Practical_Antenna_Handbook_0071639586

C h a p t e r 1 1 : V e r t i c a l A r r a y s 273
A Phase B Phase C Phase Heading (Degrees) Gain (dBi) Elev. F/B (dB)
0° 0° Passive S (180°) 5.4 28° 3.8
Passive 0° 0° NE2 (060°) 5.4 28° 3.8
0° Passive 0° NW1 (300°) 5.4 28° 3.8
0° 180° Passive E&W (090° and 270°) 3.1 23° 0
0° Passive 180° NE1 & SW1 (030° and 210°) 3.1 23° 0
Passive 180° 0° SE2 & NW2 (150° and 330°) 3.1 23° 0
0° Passive Passive SW2 (240°) 4.2 28° 10
Passive 0° Passive NW1 (300°) 4.2 28° 10
Passive Passive 0° N (000°) 4.2 28° 10
0° 0° 180° E&W (090° and 270°) 4.8 23° 0
0° 180° 0° SE2 & NW2 (150° and 330°) 4.8 23° 0
180° 0° 0° NE1 & SW1 (030° and 210°) 4.8 23° 0
0° 0° 0° Omnidirectional 1.8–2.0 44° 0
Table 11.1 Pattern Gains for Three-Element Equilateral Array of Fig. 11.5.
In practice, at any specific location the triangular footprint of the three verticals can
be rotated up to 60 degrees in either direction relative to the compass headings shown
to orient the best patterns where they can be of most help. (Notice that each type of pattern
repeats every 120 degrees.) Some of the patterns listed in the table have little to
recommend them, but, as a general rule, most amateur DXers on the MF and HF bands
find that breaking the 360-degree compass rose into six 60-degree directions is more
than adequate.
If space is at a premium, or if multiband operation of an array of trap verticals is
desired, the same equilateral triangle can be used with shorter spacings between the
verticals. For years, Hy-Gain published an application note that detailed how to feed
three of their 18-AVQ trap verticals spaced l/8 apart at the lowest frequency (80 m)
they covered. One of the difficulties with spacing other than l/2 is that 180-degree
phase shifts in drive current are no longer matched by the phase shift undergone by the
radiated wave as it heads from one element to another; instead, spatial phase shifts of
45 degrees and 90 degrees are involved on 80 m and 40 m, respectively, and multiples
of 360 degrees (i.e., in phase) on bands above 20 m. Nonetheless, directivity and gain
are possible with such a system.
Four-Square Array
Another popular way to obtain directivity that can be switched to multiple compass
headings is the four-square (or 4-square) array. In its original implementation, four l/4
vertical monopoles are located at the corners of a square that is l/4 on a side. Drive
currents of equal amplitudes are applied to all four elements, but with three different