There is an excellent article in the July/August 2012 issue of QEX describing
how the author improved the performance of a Beverage by breaking it into two
in-line segments coupled by a pair of conventional Beverage matching
transformers. He also provides detailed construction information for a
switched bidirectional Beverage that he built.
He observes that:
- a long Beverage produces a stronger signal and better directivity when it is
broken into two segments and a proper length phasing line is inserted between
the two matching transformers.
- a short Beverage (two coupled inline Beverages, each either 0.15 or 0.2
wavelengths long) produces improved directivity when a 1/8 wavelength coax
phasing line in inserted between the matching transformers producing
directivity similar to a conventional Beverage of twice the length of two
coupled short Beverages.
The article has many azimuthal and elevation plots and tables from his EZNEC
modelling, for example:
Beverage length Gain (dBi) F/B 3 dB beamwidth
2 x 0.15 -21.1 9.9 92 degrees
0.5 -15.1 13.5 93
2 x 0.2 -18.0 11.0 87
0.75 -11.5 10.7 93
1.0 -9.3 15.5 79
73
Frank
W3LPL
---- Original message ----
>Date: Tue, 24 Jul 2012 09:12:42 -0400
>From: "Tom W8JI" <w8ji@w8ji.com>
>Subject: Re: Topband: Beverage antennas
>To: "Guy Olinger K2AV" <olinger@bellsouth.net>
>Cc: topband@contesting.com
>
>Hi Guy,
>
>Good topic.
>
>> The losses as a reversible beverage would far exceed those used as a
>> balanced feedline, because of the balance partly cancelling fields in the
>> dielectric between them.
>
>
>Slow wave structures are more common in microwave. Anything that increases
>capacitance or inductance per unit foot will slow wave propagation. The
>formulas are 1/f*sqrt LC for wavelength, and 1/sqrt LC for phase velocity.
>
>I built short Beverages with multiple ferrite sleeves, because a thick
>dielectric was impossible. There are limitations for how slow we can make
>the wave that change with antenna length, after which the antenna reverses.
>
>This is why most slinky Beverage "theory" put out was nonsense, because the
>real action isn't packing a wave of wire in a small area....but rather
>slowing the velocity factor a correct amount. I *think* the limit for a half
>wave structure is a Vp of about .5 before the system reverses and starts
>firing backwards, but it has been years since I looked at slow wave
>structures for 160.
>
>Velocity factor in this case is caused by the interaction of dielectric and
>electric field, specifically the increase in capacitance. The electric field
>is actually more concentrated between the two wires when voltages are
>out-of-phase, so the dielectric has more effect.
>
>When in parallel mode, more of the field is outside the line dielectric in
>air, although the field is more intense near the conductor.
>
>By the way, this is one of the "rubs" in making a Beverage really long.
>
>> Is there really anyone actually using this stuff for reversible beverages
>> on 160??? This stuff could make you plumb deaf on 160.
>
>I would avoid things that slow the wave in longer antennas. Phase is already
>bad enough in long low wires. A very thick dielectric certainly would not
>help as an antenna becomes longer. As it becomes longer, the arriving wave
>would be increasingly out-of-phase with current in the wire. But the primary
>velocity factor and loss effects are going to be in differential mode, where
>the field is most concentrated between conductors.
>
>73 Tom
>
>_______________________________________________
>UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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