----- Original Message -----
From: "Keith Dutson" <kjdutson@earthlink.net>
To: <towertalk@contesting.com>
Sent: Wednesday, June 30, 2004 8:08 PM
Subject: RE: [TowerTalk] Funniest thing I've seen in weeks
> Interesting critique of a published article. Thanks.
If I can find out how to contact the author/publisher (presumably someone in
DoD, since it's a Navy publication) I'll be working on a suitable
correction.
>
> However, I did not post the link to provoke a response. I think it is a
> good, simple explanation for the original post:
It's a good explanation of why the terminology exists. It's a terrible
explanation of how it actually works. Such "explanations", while initially
satisfying, can lead one down a horrible path (been there, done that..).
There is much "lore" in the antenna business like this, that is a reasonable
sounding explanation, but factually incorrect. Another example might be the
current distribution in antennas with lumped loading. Lots of very
plausible sounding explanations, many of which have problems. Part of the
problem is that simple antennas (such as hams use) were largely developed
before modern analytical techniques existed. There was a huge amount of
experimentation that produced antennas that work fairly well, and some
"explanations of why they work" came about at the same time. Trying to
derive Yagi optimization and design principles from analytic expressions of
mutual impedances is a nightmare and a LOT of hard work. In many cases,
simplifying assumptions are made to make it tractable for a limited set of
cases.
>
> -----------------------------
> Why is the longer element called a reflector while the shorter element
> called a director> What is the origin of the naming?
> Basically, don't they both re-radiate because of the same reason?
> -----------------------------
>
> I doubt that our dialog has helped much.
I think it has...
The naming is historical, based on a misunderstanding of how Yagi's work.
And, Basically, they all radiate for the same reason.. accelerating charge.
>
> Keith
>
> -----Original Message-----
> From: Jim Lux [mailto:jimlux@earthlink.net]
> Sent: Wednesday, June 30, 2004 9:56 PM
> To: keith@dutson.net; towertalk@contesting.com
> Subject: Re: [TowerTalk] Funniest thing I've seen in weeks
>
> 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.
>
> For example:
> "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
> approximation")
>
> Again:
> "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,
> etc.
>
> In modern usage, the interactions between the elements are described as a
> matrix of mutual impedances (or admittances).
>
>
>
> ----- Original Message -----
> From: "Keith Dutson" <kjdutson@earthlink.net>
> To: <towertalk@contesting.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.
> >
> > http://www.tpub.com/neets/book10/42l.htm
> >
> > Keith
> >
> >
>
> _______________________________________________
>
> 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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_______________________________________________
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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