Practical_Antenna_Handbook_0071639586

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design frequency is a fraction of a decibel less than in either of the other two
designs, and F/B ratio can drop from a maximum of 30+ dB at 14.200 to 15 to 20
dB at the frequency extremes. The 50-Ω SWR is under 3.0:1 anywhere between
14.000 and 14.350.
• The “Phone” column is a design that keeps the SWR under 2.0:1 across the
14.150 to 14.350 range that corresponds to the full Amateur Extra license
privileges in the United States. Front-to-back ratio is in excess of 30 dB at 14.250
and stays above 20 dB throughout the full 200-kHz segment.
The element tapering of Fig. 12.4A is used for this three-element Yagi. All half-Â
elements of all three designs in Table 12.2 use identical taper schedules except for the
length of the tip (½-in OD) sections, which is given in Table 12.2. Only the longest tip of
all—the half-element for the “CW” Reflector—is shown in Fig. 12.4A. Thus, assembly
of the six half-elements is identical, and uses exactly the same lengths of tubing, except
for the final setting of the tips.
In all three designs, the input impedance is very close to 75 Ω at the design frequency
(after passing through a 1:4 step-up balun or other transformer). Thus, the SWR
bandwidths in systems employing 75-Ω feedlines are all noticeably wider than the
stated 50-Ω bandwidths above. Free-space gain at the design frequency is approximately
8.5 dBi, typically varying ±0.25 dB throughout the useable bandwidth, but the
designs have been tweaked for mounting 70 ft (approximately one wavelength) above
flat ground consisting of “average” or “medium” earth.
Multimode DXers or contesters might choose the “CW” settings if the bulk of their
voice-mode activity is apt to occur below 14.220. Alternatively, adding an adjustable
ATU or fixed-component matching network at the equipment end of the transmission
line should easily permit reduction of the SWR at the transmitter or amplifier to under
2.0:1 across the entire 20-m band. One setting of the ATU or matching network is
Âsufficient—use a coaxial bypass relay or two-position switch to switch the matching
network in and out of the circuit for the two halves of the band with either the “CW” or
“Phone” design. With this approach, the main reason to choose the settings of one design
over the other is to place the best F/B ratios in either the low or the high end of the
20-m band.
To recap, the six element halves of each design differ in only two ways:
• The lengths of the outermost sections, or tips, of the three elements vary
according to the selected design frequency and which element they are part of
(as given by Table 12.2).
• The director and reflector are continuous tubing as they cross the boom, while
the driven element is split at the boom to allow connection of a 4:1 balun.
In general, where two tubes are telescoped, the length of internal “overlap” should
be 3 in or greater. For a detailed discussion of techniques for securing telescoping sections,
refer to the earlier section in this chapter on rotatable dipoles.
Wire Yagis
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 two-element wire beam is shown in
Fig. 12.5. The wire beam is made as if it were two half-wavelength dipoles, installed