While most of the NEETS documents are fine, as far as explaining what's
going on, quite simply, the explanation in the referenced link is incorrect.
"When the parasitic element is placed so that it radiates away from the
driven element, the element is a director. When the parasitic element is
placed so that it radiates toward the driven element, the parasitic element
is a reflector. "
An individual element radiates the same whether it is in an array or not,
that is, it has the usual dipole pattern (or one quite similar to it,
depending on its length). The magnetic field surrounding a current carrying
conductor is symmetric around the conductor. This principle (which can be
derived at some length from first principles) is what things like method of
moments programs are based on, and for that matter, most of modern antenna
design. The overall pattern of the antenna is the superposition of the
contributions of all the elements, just summed up (in fact, you can treat a
long element as a series of much shorter segments strung together, which is
what the MoM programs do)
There is almost NO way that an element can be made to radiate preferentially
in one direction. (the exception would be where the current distribution is
nonuniform around the conductor, thus violating the "thin wire
"The parasitic element is effectively a tuned circuit coupled to the driven
element, much as the two windings of a transformer are coupled together. The
radiated energy from the driven element causes a voltage to be developed in
the parasitic element, which, in turn, sets up a magnetic field. This
magnetic field extends over to the driven element, which then has a voltage
induced in it. The magnitude and phase of the induced voltage depend on the
length of the parasitic element and the spacing between the elements. In
actual practice the length and spacing are arranged so that the phase and
magnitude of the induced voltage cause a unidirectional,
horizontal-radiation pattern and an increase in gain. "
I don't know that I'd describe two coupled pieces of a radiator as two
coupled tuned circuits. The transformer analogy is more correct, especially
with respect to the idea of the magnetic flux from one element coupling to
another. As a practical matter, voltages do not result in magnetic fields,
currents do. The magnetic field from the driven element induces a voltage in
the parasitic element, which then results in a current (since the element
is, after all a conductor), which in turn creates another magnetic field,
In modern usage, the interactions between the elements are described as a
matrix of mutual impedances (or admittances).
----- Original Message -----
From: "Keith Dutson" <email@example.com>
Sent: Wednesday, June 30, 2004 4:50 PM
Subject: RE: [TowerTalk] Funniest thing I've seen in weeks
> In the following link, read the section titled Operation.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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