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