The bisquare antenna, shown in Fig, is similar to the other large loops, except that it is wavelength/2 on each side, making a total wire length of two wavelengths. This antenna is built like the diamond loop shown earlier (i.e., it is a large square loop fed at an apex that is set at the bottom of the assembly). In this case, the loop is fed either with an antenna tuning unit (to match a 1000-Ω impedance) or a quarter-wave length matching section made of 300-Ω or 450-Ω twin-lead transmission line. A 1:1 balun transformer connects the 75-Ω coaxial cable to the matching section.
The bisquare antenna offers as much as 4-dB gain broadside to the plane of the antenna (i.e., in and out of the book page), in a figure-8 pattern, on the design frequency. It is horizontally polarized. When the frequency drops to one-half of the design frequency, the gain drops to about 2 dB, and the antenna works like the diamond loop covered previously.
The image is misleading to the unwary: the loop isn't actually closed-circuited; it's open circuited at the top.
ReplyDeleteFeeding with ladder line to a tuner is also often recommended over coax.
Yep, blindly reproducing plans from elsewhere doesn't help the inexperienced - a bisquare is not a loop at all but two full wave elements mounted in an angled configuration opposite each other in a diamond shape, open at the top. Feed it with 300 ohm line via an ATU - about 8-9 dBi gain and very effective.
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