Practical_Antenna_Handbook_0071639586

302 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
Two 40-m beams with linear loading currently on the market are the MFJ/Hy-Gain
Discoverer series (in one-, two-, and three-element versions) and the M 2 40M2LL and
40M3LL two- and three-element beams. The Discoverer DIS-72 2-element beam supersedes
the venerable 402BA. All these antennas insert an insulator midway out each
shortened half-element; a loading wire runs from one side of the insulator back toward
the boom, then back out to the other side of the insulator, thus effectively lengthening
the element by folding a portion back on itself. Note, however, that linear loading wires
are inherently low-Q components because their inductance is spread out over a large
area and they have more loss resistance than high-Q coils for an equivalent shortening
of the elements. They may also be subject to a slight (but temporary) shift in inductance
with icing.
Feeding the Yagi
The feedpoint impedance of a dipole is on the order of 73 Ω in free space, although the
actual impedance will vary above and below that figure for antennas within a wavelength
of the earth’s surface or other ground plane. The addition of parasitic elements
properly spaced and adjusted for maximum forward gain and front-to-back ratio reduces
the impedance even more. Often the feedpoint impedance of the antenna is too
low—typically between 15 and 30 Ω—to be directly fed with coaxial cable, so some
means of impedance matching is needed. For those designs, three feedpoint matching
systems are in common use at HF:
• Balun
• Gamma match (and its cousin, the T-match)
• Hairpin match
Balun Feed
Some people feed the antenna through an impedance-matching balun transformer.
However, not everyone appreciates that the balun for most three-element direct-feed
Yagis needs to be a step-down configuration, dropping the 50- or 70-Ω impedance of the
coaxial cable or hard line to one-fourth of either value, depending on the approximate
input impedance of the specific antenna involved. See Chap. 4 for details.
Of course, the primary purpose of a balun—and the origin of its name—is to drive
balanced feedpoints, such as those found on most horizontal antennas, including dipoles
and the driven elements of Yagis, with unbalanced feedlines, especially coaxial
cables. When used, the balun should be as close to the feedpoint as possible, such that
the wires from the balun terminals to the feedpoint are almost nonexistent. Otherwise,
the wire lengths need to be included in the calculations associated with the
length of the driven element itself. Ideally, the “ground” terminal on the unbalanced
side of the balun should be directly grounded to the boom of the Yagi at the balun, to
minimize the impact of lightning-induced surges on the transmission line.
Gamma Match
Although our earlier discussion of the rotatable dipole—and later the driven element of
multielement Yagi beams—was based on a l/2 dipole split at the center to allow a direct
series connection to a feedline, that is not the only possible way to feed the dipole.