[TenTec] vertical dipoles

["Rob Atkinson, K5UJ" <k5uj@hotmail.com>]

my hunch is that ground proximety on one side of a vert. dipole may do some 
unbalancing but it probably isn't significant unless the pipe making the 
bottom half is sticking down in salt water :  ) okay that's an exageration.  
  if it's 10 feet up it's probably no big deal.    what's more of a trick 
is figuring out how to physically support the bottom end up off the ground 
but keep it insulated as a voltage maximum.   I'm focusing on rigid pipe 
vertical dipoles; not hung wires.    One method would be to put a treated 
pressed 4x4 into the ground about 3 or 4 feet (you need one that is around 
15 feet long) and top it with a glass or porcelain insulator that the bottom 
of the vertical pipe can fit over.  Of course the antenna has to be guyed 
with dacron because the insulator can only support it vertically as is the 
case with broadcast tower insulators.

I have a house and garage with aluminum siding; when I put up my loop I 
worried that since part of it would be along and over the house and garage 
and other parts of it would be out over the lawn, the different materials 
under it would unbalance it but so far I have not had a problem I'm aware 
of.  I think there is a sliding scale as opposed to either/or (it's either 
perfectly balanced or it isn't and if it isn't it will radiate horribly).   
I bet most balanced systems are not perfectly balanced but rather a bit off, 
but the amount of lost rf is negligible.

One thing that helps is to keep the open wire feed outside.  The way I have 
mine set up is the open wire enters the house through a basement window and 
comes in about one foot.  I have my balanced transmatch positioned there at 
the window to match the line and coaxial feed the rest of the way to the 
operating position, a length of about 10 or 15 feet.  So the bal. line is 
mostly outside.  Unless running exciter power only, I would not recommend 
extending balanced feed all the way to the operating position.   Of course 
this means that when I want to change bands or qsy 50 khz I have to get up 
and walk over to the tuner and adjust it.  but this is okay for me as I do 
not often change frequencies; i tend to park somewhere and ragchew for 
awhile.

rob/k5uj

<<<I agree that this is excellent information. However there is not much
discussion about feedline current balance, and the effect of the ground
being much closer to one end of the antenna than the other. The only
reason given for the ten foot height above ground of the bottom end of
the antenna, was for safety, keeping the high voltage at the end away
from people. Surely the greater capacitance to ground of the lower end
of the antenna is going to have some effect on the current balance, and
the currents in the two wires of the feeder will not be equal and
opposite. While I doubt it's a problem, that is where this discussion
started, as someone claimed excessive radiation from open wire feeder.

Feedlines with equal and opposite RF currents in their two conductors
(either open wire or coax) don't radiate. Getting the currents to be
equal and opposite requires care. In the absence of nearby objects, it
would seem to me easier to accomplish this with a horizontal dipole.
Orienting the dipole vertically seems like a way to guarantee that the
currents will not be equal.

Then again, how often are we luck enough to be able to put up a
horizontal dipole that does not have one leg closer to a tin roofed
building or metal pole or tower than the other leg.

None of this would keep me from building whichever version, vertical or
horizontal, fits the space available. I have always found ways to
mitigate RF in the shack problems. I don't transmit on TV channel
frequencies (at least not at a significant power level) so if my
radiating feedline makes someones TV set have problems, I guess there is
a problem with the TV.

DE N6KB>>>

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