Question on dipole SWR problem
 

Tam/WB2TT wrote:

If I had a huge metal roof like that, I would be tempted to try one of the
shortened verticals. The kind that requires radials - which would be your
roof.

Unfortunately, a vertical makes a poor NVIS antenna.

. . .

Roy Lewallen, W7EL

Question on dipole SWR problem
 

On Tue, 16 Oct 2007 16:07:40 -0400, "Tam/WB2TT"
wrote:

I guess I should have called it a transformer. Although, it could be a
balun, like they use on output stages of RF amplifiers; 5.6 Ohm balanced to
50 Ohm unbalanced.

Hi Tam,

Lift one wire from ground and you've got that BalBal.

73's
Richard Clark, KB7QHC

Question on dipole SWR problem
 

Ed G wrote in
.89:


Ed,

The way in which you dealt with the transition from the open wire
feedline to the SGC237 unbalanced tuner is relevant. As is whether or
not you bonded the tuner 'ground' terminal to the roof.

Your description is short on relevant detail.

Owen



SG237 tuner is not "bonded" to the roof ground, SGC does not
suggest
doing such a thing, and in this case, it would not be possible anyway
since the "ground" side of the tuner is also one side of the balanced
antenna feedpoint. The SG237 is feeding about 14' of 600 ohm open
ladder line, spaced about 1 foot off the aluminum mast, up to the
feedpoint of the antenna, a 75M dipole ( 61' per leg). The tuner is
sitting in a plastic box, about 1 foot off the roof.

Ed

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.

Owen

Question on dipole SWR problem
 

"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

Question on dipole SWR problem
 

"Roy Lewallen" wrote in message
...
Tam/WB2TT wrote:

If I had a huge metal roof like that, I would be tempted to try one of
the shortened verticals. The kind that requires radials - which would be
your roof.

Unfortunately, a vertical makes a poor NVIS antenna.

. . .

Roy Lewallen, W7EL

Still, a waste of a ground plane.

Tam/WB2TT

Question on dipole SWR problem
 

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

Question on dipole SWR problem
 

If I had a huge metal roof like that, I would be tempted to try one of
the shortened verticals. The kind that requires radials - which would
be your roof.


That is something we had not considered. Will keep that in mind.
Tnx.


You are running coax from the tuner to the shack, right; with just the
14 feet of ladder line?

Yes, 14' ladderline from tuner to antenna center. About 100 ' of
RG-213 on roof and another 30' or so going down to station inside
building. We have a coax choke balun located in line on the roof about
8' before the coax goes down in the building.


Ed

Question on dipole SWR problem
 

Is the 237 designed for balanced antennas or single random wires, or
either?

bob
k5qwg

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.


Ed

Question on dipole SWR problem
 

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

Question on dipole SWR 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

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.