Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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222 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s ATU will allow wide-range impedance matching of both balanced and unbalanced loads (antenna + feedline). High-frequency 1000W or greater ATUs on the market at this writing include units from Nye Viking, LDG, Ameritron, MFJ, Palstar, Vectronics, and others. Two excellent units from “yesteryear” that can still be found at hamfests, flea markets, and on the various Internet used equipment sites are the E. F. Johnson Kilowatt Matchbox (which can match a distinctly wider range of impedances than its little brother, the 275-watt Matchbox) and the Dentron Super Super Tuner. Unfortunately, the Johnson Matchboxes are not of much use below 3.5 MHz. Be aware, when searching for the proper ATU, that “low loss” and wide matching range translate into a large enclosure. In today’s world, a low-loss, legal-limit ATU is often the largest “box” in the radio shack, dwarfing transceivers and power amplifiers alike! The antenna of Fig. 8.2 is a center-fed dipole that exhibits the familiar figure eight or doughnut-shaped radiation pattern at or near its fundamental frequency—i.e., when each side of the antenna is approximately l/4 in length. As discussed in Chap. 6, as the operating frequency is increased, the dipole legs become longer and longer in terms of wavelength. The effect of this is to cause the peak amplitude of the main radiation lobe first to grow even larger than it is for a l/2 dipole and then to decline and break into additional lobes that begin to pop up at other angles relative to the axis of the antenna. Fig. 8.3 shows the radiation patterns for an 80-m dipole operated in free space at a few selected higher frequencies. When used closer to earth, however, the nulls in the radiation pattern are nowhere near as sharp and as deep as shown here. As a result, this antenna will actually be useable regardless of the direction of the signal you’re attempting to hear or work. At higher frequencies, a dipole in free space that is resonant on 3.6 MHz will exhibit resistive input impedances ranging from 40 or 50 Ω up to 5K Ω or so (the second harmonic is often the worst), and reactances up to perhaps 2000 Ω (both positive and negative). Adding a 40-m dipole to the same feedpoint can substantially lessen the range of impedances that must be matched on the higher bands. There is no requirement that a dipole be fed in the center; that’s simply a convenience to simplify matching of transmitters to feedlines and feedlines to antenna feedpoint impedances. Nor does the pattern of a l/2 wire change as a result of where the feedpoint is located. Feeding a l/2 dipole at one end means the feedpoint impedance will be very high (a few thousand ohms), but a good ATU should be able to handle this. Figure 8.4 shows the once-popular end-fed Zepp antenna. This antenna uses a half-Â wavelength radiator but it is fed at a voltage node rather than a current node (i.e., the end of the antenna rather than the center). Typically, 450- or 600-Ω parallel-conductor air-dielectric open-wire transmission line is used to feed the Zepp because of the high voltages on the line as a result from the extreme impedance mismatch between the line and the antenna. In theory, the line can be any length, but the task of the ATU is simplified if the length is an odd number of quarter-wavelengths for those bands where the antenna length is a multiple of l/2 at the operating frequency. When that condition is met, the transmission line transforms the high feedpoint impedance to a much lower value that is more apt to fall within the ATU’s range. For example, a l/4 section of 600-Ω open-wire line will transform a 3000-Ω feedpoint impedance at one end of a l/2 dipole down to 120 Ω—usually an easy match for an ATU! As the operating frequency is raised above the point where the wire is l/2 in length, the radiation pattern begins to depend on the location of the feedpoint. At the fundamental operating frequency (80 m in our example), there is no difference in radiation

C h a p t e r 8 : M u l t i b a n d a n d T u n a b l e W i r e A n t e n n a s 223 A. B. C. D. E. F. G. H. Figure 8.3 Patterns for an 80-m center-fed dipole on higher frequencies (solid line) versus its 80-m pattern (broken line).

C h a p t e r 8 : M u l t i b a n d a n d T u n a b l e W i r e A n t e n n a s 223<br />

A.<br />

B.<br />

C.<br />

D.<br />

E.<br />

F.<br />

G. H.<br />

Figure 8.3 Patterns for an 80-m center-fed dipole on higher frequencies (solid line) versus its 80-m pattern<br />

(broken line).

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