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 275
two end-fire arrays are then combined in a two-element broadside array, thus enjoying
high forward gain and deep nulls to the rear, thanks to the principle of pattern multiplication.
Both the eight-circle and the four-element combined end-fire/broadside array
building block are described in detail by John Devoldere in his excellent book ON4UN’s
Low-Band DXing, published by ARRL.
Although circle arrays for a given frequency range require less acreage than four
bidirectional Beverage antennas, an absolute minimum for a 1.8-MHz eight-circle is
about three acres, assuming ideally shaped property. Recently, John Kaufman, W1FV,
has designed and published (through ARRL) a compact dual-band version of a ninecircle
array requiring a very modest 140-ft-diameter footprint (corresponding to slightly
more than a half-acre). The W1FV array consists of eight elements around the periphery
of the circle, surrounding a single element at its center. It functions as a three-element
in-line array that—just like the eight-circle—can be switched in 45-degree heading increments.
The 160-m design acquits itself quite nicely on 80 m, as well, thus eliminating
the need for the added costs (and acreage,) of a separate 80-m receiving array.
As we have said many times throughout this book, there is no “free lunch”. Highperformance
arrays of short receiving elements are precision designs, requiring close
matching of signal amplitudes and differential phase shifts from element to element.
Poor feedline installations and sloppy grounding techniques, among other things, can
lead to common-mode signal pickup and other ills that will easily nullify the directivity
and SNR improvements possible with these antennas.
Grounding
It goes without saying that no ground-mounted or ground-plane vertical will perform
very well without an adequate ground system beneath it. This is especially true for arrays
of verticals, as well. Each element of an array of vertical monopoles should have its
own extensive ground system (the W1FV array is an exception to this), whether composed
of sheets of copper flashing, two dozen or more radials each generally as long as
the vertical is tall, or saltwater. If radials or other conductors are employed, they should
not be allowed to overlap unless they are insulated or unless all points of possible contact
have been soldered or mechanically joined. Intermittent or oxidized joints can raise
the background noise level when receiving and result in the radiation of spurious outof-band
signals when transmitting. Refer to the grounding information in Chaps. 9 and
30 for a more extensive discussion of grounding techniques.