On Fri, 09 Jan 2009 08:04:23 +0000, Paul Ormandy wrote:
>Would be interested to hear any experiences or dimensions,
The literature is pretty clear on this, and NEC is pretty good
at doing the math. The goal of the top loading is to get the
ELECTRICAL length close to a quarter wave. Essentially the top
loading can take any form, but the more symmetrical it is, the
more the radiation from it cancels and you're left with the
vertical portion. The vertical portion is low angle, the
horizontal portion is high angle.
A VERY rough approximation to get you to resonance is to view
the sum of whatever wire is on top as being added to the
vertical length. This will NOT be exact, and you should model
it in NEC to get a better answer. NEC is not at all costly, and
not very hard to learn to use. You plug x, y, and z coordinates
of the conductors into the software, push some buttons, and you
get plots of SWR and the radiation patterns. You can also move
the cursor to any point on the SWR curve and read the feedpoint
impedance. One very nice version was developed and is sold by
W7EL. Google to find it.
The good news is that you don't necessarily have to hit that
quarter wave resonance, you only need to get the impedance at
the feedpoint within the range of your antenna tuner. Loss in
coax is rather minimal at 1.8 MHz, so don't lose a lot of sleep
about matching as long as the line is not VERY long and you can
make your transmitter happy.
What's FAR more important is a lot of copper on the ground
under it.
73,
Jim K9YC
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