Practical_Antenna_Handbook_0071639586

C h a p t e r 1 3 : C u b i c a l Q u a d s a n d D e l t a L o o p s 319
F/B ratio at 21.2 MHz. Perhaps more important, however, the forward gain
varied by less than 0.6 dB across the entire band and the VSWR never rose
above 1.5:1 anywhere in the band when fed directly with 50-Ω cable (i.e., with
no matching network required).
• A two-element quad modeled with 0.21l (about 10 ft) spacing exhibited 7.6 dBi
forward gain with F/B and F/S rejection comparable to the Yagi just described.
Its input impedance at the design frequency was 120 Ω, which can be
transformed down to 48 Ω with a l/4 section of 75-Ω cable located at the
feedpoint. However, there was substantial variation in the input impedance
across the band, resulting in a much narrower VSWR bandwidth, and the F/B
and F/S ratios showed a 2:1 variation, as well.
• At element spacings ranging between 0.11l (about 5 ft) and 0.21l it is possible
to maximize some performance specs near the design frequency at the expense
of others, but the two-element wire quad always shows a greater variation in
input impedance (hence, VSWR bandwidth) and F/B or F/S ratios with
frequency than does the two-element Yagi constructed of either copper wire or
aluminum tubing.
• On the whole, it is easier to maintain F/B and F/S ratios in excess of 10 dB at the
design frequency with the two-element quad, but these ratios show much
greater variation across the band than do those of the Yagi.
• It is somewhat easier to adjust the Yagi driven element (in both the model and
real life) by lengthening or shortening the element. Similar adjustments to the
quad driven element affect all four legs unless the supporting frame is distorted.
Like the basic 1l loop, one advantage of the cubical quad is footprint. The “wingspan”
of each quad element is roughly half that of each corresponding element of a
full-size Yagi, so the turning radius of a two- or three-element quad will almost always
be somewhat smaller than that of a Yagi of the same number of elements. The advantage
is somewhat less when the quad is compared with a Yagi employing traps, loading
coils, or linear loading wires to shorten its elements.
Proximity to earth is no boon to the quad. With the top leg of each element at 1l
(e.g., 46 ft on 15 m), the two-element quad exhibits almost exactly 1-dB advantage over
the Yagi at the same height over average ground, albeit at a slightly higher wave angle
(15 degrees versus 14 degrees). As both antennas are brought closer to ground, the quad
begins to lose its gain advantage, and by a height of l/2 the Yagi actually enjoys a
0.5-dB advantage in forward gain, which occurs at 26 degrees versus the quad’s peak at
29 degrees. Again, this is not surprising, since part of the power fed to the quad goes
into the lower leg, where the greater proximity to earth causes it to favor relatively high
wave angles.
Feeding the Quad
The driven element of a cubical quad can be fed in the center of a horizontal side (Figs.
13.1A and 13.3A), in the center of a vertical side (Fig. 13.3B), or at a corner (Fig. 13.3C). In
all cases, the currents on opposing sides of the quad geometry are in phase; thus, maximum
gain is along the axis of the loop—that is, on a line perpendicular to the plane of
the loop. Thus, for all the quads of Fig. 13.3, maximum gain is into and out of the page.