SGC coupler to Dipole feedling question
 

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



I would have preferred an explanation of your view on why it wouldn't
be, but I thank you for all your prior discussion.


Ed K7AAT

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in
. 192.196:



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



I would have preferred an explanation of your view on why it
wouldn't
be, but I thank you for all your prior discussion.

From my first post on the topic:

"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."

Sure, you can fabricate a parallel line from two coaxial lines, it just
has much more loss than a conventional air spaced line... and although it
is short, you intend operating it at extreme VSWR. The shielded twin line
you synthesise does not have any magic properties in supressing or
shielding feed line radiation.

Note that I am avoiding the term balanced line that some have used.
Balance is not forced by line geometry, but is a result of the
environment, so balanced lines are balanced by external factors, not the
line geometry.

Owen

PS: I wonder if you had considered end feeding the Inverted V with the
ATU. IIRC you had a sheet metal roof. You could just fix the tuner to the
roof, connect the ground terminal to the roof sheet, and take a wire from
the ATU output terminal to the end of the inverted V (which is a
continuous conductor across the apex). This is an unbalanced load
connected to an unbalanced output. Is that too easy?

SGC coupler to Dipole feedling question
 

Ed_G wrote:

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?

The answer is that making the feedline physically symmetrical doesn't
make it "act as a balanced feedline". A feedline is balanced and not
radiating only when the common mode current is zero, i.e., the currents
on the two conductors are equal in magnitude and opposite in phase.
Making a line physically symmetrical doesn't guarantee or cause this.
Nor, for that matter, does making a line physically asymmetrical (e.g.,
coax) necessarily cause a line to become unbalance. There's more about
this at http://eznec.com/Amateur/Articles/Baluns.pdf.

Roy Lewallen, W7EL

SGC coupler to Dipole feedling question
 

Perhaps a little amplification of what Owen has said will help clarify
the situation.

Suppose we have a twinlead transmission line, one conductor of which is
carrying a current of 2 amps at one point, and the other 3 amps at the
same point. For simplicity, we'll assume that the currents are exactly
out of phase. The common mode current (as we'll define it*) is 3 - 2 = 1
amp. (We can directly subtract them due to the assumption that they're
exactly out of phase; otherwise we'd have to do a vector addition.) So
the line will radiate exactly as though there was a single conductor
carrying one amp. This is an unbalanced, radiating feedline.

Now let's replace the line with a coax line of the same impedance so it
doesn't otherwise alter the system. What we'll find is that the center
conductor will carry 2 amps. The inner surface of the outer conductor,
which is always forced to be equal and opposite, carries 2 amps of
opposite polarity, that is, 2 amps going exactly the opposite direction.
On the outside of the shield is one amp, our common mode current. The
inner and outer shield currents combine at the cable ends to become 3
amps. This line will also radiate just like a single conductor carrying
one amp.

Finally let's look what happens when we use two coax lines with the
shields connected but floating. Suppose the 2 amps is on the center of
coax A and 3 amps on the center of coax B. On the inside of the coax A
shield is 2 amps flowing one way (the direction opposite the current on
the center conductor). On the inside of the coax B shield is 3 amps,
flowing the other way. What happens at the ends of the shield? At each
end, the 2 amps flowing one way will add to the 3 amps the other way
(since they're connected at the ends so there's a path from one to the
other), resulting in a 1 amp current which flows down the outside of the
shield. This radiates just the same as the others, like a one amp
current flowing on a single conductor. Using dual coax has accomplished
nothing.

The way to prevent the feedline, whatever the type, from radiating, is
to force the currents on the two conductors to be equal and opposite.
This can be done by making both the antenna and the tuner symmetrical,
in which case any of the three lines will be balanced and not radiate.
Another way is to use one or more common mode chokes (current baluns)
which will also balance any of the three line types. But just changing
from one type of line to another doesn't do it.

I've simplified this analysis to deal only with constant currents, such
as you'd approximately have with an electrically short transmission
line. But the individual currents maintain the same ratio all along
longer lines, so the same result occurs.

(*) Common mode current is sometimes defined as half the vector sum of
the two conductor currents, rather than simply the sum as done here. If
you use the other definition, you assume that the common mode current is
flowing on each of the two conductors to determine the amount of
radiation you'll get. The end result is the same either way.

Roy Lewallen, W7EL

SGC coupler to Dipole feedling question
 

Ed_G wrote:

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?

I hope my recent postings have helped answer your question.

Roy Lewallen, W7EL

SGC coupler to Dipole feedling question
 

Dave Platt 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 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. . .

There's a semantic problem here. I and many others consider "balanced"
to mean non-radiating, which requires that the two conductors carry
equal and opposite currents. Others call any physically symmetrical line
"balanced". As I explained in another recent posting (and in the article
at http://eznec.com/Amateur/Articles/Baluns.pdf), making a line
symmetrical doesn't make it balanced -- that is, it doesn't guarantee
equal and opposite conductor currents and therefore doesn't guarantee
that it won't radiate. A coax line can be balanced and a symmetrical
twinlead line can be unbalanced.

Roy Lewallen, W7EL

SGC coupler to Dipole feedling question
 

Using the center conductors of two pieces of coax, with shields
bonded together, does create a balanced transmission line. . .

There's a semantic problem here. I and many others consider "balanced"
to mean non-radiating, which requires that the two conductors carry
equal and opposite currents. Others call any physically symmetrical
line "balanced". As I explained in another recent posting (and in the
article at http://eznec.com/Amateur/Articles/Baluns.pdf), making a
line symmetrical doesn't make it balanced -- that is, it doesn't
guarantee equal and opposite conductor currents and therefore doesn't
guarantee that it won't radiate. A coax line can be balanced and a
symmetrical twinlead line can be unbalanced.

Roy Lewallen, W7EL



In the past, I normally have always used a balanced tuner on my
balanced antenna systems..... this one under discussion, being one of
those SGC antenna couplers, is NOT balanced..... It did not occur to me
that the currents through the twin coax feedlines to the balanced
antenna would not be equal. So, to make sure I am understanding you
correctly, this un-symmetrical current could also occur in open ladder-
line?

Obviously the weakness of my antenna knowledge has been exposed. My
thanks to Roy, AND to Owen.


Ed

SGC coupler to Dipole feedling question
 

Owen Duffy wrote in
:

"Ed_G" wrote in
. 192.196:



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



I would have preferred an explanation of your view on why it
wouldn't
be, but I thank you for all your prior discussion.

From my first post on the topic:

"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."

Sure, you can fabricate a parallel line from two coaxial lines, it
just has much more loss than a conventional air spaced line... and
although it is short, you intend operating it at extreme VSWR. The
shielded twin line you synthesise does not have any magic properties
in supressing or shielding feed line radiation.

Note that I am avoiding the term balanced line that some have used.
Balance is not forced by line geometry, but is a result of the
environment, so balanced lines are balanced by external factors, not
the line geometry.

Owen

PS: I wonder if you had considered end feeding the Inverted V with the
ATU. IIRC you had a sheet metal roof. You could just fix the tuner to
the roof, connect the ground terminal to the roof sheet, and take a
wire from the ATU output terminal to the end of the inverted V (which
is a continuous conductor across the apex). This is an unbalanced load
connected to an unbalanced output. Is that too easy?




Owen, after reading Roy's explanation, I came back to this one
and what you have been saying is now more clear to me.

I will have to check what is available for end feed on the roof,
but it is a completely rubber covered roof and I doubt there is any
convinent way to access a ground. Tnx.

Ed

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in message
. 192.196...
Owen Duffy wrote in
:

"Ed_G" wrote in
. 192.196:



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



I would have preferred an explanation of your view on
why it
wouldn't
be, but I thank you for all your prior discussion.

From my first post on the topic:

"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."

Sure, you can fabricate a parallel line from two coaxial
lines, it
just has much more loss than a conventional air spaced
line... and
although it is short, you intend operating it at extreme
VSWR. The
shielded twin line you synthesise does not have any magic
properties
in supressing or shielding feed line radiation.

Note that I am avoiding the term balanced line that some
have used.
Balance is not forced by line geometry, but is a result
of the
environment, so balanced lines are balanced by external
factors, not
the line geometry.

Owen

PS: I wonder if you had considered end feeding the
Inverted V with the
ATU. IIRC you had a sheet metal roof. You could just fix
the tuner to
the roof, connect the ground terminal to the roof sheet,
and take a
wire from the ATU output terminal to the end of the
inverted V (which
is a continuous conductor across the apex). This is an
unbalanced load
connected to an unbalanced output. Is that too easy?




Owen, after reading Roy's explanation, I came back
to this one
and what you have been saying is now more clear to me.

I will have to check what is available for end feed on
the roof,
but it is a completely rubber covered roof and I doubt
there is any
convinent way to access a ground. Tnx.

Ed



-------------------


What about running radials made of lengths of burlar alarm
metal tape? A coat of black spray paint will make them
nearly invisible on a black roof.

I didn't see your frequency requirements, but I have had
excellent results from a Cushcraft R7. I am amazed at just
how well it works, even on 75m (by accident!).

Ed, NM2K

SGC coupler to Dipole feedling question
 

Ed_G wrote:
In the past, I normally have always used a balanced tuner on my
balanced antenna systems..... this one under discussion, being one of
those SGC antenna couplers, is NOT balanced..... It did not occur to me
that the currents through the twin coax feedlines to the balanced
antenna would not be equal. So, to make sure I am understanding you
correctly, this un-symmetrical current could also occur in open ladder-
line?

Yes, that's correct. I suggest taking a look at the article I posted the
link to -- it discusses this in more detail.

Obviously the weakness of my antenna knowledge has been exposed. My
thanks to Roy, AND to Owen.

Glad to help.

Roy Lewallen, W7EL

SGC coupler to Dipole feedling question
 

In article . 196,
"Ed_G" wrote:

"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

Most of the discussion here is more esoteric, than practical. You have
an imposed Antenna Requirement, that gives you little wiggle room in
what you can do. I have given you the ideas, that are proven in the
Real World, for installations with similar requirements. As I stated
in my first post, these type systems, ALL preform equally BAD, when
compared to the Antenna Systems, that these type tuners were DESIGNED
to feed. They are intended to feed a Marconi Vertical, with a very
Low Impedance RF Ground. That is where they work BEST. Everything else
will be a poorer situation. How poor, will depend on a BIG pile of
variables, most of which are esoteric in significance, and will
not make a significant difference in actual Practical Performance of the
Station. Again as I stated in my last post, Capitative Coupling to
RF Ground is the BIG Killer, the second largest problem is selecting
antenna length, so as the Natural Impedance of the antenna is NOT
anywhere near a frequency that you need to operate on. All the rest,
is Picking Nits, and can be discussed at infinitum, without changing
the actual performance in any significant way. One of the posters
suggested building an "counterpoise", (God, I hate that Term) RF Ground
from Aluminum Alarm Tape, on the roof, and then feeding what would be
essentially a end feed wire antenna. That would be preferable, IF you
could get the Biggies to go along, but that was NOT, part of the
original Problem. In the real WORLD, your Comm's are going to depend
more on, if the Band is Open, for your Distance, and Frequency, to
the destination, than the Antenna design, and if the Band is NOT open,
you aren't going to talk, as Ground Wave Comm's are not usually a
significant part of HF Communications.

Just a note, for historical purposes. There was a highly modified
SEA1612B Tuner that was designed to feed a Balanced Antenna. It was
fabricated out of Two, (2) Tuner Boards, using a Common CPU/Sensor
System, that determined the configuration of the Switched Elements
on one side of the Antenna, and then locked the same configuration
on the Second board for the other side. This scheme was developed
by Don Hollingsworth Sr. at G&L Marine Radio, in Seattle, Washington,
and was deployed in one or two installations of Maritime Mobile Limited
Coast Stations. I never did hear just how well they worked.

--
Bruce in alaska
add path after fast to reply

SGC coupler to Dipole feedling question
 

"Ed_G" wrote in
. 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

Ed,

I would consider the following:

Mount the ATU at an accessible place on the masting near the lowest end.
Bond the ATT's ground terminal to the mast.

If there is a lighting protection conductor, steel rain gutter, steel
roof in proximity of the base of the mast, bond the base of the mast to
them also using substantial conductors (16mm^2).

At the top of the mast, bond one side of the dipole to the mast.

Run an RG213 or better coax from the ATU to the top of the mast, bond the
shield at both ends to the mast. Connect the inner conductor at the top
to the other side of the dipole, and at the bottom to the output terminal
on the ATU. You must treat the ends of the coax to prevent water ingress.
LDF4-50 would be an even better choice because its closed cell bonded
foam dielectric is better protection against water ingress (it also has
lower loss and higher breakdown voltage). I would not use a foil shielded
coax.

Connect the DC / control wires and input coax to the base of the tuner,
but route them through a common mode choke. You may well be able to use
RG58C/U for the input feed line (depending on length). Wind several turns
of the coax and DC / control wires together through a large ferrite
toroid (#43 should be fine). Two or three of these chokes should probably
be adequate. You may also want one or two chokes where you enter the
equipment room.

Treat all connections to prevent corrosion, especially considering
dissimilar metals.

There is likely to be common mode current on the feedline / mast above
and below the tuner. The chokes reduce the extent of it to minimise the
contibution of the feedline to the radiation system / RFI and conversely
feed line noise pickup, and act to reduce RF "flowing into the shack".


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.

You can take measures as above to minimise the downsides of the lack of
system symmetry.

SGC have recommended attaching symmetric loads to their assymetric tuners
for a long time (they sell tuners, and didn't at the time have a balanced
tuner), and hams have proved it "works" whatever that means.


Also, what recomendations do you guys have for use of a balun?

See the discussion above about a common mode choke.

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.

See above.

Owen

SGC coupler to Dipole feedling question
 

In article . 196,
"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

hi Ed

you might try using a plastic mast (pvc or fiberglass) with enought
strength to survive the load and weather thats hollow and try running
laddar line inside

you can use some solid plastic pipe some ham places sell it or
even a short piece ofmetal mast if you can cut a small slot in
the plastic such that it's integraity is good or u can run the
laddar line down the outside of the plastic pipe pprobably easist
and paint it all black w/rf ok paint very hard to see the feed line
then


way back when i spoke to somone that was 'smart' at sgc and they said
using a short piece of laddar line to feed the dipole (as they way i
happened to do mine almost exactly as you described) they said it was
less than ideal but not bad as long as the laddar line was i believe
less than 10??ft long they said the output definately shouldn't
have any balun as far as the coax feeding the tuner?? most
seemed to agree it wasn't needed but i thru one in anyway both on
the roof and in my shack


my set up thou slightly different than yours unscientifically seems to
work very well, i don't have nasty stuff on my coax run to the tuner
my swr even w/a very short dipole is nearly almost always max at 1.2
worst case the tuner tunes in seconds and i've enjoyed some pretty
good fun the total length of my dipole is maybe less than 40ft

is my set up ideal? or good as a beam? i'd guess not but i had co
op board roof constraints as you sorta have so it's best effort
and i've had a blast with it from 160 to 6m


i got a bunch of my ideas for it's actual design and final construction
from my fooling aournd , here in this group , sgc personal I think i
spoke to the original owner once, there manuals and googling


goodluck Ed

SGC coupler to Dipole feedling question
 

In the past, I normally have always used a balanced tuner on my
balanced antenna systems..... this one under discussion, being one of
those SGC antenna couplers, is NOT balanced..... It did not occur to me
that the currents through the twin coax feedlines to the balanced
antenna would not be equal. So, to make sure I am understanding you
correctly, this un-symmetrical current could also occur in open ladder-
line?

Obviously the weakness of my antenna knowledge has been exposed. My
thanks to Roy, AND to Owen.


Ed
Just curious, how did this project turn out?
John Ferrell W8CCW
Beware of the dopeler effect (pronounced dope-ler).
That's where bad ideas seem good if they come at you
fast enough.

SGC coupler to Dipole feedling question
 

In the past, I normally have always used a balanced tuner on my
balanced antenna systems..... this one under discussion, being one of
those SGC antenna couplers, is NOT balanced..... It did not occur to
me
that the currents through the twin coax feedlines to the balanced
antenna would not be equal. So, to make sure I am understanding you
correctly, this un-symmetrical current could also occur in open
ladder-
line?

Obviously the weakness of my antenna knowledge has been exposed. My
thanks to Roy, AND to Owen.


Ed
Just curious, how did this project turn out?
John Ferrell W8CCW
Beware of the dopeler effect (pronounced dope-ler).
That's where bad ideas seem good if they come at you
fast enough.


Appreciate the inquiry, but due to my pending move in early May (
local ) We have not yet completed this project. Rest assured, I
remember the information presented in the discussion on this thread and
intend to proceed..... probably later in May when my move is completed
and I am available to assist the other members of the team that will be
installing the aformentioned antenna. TNX

73 de Ed K7AAT