Practical_Antenna_Handbook_0071639586

C h a p t e r 1 8 : a n t e n n a s f o r 1 6 0 M e t e r s 415
If the decision to insulate the tower from ground has been made, the insulator section
is not limited to being located at ground level. A number of DXers and contesters
are using elevated-base ground-plane antennas on 160 m. One advantage is that fewer radials
are needed; if the base of the ground plane is l/8 or higher above earth ground,
somewhere between four and eight elevated radials will provide ground efficiencies
comparable to 20 or more radials lying on the ground for a vertical monopole whose
(insulated) base is at ground level. The big disadvantage, of course, is that, all other
things being equal, the overall height of the tower increases by as much as l/8, or 65 ft!
But see Chap. 30 for a clever alternative.
Finally, it’s worth explicitly noting when considering any form of insulated-base
vertical:
• Insulated-base towers are more expensive.
• Lightning protection is simpler with a grounded-base tower.
Phased Verticals
Almost all serious topband DXers employ arrays of phased verticals to add transmit
gain in switch-selectable compass directions. To maximize radiation efficiency and
bandwidth, most try to make the array elements l/4 tall or higher, but arrays of shortened
verticals have merit, as well, as long as the user takes pains to minimize losses in
ground systems and matching units.
One favorite form of array on 160 m (and 80 m, too) is the four-square (also written
4-square). In general, these arrays are base-insulated and series-fed; because of this,
some amateurs install them as much as l/8 above ground and use elevated radials.
Whether elevated or ground-mounted, a 160-m four-square of l/4 elements is a marvel
to behold!
Typically, feedpoint currents are phased as 0 degrees in the rear element, –90 degrees
in the two side elements, and –180 degrees in the front element (i.e., the element
nearest the desired direction for maximum gain), but many variations in both element
phasing and interelement spacing to optimize one or another parameter exist. The definitive
compendium of the many forms these arrays can take is “ON4UN’s Low-Band
DXing” by John Devoldere, available from ARRL.
Four-squares are all-driven arrays, but parasitic arrays are equally useful and often
quite a bit less expensive or time-intensive to install. For most of us, any array of fullsize
l/4 elements does not fall in our definition of “practical”, so we won’t devote any
more space to them. However, some very worthy—and practical— alternatives exist.
Chief among those—whether employed in an all-driven or a parasitic array—is the
inverted-L, described later in this chapter.
Of course, the use of directional arrays for transmitting antennas enhances their
utility as receiving arrays. Some operators have run A/B tests comparing full-size foursquares
with dedicated receive antennas, such as 1l to 2l Beverages and concluded that
these transmitting arrays are usually as good as or better than the specialized receiving
antennas. But then again, as the saying goes, “You can never have too many antennas!”
Horizontal Antennas
Although subject to the limitations discussed at the beginning of this chapter, a horizontal
dipole can be an inexpensive solution to the problem of putting out a decent