[Antennaware] Re: When is a stub not just a stub
n4zr at contesting.com
Mon Jun 2 18:18:14 EDT 2003
At 02:58 PM 6/2/03 -0400, Guy Olinger, K2AV wrote:
>The discussion originated around evaluating a lazy V, four vertical
>doublets distributed around the tower, pulled out from the top of a tower
>to a feedpoint, and back to the tower, with four coaxial feedlines from
>the doublet centers back to the tower and the switch. One feedline fed at
>a time, the rest passive and providing gain and front to back. Then add
>the issue of leaving the unfed feedlines' shields connected to the switch
>vs. isolating them.
It seems to me important to distinguish two distinct issues. The first is
whether, for completeness, one needs to model the shield of the coax
feedlines as a separate wire. Per observations by W7EL and others, I think
the answer is yes. EZNEC ships with an example of how this is done in a
simple dipole model. I have not done this in my model of the antenna,
because the dipoles are symmetrical with respect to the feedlines, so I
assumed cancellation (each feedline has a choke balun at the feedpoint as
well). Frankly, there are far more serious sources of infidelity in the
model (the elements aren't really symmetrical, for example), and the darned
thing works very much as modeled, notwithstanding those differences, so...
The other issue is why it is required that both the shield and the center
conductor of each of the three unfed dipoles be "floated" at the switchbox,
rather than just floating the center conductors. This seems to be
necessary in practice, but why? Does leaving the shields connected to one
another, to the shield of the one dipole that is driven, and to ground
through the mounting of the relay box, compromise the magnitude of loading
provided to each undriven dipole by the feedlines functioning as open
stubs? Or does it do away with that loading effect altogether? If so, how?
73, Pete N4ZR
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