Owen Duffy wrote in
:

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

I will answer my own question...

In the absence of an effective device to prevent common mode feedline
current, your configuration where one side of the ladder line connects to
the 'ground' side of the nominally unbalanced tuner, and that same tuner
terminal connects to the outside of the feedline coax and control wires
(at RF), then you have a continuous RF common mode path to wherever the
coax and control lines connect and so on, they are just as much a part of
your radiator as the dipole wire.

Analysis of the 122' entre fed wire in isolation of the rest of the
antenna system are inadequate in explaining the difficulties you
encounter.

Owen

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'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.


Ed

"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

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

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

"Ed G" Huckleberry_ large SWR when operating below about 3825 KHz.

We recently replaced the entire antenna assy with a 122' half wave
dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the
base of the center mast. The SGC 237 coupler tunes the antenna
well above 3825, and all other bands, too. However, we still exhibit a
very high SWR when going below about 3825 and the tuner fails to tune...
in fact the Radio's power drops
way down, possibly preventing the tuner from working properly. A
different radio shows the same problem.

Ed, sounds like your radios are into VSWR turndown from the inability of the
tuner to match at the low end. It could be that the Z at 3825 is going high
which a lot of tuners can't handle--or could be very low, same problem, no
match. Try adding some more length to the dipole if you can and see if it
tunes better at the low end. Even if you only drape pieces down at the ends
would help if you can't extend more horizontally.
73,Bill W0BVR

Ed, sounds like your radios are into VSWR turndown from the inability
of the tuner to match at the low end. It could be that the Z at 3825
is going high which a lot of tuners can't handle--or could be very
low, same problem, no match. Try adding some more length to the
dipole if you can and see if it tunes better at the low end. Even if
you only drape pieces down at the ends would help if you can't extend
more horizontally. 73,Bill W0BVR

You, and others earlier on in this thread, have suggested such a
thing. I have that on my list of things to try when the weather allows.
Tnx.