Practical_Antenna_Handbook_0071639586

324 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
Driven element
Reflector
S
Tuning stub
Figure 13.6 Quad beam antenna.
0.11 S 0.22
One method for constructing a single-band, two-element quad is shown in Fig. 13.7.
This particular scheme uses a 12- × 12-in wooden plate at the center, bamboo (or fiberglass)
spreaders, and a wooden (or metal) boom. The construction must be heavy-duty
in order to survive wind loads. A better solution might be to buy a quad kit consisting
of spreaders and spider for either a two- or a three-element quad.
With most kits, more than one band can be installed on a single set of spreaders. The
size of the spreaders is set by the lowest band of operation, so higher-frequency bands
can be accommodated with shorter loops on the same set of spreaders. And while the
use of a spider allows element spacing proportional to wavelength on multiple bands,
there is no shortage of two-, three-, four-, and five-element quads that forsake proportional
spacing and simply nest concentric loops for three, four, and five bands on
spreaders that are perfectly vertical. Thus, there is no reason the construction technique
of Fig. 13.7 can’t be used to support loops for multiple bands as long as it is understood
that VSWR bandwidth and the variation of other key performance parameters such as
forward gain, F/B and F/S ratios across the operating band will be different on every
band.