Practical_Antenna_Handbook_0071639586

CHAPTER 10
Wire Arrays
When a single wire gets substantially longer than a half-wavelength, or when
multiple wires are employed, we enter the realm of wire arrays. In general,
the purpose of using an array instead of a simpler antenna is to get higher
forward gain, better rejection off the back and/or sides, or a combination of both. Few
antenna projects bring as much reward at so low a cost as wires, and that’s especially
true of wire arrays.
Longwire Antennas
The end-fed longwire (or, more simply, longwire) is a name given to any of several types
of resonant and nonresonant antennas. The longwire antenna is capable of providing
gain over a dipole and (if it is long enough) a low angle of radiation (which is great for
DX operators!). But these advantages become substantial only when the antenna is several
wavelengths long. Nonetheless, longwire antennas have been popular for decades—often
because the layout of a particular property lends itself far more readily to
locating one end of an HF wire close to the radio room, if not actually in it!
Any given longwire antenna may be either resonant or nonresonant, depending
upon the operating frequencies used. In the old days, most longwires for use on the
amateur MF and HF bands could be cut to be reasonably close to resonant on all the HF
bands because those bands were harmonically related to each other. But with the addition
of the 5-, 10-, 18-, and 24-MHz band segments, that’s not possible anymore.
Figure 10.1 shows the classic random length, nonresonant longwire antenna. It consists
of a wire radiator that is at least a quarter-wavelength long but is most often much
longer. The specific length is not critical, but by convention it is usually assumed to be
a wavelength or more at the lowest frequency of operation anticipated. Nonetheless, a
70-ft wire fed at one end against a good RF ground will acquit itself well on all HF
bands above 3.5 MHz.
The longwire is end-fed. Therefore, when it is an even number of quarter-wavelengths
it will exhibit a very high feedpoint impedance (a few thousand ohms), while
for lengths that are odd multiples of l/4 it will present a much lower impedance (perhaps
25 to 100 Ω). Thus, it is imperative that an antenna tuner (ATU) be used, since even
those transceivers with built-in antenna tuners typically cannot match the very high
feedpoint impedances of end-fed wires that are approximately a multiple of l/2 in
length.
Often, the most important factor affecting the performance of an end-fed longwire
is not the antenna—it’s the ground system it’s fed against! Although we may be aware
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