Question on dipole SWR problem
 

"Ed G" wrote in message
.81...


The feedline is doing more than transporting energy to the dipole
centre, it almost certainly carries a significant common mode current
and in that case is just as much a part of the radiating system as the
dipole itself.

So, isn't the coax and control line to the SG237 carrying current mode
RF current, ie contibuting to radiation.

Inch by inch, the detail unfolds.


Yeah, inch by inch. I hate to fill up the post with too many
details as some people grab something insignifcant and go off on a
tangent.... however, the devil may be in the details.....

You are right here. I have a feeling some people think you are running
ladder line to the radio.


You're right on the current in the shield. We have a choke balun,
currently located 85' down the coax toward the transmitter. I have it
scheduled, when it stops raining, to move it up to the input of the SG-
237 coupler unit.

This has got to be a step in the right direction.

Tam/WB2TT

The Choke balun is homemade, 6" diamter, about 11
turns of RG-8X coax. Does that sound sufficient? I didn't make it or
provide it. There is no control line for the SGC coupler unit. It only
requires 12VDC @ 300ma and we are providing that by using a couple MFJ
DC adapter units to feed the 12VDC up the coax.


Ed

Question on dipole SWR problem
 

Ed G wrote in
.81:



The feedline is doing more than transporting energy to the dipole
centre, it almost certainly carries a significant common mode current
and in that case is just as much a part of the radiating system as the
dipole itself.

So, isn't the coax and control line to the SG237 carrying current mode
RF current, ie contibuting to radiation.

Inch by inch, the detail unfolds.


Yeah, inch by inch. I hate to fill up the post with too many
details as some people grab something insignifcant and go off on a
tangent.... however, the devil may be in the details.....

Yes, the people who modelled your antenna based on half the information
got an incorrect answer.


You're right on the current in the shield. We have a choke balun,
currently located 85' down the coax toward the transmitter. I have it
scheduled, when it stops raining, to move it up to the input of the
SG-
237 coupler unit. The Choke balun is homemade, 6" diamter, about 11
turns of RG-8X coax. Does that sound sufficient? I didn't make it or
provide it. There is no control line for the SGC coupler unit. It
only
requires 12VDC @ 300ma and we are providing that by using a couple MFJ
DC adapter units to feed the 12VDC up the coax.

You need to visualise that you what you have looks like a dipole with a
wire from one side of the centre following the path of the feedlines to
the choke and from the choke on to wherever.

The choke balun introduces a common mode impedance in that equivalent
wire, but it does not necessarily eliminate common mode current, rather
it modifies the amplitude and distribution of the common mode current.

Owen

Question on dipole SWR problem
 

On 17 Oct 2007 22:40:44 GMT, Ed G
wrote:

It only
requires 12VDC @ 300ma and we are providing that by using a couple MFJ
DC adapter units to feed the 12VDC up the coax.

Hi Ed,

This is another antenna element unless you choke it too.

73's
Richard Clark, KB7QHC

Question on dipole SWR problem
 

Ed G wrote:
Not sure what SGC recommends regarding balanced antennas.... will
review their info. However, I have read numerous reports from hams who
have used SGC and other similar tuners to feed balanced lines. The only
important condition was to use a choke balun to keep RF from coming back
down the coax shield.

My SG-230 manual says NOT to connect any transmission
line to the tuner output - to connect only an antenna.
--
73, Cecil http://www.w5dxp.com

Question on dipole SWR problem
 

..

Hi Ed,

This is another antenna element unless you choke it too.



Richard, I don't think this one is. Its just a "barrel" type device
located in the coax run to the SGC coupler, about 1 foot from the input
to the SGC coupler.. There are no other connections or long runs of
anything going away from the coupler or the antenna. The MFJ adapter
merely filters the 12VDC off the coax center conductor and routes it
through a wire a couple feet long into the coupler 12VDC input.


Ed

Question on dipole SWR problem
 

Not sure what SGC recommends regarding balanced antennas.... will
review their info. However, I have read numerous reports from hams
who have used SGC and other similar tuners to feed balanced lines.
The only important condition was to use a choke balun to keep RF from
coming back down the coax shield.

My SG-230 manual says NOT to connect any transmission
line to the tuner output - to connect only an antenna.

SGC manual for the SG-237 says it is recommended to mount the coupler at
the antenna feedpoint, but it will work on anything, including balanced
line. They show a similar example to what we are doing in the manual.


Ed

Question on dipole SWR problem
 

Bob Miller wrote:

Maybe one could say the ground side of the balanced line and its
dipole quarter wave are acting as an elevated counterpoise to the
"random wire" on the other side?

Antenna systems are often a lot easier to analyze and understand if you
put aside concepts like "counterpoise" and "ground".

An antenna is a two terminal device, even if it's "end fed". The
transmitter is also a two terminal device. Connect the two together and
you have an electrical circuit. Like any circuit, the current leaving
one terminal has to equal the current going into the other terminal.

So what happens with an end fed antenna? Well, whatever the current
going into the antenna (and current must go into it, since the power
into it is I^2 * R, where R is the sum of radiation and loss
resistance), an equal and opposite current must go somewhere else. If
you succeed in completely choking off the current going somewhere else,
you've also succeeded in choking off the current going to the antenna.
So you don't want to do that.

Conductors don't care what label you put on them -- calling one a
"ground" or "counterpoise" doesn't give it magical properties. When a
current flows on a conductor, it creates a field. This field will
radiate unless canceled by other fields. So the "somewhere else" that
the current flows is just as much an antenna as the supposed antenna is.

If the current has nowhere else to go, it'll go down the outside of the
coax, which will effectively become the other half of a dipole. If you
choke off the current on the outside of the coax, it'll go somewhere
else if it can. But if there is no other place, then the current to the
antenna will drop -- the feedpoint impedance will increase.

If the current goes into two or more radial wires which are
symmetrically placed, the fields from the wires will largely (but not
completely) cancel, so the net radiation from the radials will be small.
This can reasonably called a "counterpoise" -- a place for the current
to flow without creating much radiation. Or you can connect the antenna
to a buried radial field ("ground"), which behaves much the same way,
but with even better field cancellation. But people often put these
names on other configurations, expecting the currents or fields to
behave differently than on other conductors.

But the things to remember are that all antennas have two terminals, and
the current into one equals the current out of the other. And current
flowing along any conductor creates a field, whether you consider it to
be an "antenna" or not. Keeping this in mind helps a lot in
understanding end fed and other antennas.

Roy Lewallen, W7EL

Question on dipole SWR problem
 

On 18 Oct 2007 01:39:41 GMT, Ed G
wrote:

.

Hi Ed,

This is another antenna element unless you choke it too.



Richard, I don't think this one is. Its just a "barrel" type device
located in the coax run to the SGC coupler, about 1 foot from the input
to the SGC coupler.. There are no other connections or long runs of
anything going away from the coupler or the antenna. The MFJ adapter
merely filters the 12VDC off the coax center conductor and routes it
through a wire a couple feet long into the coupler 12VDC input.

Hi Ed,

I stand corrected. You seem to have that one under control.

73's
Richard Clark, KB7QHC

Question on dipole SWR problem
 

Ed G wrote:
w5dxp wrote:
My SG-230 manual says NOT to connect any transmission
line to the tuner output - to connect only an antenna.

SGC manual for the SG-237 says it is recommended to mount the coupler at
the antenna feedpoint, but it will work on anything, including balanced
line. They show a similar example to what we are doing in the manual.

Perhaps the SG-237 has some protection that the SG-230
didn't have? The wording in the SG-230 manual suggests
that, since the autotuner is capable of developing
voltages high enough to cause open-circuited coax
to arc, discretion is advised.
--
73, Cecil http://www.w5dxp.com

Question on dipole SWR problem
 

You're right on the current in the shield. We have a choke balun,
currently located 85' down the coax toward the transmitter. I have it
scheduled, when it stops raining, to move it up to the input of the SG-
237 coupler unit.

This has got to be a step in the right direction.

Tam/WB2TT

I think I misunderstood by what you meant by INPUT. The choke goes between
the antenna and the tuner. Generally at the antenna feed point.

Tam/WB2TT