly concerned with the visual impact of things, but beggars
can't be choosers! Tnx.

Ed

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in message
. 192.196...

If the goal of those guys is to sabotage amateur
radio communications, they have probably succeeded.
It's like them going to fight a fire with big holes
in the hoses.


It the goal was to sabotage our communications, they would have denied
permission to use their facilities totally, let alone, not have purchased
and fabricated the nice aluminum mast which they did for us. Admittedly,
they seem overly concerned with the visual impact of things, but beggars
can't be choosers! Tnx.

Ed


Right off I would say they are the beggars. It is your group that is
supporting them.

SGC coupler to Dipole feedling question
 

Bruce in alaska wrote in news:fast-25B5D6.09195501032008
@netnews.worldnet.att.net:

....
Ed, you would be Much Better Off, if they would allow you to use (2)
runs of Coax, side by side up the mast, and connect only the Center
Conductor of each, to the tuner, with the shield left open on each end
and sealed against water intrusion.

What does this do, what does it achieve?

Owen

SGC coupler to Dipole feedling question
 

Ed, you would be Much Better Off, if they would allow you to use (2)
runs of Coax, side by side up the mast, and connect only the Center
Conductor of each, to the tuner, with the shield left open on each
end and sealed against water intrusion.

What does this do, what does it achieve?

Owen



My expertise is weak in this area, but just guessing.... using twin
coax in the above configuration, if the shields were grounded, would
allow the feedling between the antenna coupler and the feedpoint to be
'balanced' and yet the shields would not radiate as they would with a
single coax run.

Perhaps others, here, will either expand on this, or correct my
misconception.

Ed K7AAT

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in
.91:


Ed, you would be Much Better Off, if they would allow you to use (2)
runs of Coax, side by side up the mast, and connect only the Center
Conductor of each, to the tuner, with the shield left open on each
end and sealed against water intrusion.

What does this do, what does it achieve?

Owen



My expertise is weak in this area, but just guessing.... using
twin
coax in the above configuration, if the shields were grounded, would

But that is not what was said. I read it to say "with the shield left
open on each end".

allow the feedling between the antenna coupler and the feedpoint to be
'balanced' and yet the shields would not radiate as they would with a
single coax run.

Bunk. The only reason the shields would not radiate would be if they
carried equal but opposite currents. That is most unlikely in this case.

Let us just consider a simple example. Assumption is that skin effect on
the coax is fully effective, a reasonable assumption at HF.

Make a quarter wave vertical of a piece of RG213 supported on sky hooks.
Make no connection to the shield at either end, and connect the feed line
between a ground plane / counterpoise / whatever and the centre conductor
of the vertical piece of coax.

What current flows on the outside surface of the vertical coax?

The current on the outside surface of the vertical coax adjacent to the
bottom end of the isolated shield is the same as the current flowing on
the inner conductor adjacent to the same end of the shield.

Does the outer conductor 'shield' the vertical so that it will not
radiate?

No, the outside surface of the shield is the radiatior, it just has a
quarter wave o/c stub in series from the feedline to the radiating
element.



Perhaps others, here, will either expand on this, or correct my
misconception.


Owen

SGC coupler to Dipole feedling question
 

Ed_G wrote:
My expertise is weak in this area, but just guessing.... using twin
coax in the above configuration, if the shields were grounded, would
allow the feedline between the antenna coupler and the feedpoint to be
'balanced' and yet the shields would not radiate as they would with a
single coax run.

Side-by-side runs of RG-11 (Z0=150 ohms) would reduce
the SWR on the coax and change the impedance seen by
the tuner from 0.3-j69 ohms to 1.9-j154 ohms which is
quite an improvement. You can increase that feedpoint
impedance even more to 2.5-j170 ohms by using side-by-
side runs of RG-133 (Z0=190 ohms) if you can find it.
--
73, Cecil http://www.w5dxp.com

SGC coupler to Dipole feedling question
 

Owen Duffy wrote:
Does the outer conductor 'shield' the vertical so that it will not
radiate?

No, the outside surface of the shield is the radiatior, it just has a
quarter wave o/c stub in series from the feedline to the radiating
element.

Owen, feedline radiation in this particular system might
increase the efficiency which would be a good thing.
--
73, Cecil http://www.w5dxp.com

SGC coupler to Dipole feedling question
 

Cecil Moore wrote:
Ed_G wrote:
My expertise is weak in this area, but just guessing.... using twin
coax in the above configuration, if the shields were grounded, would
allow the feedline between the antenna coupler and the feedpoint to be
'balanced' and yet the shields would not radiate as they would with a
single coax run.

Side-by-side runs of RG-11 (Z0=150 ohms) would reduce
the SWR on the coax and change the impedance seen by
the tuner from 0.3-j69 ohms to 1.9-j154 ohms which is
quite an improvement. You can increase that feedpoint
impedance even more to 2.5-j170 ohms by using side-by-
side runs of RG-133 (Z0=190 ohms) if you can find it.

I forgot to say that these figures are from EZNEC which
assumes lossless feedlines. Real-world impedances would
be much higher due to coax losses.
--
73, Cecil http://www.w5dxp.com

SGC coupler to Dipole feedling question
 

Bunk. The only reason the shields would not radiate would be if they
carried equal but opposite currents. That is most unlikely in this
case.



If both shields, ( ungrounded ) are tied together, and the two
center conductors are acting as a 'balanced' feedline, how can current
flow on the outsides of the shields, if the interior currents of the two
center conductors are always 180 out of phase?


Ed

SGC coupler to Dipole feedling question
 

In article . 196,
"Ed_G" wrote:


Ed, you would be Much Better Off, if they would allow you to use (2)
runs of Coax, side by side up the mast, and connect only the Center
Conductor of each, to the tuner, with the shield left open on each end
and sealed against water intrusion. Even if taped to the aluminum
mast.
This would provide a much better situation than a single coax feed.
Also if you are stuck with a single coax, then make SURE, that the
shield side of the coax is connected to the Ground Stud of the tuner.



Well! That was part of my original question in this thread! I had
intended to do just that, but some here seemed to steer me toward a
single coax, including SGC personnel. I do NOT think we have any
decent ground at all available anywhere near the roof location where
this antenna mast will be located, so I was unsure if the twin coax
feed would be OK without grounding the shields. Left floating, I
don't know what effect, if any, we will have for incidental radiation.
One thing for sure, it won't be any worse than using a single coax with
no balun at the feedpoint.


Ed

"Left Floating" so that there is less Capacitance to RF Ground, and more
distance between the vertical parallel Feedlines. It would even help
if you can use twin Coax Runs, to put them on opposite sides of the
Aluminum Mast, which would give you more separation with the same
Coupling Capacitance to the Mast. Capacitive coupling to RF Ground,
is the Killer here, and you MUST reduce that, as much as possible,
if your system is going to have any chance at reasonable operation.

--
Bruce in alaska
add path after fast to reply

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in
. 192.196:


Bunk. The only reason the shields would not radiate would be if they
carried equal but opposite currents. That is most unlikely in this
case.



If both shields, ( ungrounded ) are tied together, and the two

At both ends?

This is the first mention of shields tied together, I certainly didn't
read that into Bruce's "with the shield left open on each end
".

center conductors are acting as a 'balanced' feedline, how can
current flow on the outsides of the shields, if the interior currents
of the two center conductors are always 180 out of phase?

The analysis in this case is different, but if I understand your
scenario, the outer surface of the two coaxes which are tied together at
both ends but connected to nothing else still carries the common mode
current. that exists on the two open wire conductors just prior to
entering the coax assembly. No, you cannot guarantee that those currents
are equal and opposite, ie that there is no common mode current, and the
common mode current will flow entirely on the outside surface of the
outer conductors of the coax assembly when connected as you now propose.

Bruce hasn't explained what his configuration is supposed to do, so we
are still guessing about that one.

There is no answer to this problem, because the problem is ill defined.
You have just added a new element in tying the shields together. Other
questions exist like what other connections exist between tx feed line,
ATU, ant feed line, mast, roofing / rain gutters, any other conductors.

Somethimes knowing how to describe a problem is knowing the answer to the
problem... or conversely, not knowning the answer is the result of not
knowing how to describe the problem.

Owen

SGC coupler to Dipole feedling question
 

If both shields, ( ungrounded ) are tied together, and the two

At both ends?

This is the first mention of shields tied together, I certainly didn't
read that into Bruce's "with the shield left open on each end
".


That's because I added that to the mix. Bruce's comment was a
suggestion for me. I have not done this yet, but my original post
under this thread was solliciting comments on using twin coax to feed a
balanced antenna, or using a single coax feed under the specific set of
circumstances I outlined.


About the common mode current.... please explain how this would be an
issue with the outer shields of two coaxes, shields tied together but
going nowhere ( no ground ), and the balanced antenna fed by the two
center conductors. I do not see common mode current being a factor, but
I'm willing to listen and learn.


Ed

SGC coupler to Dipole feedling question
 

"Left Floating" so that there is less Capacitance to RF Ground, and
more distance between the vertical parallel Feedlines. It would even
help if you can use twin Coax Runs, to put them on opposite sides of
the Aluminum Mast, which would give you more separation with the same
Coupling Capacitance to the Mast. Capacitive coupling to RF Ground,
is the Killer here, and you MUST reduce that, as much as possible,
if your system is going to have any chance at reasonable operation.



Please read my response and question just posted to Owen. With both
shields tied together, but not grounded, nor connected to the antenna
either, I do not understand how common mode current is an issue on the
shields.

We could use the mast as a physical separation as you suggested, (
the mast is not grounded, either, but again, what is the point, if the
two coax shields were "as one" anyway?


Ed
I

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in
. 192.196:

That's because I added that to the mix. Bruce's comment was a

Ok, well here is a model to shape your thinking and moving the goal
posts.

At frequencies where skin effect is fully developed, and that is a
reasonable assumption for most practical coaxial cables at HF, the
current on the inside surfaace of the outer conductor is equal to but
opposite in direction to the current on the outside surface of the inner
conductor. This is TEM mode propagation.

At the end of the isolated outer conductor, this current must flow
somewhere, and it flows around the end onto the outside surface of the
outer conductor (effectively changing direction as it does so). So, at
that point, the current flowing on the outside of the outer conductor is
exactly equal to the current flowing on the outside of the inner
conductor.

Leaving aside the effects of changing Zo by substitution of coax for
plain conductors:

If you use two coax lines in parallel with the shields isolated, it makes
very little difference, the current that would have flowed on the two
plain conductors now flows on the outer of the coax lines. The common
mode current is the sum of the currents in both coax shields, as it would
be for plain conductors.

If you join the shields together at each end, the sheilds together now
carry the common mode current. A different equivalent circuit, but almost
the same outcome.

Most of these 'shielded solutions' arise from a lack of understanding of
how the coaxial transmission line works in TEM mode.

For example, I saw an ham advise someone that station ground connections
were subject to noise pickup and the best improvement he could make was
to shield the ground lead. In his case, his shack was on the first floor
of the building, and his 7m vertical ground lead to the earth stakes etc
was a source of noise, so he used 7m of RG213 with the shield and inner
bonded to the earth stake and the shield left isolated at the top end.

Firstly, this is not a 'shield' at radio frequencies, but what he did
achieve was to insert a s/c stub in series with his station ground
conductor. The impedance of that series stub at 7.1MHz is 3056.20-
j1509.30 ohms... not a good outcome.

It might have 'fixed' his RF feeback problem, but it didn't improve the
station earth at all, it degraded it severely.

Owen

SGC coupler to Dipole feedling question
 

At frequencies where skin effect is fully developed, and that is a
reasonable assumption for most practical coaxial cables at HF, the
current on the inside surfaace of the outer conductor is equal to but
opposite in direction to the current on the outside surface of the
inner conductor. This is TEM mode propagation.

At the end of the isolated outer conductor, this current must flow
somewhere, and it flows around the end onto the outside surface of the
outer conductor (effectively changing direction as it does so). So, at
that point, the current flowing on the outside of the outer conductor
is exactly equal to the current flowing on the outside of the inner
conductor.

Leaving aside the effects of changing Zo by substitution of coax for
plain conductors:

If you use two coax lines in parallel with the shields isolated, it
makes very little difference, the current that would have flowed on
the two plain conductors now flows on the outer of the coax lines. The
common mode current is the sum of the currents in both coax shields,
as it would be for plain conductors.

If you join the shields together at each end, the sheilds together now
carry the common mode current. A different equivalent circuit, but
almost the same outcome.


The last paragraph above is where I lose you..... when the shields
are joined together. Yes, I understand inside the shield RF
current flowing around the end and to the outer side.... HOWEVER, the
OTHER center conductor is inducing RF current flowing in the opposite
direction. Since both these inside currents are 'shorted' at the ends
of the two shields, I fail to see how you can have any current flowing
on the outer shield since the two opposite currents should cancel....
????


Ed

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in
. 192.196:

of the two shields, I fail to see how you can have any current flowing
on the outer shield since the two opposite currents should cancel....
????

I already said that as I understand your variable configuration, it is most
unlikely that there is zero common mode current, or close to it.
Irrespective of the magnitude of the common mode current, the mechanism is
that the coax doesn't 'shield' it.

Owen

SGC coupler to Dipole feedling question
 

I already said that as I understand your variable configuration, it is
most unlikely that there is zero common mode current, or close to it.
Irrespective of the magnitude of the common mode current, the
mechanism is that the coax doesn't 'shield' it.


No, I wouldn't say "shield" would be a proper term, either. But I
would suggest that a "cancellation" similar to radiation in a balanced
feedline, would be pertinent.


Ed

SGC coupler to Dipole feedling question
 

My expertise is weak in this area, but just guessing.... using twin
coax in the above configuration, if the shields were grounded, would
allow the feedling between the antenna coupler and the feedpoint to be
'balanced' and yet the shields would not radiate as they would with a
single coax run.

Perhaps others, here, will either expand on this, or correct my
misconception.

Using the center conductors of two pieces of coax, with shields bonded
together, does create a balanced transmission line. Its
characteristic impedance is twice that of the coax itself. The higher
impedance of this feedline will cause the voltages on the line to be
higher than on a single coax, and the currents lower (all else being
equal, of course) and thus reduce I^2*R losses. This can be
beneficial if this line is being used between an antenna and a
transmatch/tuner/coupler, where a relatively high SWR may be expected
on the transmission line.

The OP might want to consider a two-coax run of RG-6 cable, for a
total line impedance of 150 ohms. Satellite-dish RG-6 is available in
both black and white - the white coax might help reduce visibility if
it's cable-tied to the metal mast.

--
Dave Platt AE6EO
Friends of 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!

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in
. 192.196:



I already said that as I understand your variable configuration, it is
most unlikely that there is zero common mode current, or close to it.
Irrespective of the magnitude of the common mode current, the
mechanism is that the coax doesn't 'shield' it.


No, I wouldn't say "shield" would be a proper term, either. But I
would suggest that a "cancellation" similar to radiation in a balanced
feedline, would be pertinent.

Then I will leave you to your view that the system is balanced.

Owen