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October 29th 03, 05:38 PM

On 29 Oct 2003 16:20:25 GMT, (ARDUJENSKI) wrote:

With a 1/4wl vertical (mounted close to ground level) and you add a parasitic
(either a reflector or director) do you just ground mount it or do you need
radials or does it depend upon the ground conditions?

I noticed in running a few simple examples on EZNEC that the impedance drops
with the number of parasitics and the spacing. for example the 1/4wl alone will
be close to 50 ohms and with a parasitic the impedance will drop to less than
half in some arrangements (especially with 0.15wl spacing). Is this due to the
interactions of the driven and parasitic element?

By the way I was quite impressed by the potential F/B ratio and almost
cardioid pattern with a reflector and director added

Alan KB7MBI
Woodinville, WA

Hi Alan,

The parasitic is simply an antenna that is not directly driven. In
other words, to be efficient it has to conform to all the requirements
of the active element. In more words, it also follows all the
variables that affect the active element.

Do you ground mount it w/wo radials? Yes/no. Symmetry usually brings
its reward, but there are certainly a lot of variations that maintain
symmetry without being exactly identical. Further, the addition of
the parasitic inevitably upsets the original configuration (sometimes
to the benefit of the user) through interactions. Why? Just like
paralleling resistors changes the load R. In this case, the fields do
the job of connecting up instead of wire. Here in Woodinville, trees
will do the same thing.

The F/B thing will vary with coupling (distance/wavelength) and
efficiency (back to ground conditions). Simple examples abound in the
AM band. Examine the slough south of Bellevue near the I-90
connection to Mercer Island.

73's
Richard Clark, KB7QHC