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12-1 Rotatable dipole antenna Inset A shows conventional feed; inset B shows transformer feed
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can use two smaller lengths to make the larger lengths of pipe, and cut it to size This method is only available to those readers who have a commercial or industrial metals distributor nearby, because the 16-ft lengths are not generally available from hardware stores Bands higher than 15 m (ie, 12 and 10 m) can be accommodated by using the 10-ft lengths of tubing, but without the inductor The tubing is cut to the desired half-wavelength size and used directly
Yagi beam antennas
A Yagi antenna is one member of a class of directional beam antennas that are popular in the HF bands Figure 12-2 shows the pattern (viewed from above) typical of the beam antenna The antenna is located at point P and fires signals in the direction shown by the arrow The beamwidth of the antenna is the angle a between the points on the main lobe that are 3 dB down from the center point C
258 Directional beam antennas
Direction of propagation
Main lobe
3 dB
3 dB
12-2 Pattern of beam antenna
Sidelobe
Backlobe
A perfect beam antenna will have only the main lobe, but that situation occurs only in dreams All real antennas have both sidelobes and backlobes, also shown in Fig 12-2 These lobes represent wasted power transmitted in the wrong direction during transmission and interference opportunities while receiving The goal of the antenna designer is to increase the main lobe while decreasing the sidelobes and backlobes Figure 12-3 shows, schematically, the basic Yagi-Uda antenna (usually called simply Yagi) The driven element is a simple half-wavelength dipole fed in the center There are two additional elements: reflectors and directors These are called parasitic elements because they are not directly excited by RF, but rather,
Yagi beam antennas 259 they receive energy radiated from the driven element and then reradiate it The reflector is placed behind the driven element, and is typically about 4 percent longer than the driven element The director is placed in front of the driven element (relative to the direction of propagation) The director is typically about 4 percent shorter than the driven element Although there is no fixed rule regarding the number of either reflectors or directors, it is common practice to use a single director and a driven element for two-element beams, and a single reflector and a single director, in addition to the driven element, for three-element beams Again, additional reflectors can be added for four- and more element beams, but standard practice calls for addition of directors instead The length of the elements is given by K L ft [122] FMHz where L is the length, in feet FMHz is the frequency, in megahertz K is a constant The spacing of the elements is typically from 015 to 0308 wavelengths, although 02 and 025 are the most common values
Direction of propagation
Director
S Driven element Feedpoint
Reflector 12-3 Basic Yagi-Uda antenna
260 Directional beam antennas Example Calculate the approximate element lengths for a three-element 15m beam designed to operate on a frequency of 2139 MHz Solution: 1 Driven element (K 478): L K ft FMHz 478 ft 2139 MHz 2234 ft 2 Reflector (K 492): L K FMHz ft
492 ft 2139 MHz 23 ft 3 Director (K 4615): L K ft FMHz 4615 ft 2139 MHz 2158 ft 4 Element Spacing (K 142): L K ft FMHz 142 ft 2139 MHz 664 ft The elements of a rotatable beam antenna can be built in a manner similar to the rotatable dipole described earlier In the beam antenna, however, a boom is needed between the elements to support them The boom can be made of metal or wood In the case of a metal boom, the driven element must be insulated from the boom, even though the parasitic elements can be mounted directly to it In general, it is usually better to use wood as a matter of convenience Metal boom antennas can be obtained from commercial sources The wood boom is easy to build and maintain, even though a little less durable than a metal boom The feedpoint impedance of a dipole is on the order of 72 in free space, although the actual impedance will vary above and below that figure for antennas close to the earth s surface In addition, adding parasitic elements reduces the impedance even more The feedpoint impedance of the antenna is too low to be directly fed with coaxial cable, so some means of impedance matching is needed Some people feed the antenna through an impedance matching balun transformer Figure 12-4 shows the gamma match system The driven element of the Yagi is not
Yagi beam antennas 261 broken in the center, as in the case of the simple dipole The outer conductor, or shield, of the coaxial cable is connected to the center point of the driven element The center conductor is connected to the gamma match element The dimensions of the gamma match are 1 Gamma match length: L/10 2 Gamma match director: D/3 3 Spacing of gamma match from driven element: L/70 where L is the length of the driven element D is the diameter of the driven element The capacitor in series with the center conductor of the coaxial cable has a value of approximately 8 pF per meter of wavelength at the lowest frequency in the band of operation, or approximately, C 2400 pF FMHz [123]
The capacitor must be a high-voltage transmitting variable type In general, the gamma match capacitors are either air or vacuum variables There are three aspects to the adjustment of the Yagi antenna Resonance is determined by the length of the element The length is increased or decreased in order to find the resonant point This point can be determined by the use of a noise bridge, VSWR meter or other means The capacitor, and the shorting bar/clamp, are adjusted to match the impedance of the antenna to the transmission line impedance For the dimensions shown, the coaxial cable should be 52 (RG-58 or RG-8) It is not necessary to use tubing or pipes for the antenna elements in order to obtain the benefits of the Yagi beam antenna An example of a wire beam is shown in Fig 12-5 The wire beam is made as if it were two half-wavelength dipoles, installed parallel to and about 02 to 025 wavelengths apart from each other Although multielement wire beams are possible, the two-element version is the most common Perhaps the most frequent
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