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" <[email protected]> >Subject: Re: Topband: Beverage antennas >To: "Guy Olinger K2AV" <[email protected]> >Cc: [email protected] > >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
