Thanks dave
Right now I am running an inverted vs. fan dipole with 3 elements for
everything from 10 to 160 with a tuner at 40 feet.
I will be changing my qth next year sometime and am planning on putting up a
rhon 45 but I am not sure how high it needs to be. I need to balance height
with my budget. Right now I am planning on using dipoles for everything from 10
to 160 (the 160 will most likely be a cloud warmer) so say 10 to 80m dipoles
(possibly another fan with one or three dedicated dipoles for frequencies the
fan does not tune well) I may also add a 20m yagi on top.
As you can tell, I like wire antennas. I do have an antenna analyzer to help
tune them.
I guess what I am asking is.
What is the minimum height of the tower that I can erect and expect decent
results, and what height to place my dipoles.
Thanks
Joe
Ki4ILb
Merlin,
A really great tool that is easy to use and illustrates what you are
asking is a program called HFTA. It was written by N6BV and comes free
with the ARRL Antenna Book. It might even now be free from N6BV's
website ... I'm not sure. Anyway, if you have a particular band and
antenna in mind (dipole, 2 element yagi, 3 element yagi, etc), let me
know and I'll run the plots for you at different heights. Just tell me
the band and the kind of antenna.
In general, though, the answer is yes ... you can create some problems
if you put the antenna too high. The main takeoff lobe goes really low
... lower than most usable propagation uses ... and really deep notches
start to develop in the pattern at takeoff angles that are important.
For most purposes, it is a good idea to go as high as you can without
getting too much above one wavelength. That's the case for just one
antenna, of course. The guys with lots of bucks and lots of ambition
will stack several antennas on top of each other on really tall towers,
all fed in parallel, to fill in the notches while still having a really
low angle when the need it.
The HFTA program doesn't handle verticals, though, so I can't do that
one for you. In general, a vertical wants to be mounted close to the
ground if you have a good radial system or really good ground
conductivity, or it wants to be at least a quarter wavelength high if
you are using tuned radials over poor ground
73,
Dave AB7E
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