A debt of gratitude is owed Rudy Severn for the work he did on modeling
verticals mounted near towers, not to mention all the other helpful
information posted on his site.
In your case it would seem that the next step would be to determine what the
electrical height of your tower will be with whatever antennas are mounted
on it and then compare that with the information provided by Rudy. From
there decide whether the model projected risk is worth the time it would
take to actually put the vertical in place and see what it does.
While I like to use modeling when building yagis and certainly respect the
information provided by modeling in general, I wouldn't necessarily let
modeling prevent me from doing empirical research.
Case in hand involves development of a 160M antenna to be used in contests
from NR4M.
We are currently using a 90 foot vertical wire with a 60 foot tee at the
top. It is supported by a line that runs between the top of a 120 foot tower
with three 5 el 15M yagis on it and a rotary guy ring at about 120 feet on a
160 foot tower supporting four five element 20M yagis on it. The vertical is
about 30 feet from the fifteen meter tower and is fed against 45 quarter
wave radials on the ground. The twenty meter tower is a couple hundred feet
from the vertical wire.
I won't get into the genesis of the top band vertical and we haven't done a
lot of measurement with it, but based on the fact it's fed with a 2:1
transformer I expect it's looking pretty much like 25 ohms resistive at the
base. At some point I'll have do a closer study of Rudy's work and see what
hid modeling shows.
Again, no scientific study has been done relative to pattern or anything
else with this antenna. It's been in use for a couple of years, and has been
very effective. It's proven to be competitive with many of the big guns in
and out of contests.
We have a 190 foot tower that will be used strictly for supporting low band
antennas at the farm. I've been looking at an adaptation of the Spitfire
similar to what Bill W4ZV has done for 160 on that tower. The existing
vertical works so well, we're kind of debating whether we should add a
second vertical between the 20 and 15 M towers and phase them, rather than
proceeding with the "Spitfire".
My point is if in modeling your situation doesn't look worst case you may
very well find that though it may take some work to get it to match and the
pattern may be skewed, you still might get utility from building the
vertical. It's your time!
73 de Lar K7SV
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