Does not work that way. Model it. It will surprise you.
The trick to understanding how it works is to visualize the system as
a collection of point impedances coming back from the high voltage
zero current points (tips = extremely high Z) to low Z's a quarter
wave away.
At a given point where there is more than one possible path, the path
with the electrical distance to tip that is most like an odd quarter
wavelength will take the larger proportional current.
It is only 30 feet or so from the tower to the farthest tip on the
C31. That has a LARGE impedance to it on 80, and much higher Z on 160.
It's much easier (lower Z) to run up the tower to the tips that are
more like a quarter wave away. That might be a choice between 2000
ohms and 100 ohms.
160 meter current on the lower C31 will be insignificant.
----- Original Message -----
From: "Bill Ralston" <n7vm@lgcy.com>
To: <towertalk@contesting.com>
Sent: Thursday, January 24, 2002 7:06 PM
Subject: [TowerTalk] Alternate 80m antenna thoughts
Of course, the obvious thing to do is shunt feed my planned 100' tower
for
80 & 160. But, I am planning a C31XR on a ring rotor at about 55' in
addition to a second C31XR at 95' and a 40m above that.
If the shunt feed attaches to the tower below 55', it seems like the
lower
C31XR would effectively choke off the vertical at that height with the
C31XR
acting as capacitive end loading. Maybe OK for 80m, not too cool on
160m.
If the shunt feed attaches above 55' (using some interesting
contortion
around the ring rotor), I'm not sure how the thing will behave.
This got me to thinking - would it be possible to do something similar
to
feeding the boom on a yagi for a different band? With a C31XR at 55'
and at
95', might it be possible to feed the tower through a suitable feed (L
or
Omega) at around 75' - just like one would feed the boom of a yagi,
but
oriented vertically? The 40' of tower plus capacitance of the
antennas just
might look close to resonant at 80m! Of course, the tower above and
below
the antenna will also contribute some.
One problem seems to be the feedline, but this might be solved by
placing
the feedline inside the tower; the skin effect should hide it.
Comments?
-- Bill N7VM
______________________________________________________________________
__
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
----------------------------------------------------------------------
-
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
________________________________________________________________________
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
-----------------------------------------------------------------------
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|