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One of the more saintly antennas used on the VHF boards is the halo (Fig 18-12) This antenna basically takes a half-wavelength dipole and bends it into a circle The ends of the dipole are separated by a capacitor In some cases, a transmitting-type mica button capacitor is used, but in others (and perhaps more commonly), the halo capacitor consists of two 3-in disks separated by a plastic dielectric While air also serves as a good (and perhaps better) dielectric, the use of plastic allows mechanical rigidity to the system
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The quad antenna was introduced in the chapter on beams It is, nonetheless, also emerging as a very good VHF/UHF antenna It should go without saying that the antenna is a lot easier to construct at VHF/UHF frequencies than it is at HF frequencies! Figure 18-13 shows a modest example There are several methods for building the quad antenna, and Fig 18-13 represents only one of them The radiator element can be any of several materials,
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VHF/UHF antenna examples 357
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Radials (four or more)
18-11 Small VHF groundplane construction
358 VHF/UHF transmitting and receiving antennas
Feedpoint 18-12 Halo antenna
including heavy solid wire (no 8 to no 12), tubing, or metal rods The overall lengths of the elements are given by: Driven element: L= Reflector: L= Director: L= 11,248 in FMHz [1811] 12,562 in FMHz [1810] 11,826 in FMHz [189]
There are several alternatives for making the supports for the radiator Because of the lightweight construction, almost any method can be adapted for this purpose In the case shown in Fig 18-13, the spreaders are made from either 1-in furring strips, trim strips, or (at above 2 m) even wooden paint stirring sticks The sticks are cut to length, and then half-notched in the center (Fig 18-13, detail B) The two spreaders for each element are joined together at right angles and glued (Fig 18-13, detail C) The spreaders can be fastened to the wooden boom at points S in detail C The usual rules regarding element spacing (015 to 031 wavelength) are followed See the information on quad antennas in Chap 12 for further details Quads have been successfully built for all amateur bands up to 1296 MHz
VHF/UHF antenna examples 359
Overall lengths: Reflector: L Director: L Driven element: L 12,562 in FMHz 11,248 in FMHz 11,826 in FMHz
Wire or rod
Glue
Notch
18-13 Quad loop construction on VHF
360 VHF/UHF transmitting and receiving antennas
VHF/UHF scanner band antennas
The hobby of shortwave listening has always had a subset of adherents who listen exclusively to the VHF/UHF bands In those bands are found a rich variety of services from commercial two-way radios, police and fire systems, and others Many of these people are serious DXers, but others have a little of the voyeur in them (they like to listen to what s going on in the community by monitoring the police frequencies) As long as they neither respond to the scene of a police or fire action, nor use the information gained from monitoring in an illegal manner, they are perfectly free to listen in A few people have an unusually practical element to their VHF/UHF listening At least one person known to the author routinely tunes in the local taxicab company s frequency as soon as she orders a cab She then listens for her own address She knows from that approximately when to expect the cab In the early 1960s, when all VHF monitor receivers were crystal-controlled (and relatively expensive), the listener had a very limited selection of frequencies to choose from Having worked in various shops that sold those receivers, I can recall that most of our customers tended to be police officers, fire fighters (or volunteers), or journalists covering the local crime news However, modern scanners operate on both major VHF bands (low and high), plus UHF bands and others
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