> From: W2pm@aol.com
To: <topband@contesting.com>
> Date: Thu, 31 Jul 97 13:07:07 +0000
Hi Pete,
> Does anyone have any experience with a slinky beverage?
I played around with slow wave structures (which is what that would
be) in the 70's.
> This antenna should behave like a 1 wavelength long end-fed wire and when
> terminated should provide the expected directivity. Any opposing views to
> this?
Coiling the wire does several things. One thing is it increases the
phase delay along the wire, another is it increases the
surge impedance of the wire. There are other effects also, but those
are the main issues.
500 feet of wire in a ten foot area does NOT work like 500 feet of
wire stretched out. The Beverages directivity is determined by
the spatial area it occupies as well as the phase delay along the
wire.
The ideal phase delay depends on the spatial (or physical) length of
the antenna, as well as the rate of propagation along the wire. For
a half wave long physical area, optimum Vp is about .5 freespace.
Kraus covers this in his book Antennas, as does "Kuecken" on page 72
in Antennas and Transmission Lines (published in paperback by MFJ as
book number 3305). The Kuecken book has an entire section on
travelling wave antenna theory, and Kuecken is rated in the top 1%
of inventors based on patents held.
I'm sure, from my experiments in the 70's and theory, the
impedance of a "slinky Beverage" is much higher than the same six
non-coiled conductor. I'm also sure the optimum Vp varies with the
overall length of the structure.
You'll probably just have to build one and see if it makes you happy
or not, with the understanding that it probably will never be as good
as a conventional Beverage unless you stumble on a good combo.
You could model it on a NEC based program, buy using a large number
of inductive loads along a thick wire. You can watch the pattern and
impedance change as you change the inductance.
73, Tom W8JI
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