Practical_Antenna_Handbook_0071639586

156 p a r t I I : F u n d a m e n t a l s
r
I 1 I A
) ) ) ) 2 ) ) ) )
Array axis
r
2 I 1
= I 2
180˚
Figure 5.3C Two-element end-fire array of dipoles
spaced l/2 on the array axis.
when changing the preferred direction
of maximum gain.
Nor is it necessary that the feedpoint
currents be equal in all elements.
Over the years, many AM broadcast
stations have employed multielement
arrays with unequal feed currents.
One common configuration uses a binomial
current distribution in the vertical
elements. (A five-element array
might have relative element currents
of 1-4-6-4-1, for instance.) Combined
Outer circle = 2.3 dB
more than for a single
dipole.
Figure 5.3D Overall pattern for the two-element
end-fire array in (C).
with very close element spacings, these arrays are capable of producing extremely high
gain patterns and deep nulls or side lobes. (The latter are important in the AM broadcast
band to prevent co-channel and adjacent channel interference to “protected” stations on
the same frequency in other parts of the country.)
Another form of array is the collinear. In this configuration, multiple “copies” of an
element type are laid (or stacked, if vertical) end to end and electrically connected “heel
to toe” through phasing sections designed to force the currents in all elements to be in
phase even though only one element is actually driven by a signal from the transmitter.
Many VHF and UHF verticals and mobile whips are collinears; the short lengths of
antennas for those frequencies make stacking very practical from a mechanical standpoint.
For a vertical, collinear stacking does not alter the shape of the azimuthal pattern
at all; instead, the usual objective is to “sharpen” the elevation pattern so that more gain
in all azimuthal directions is available at low elevation angles at the expense of (usually
useless) high-angle radiation. At VHF and UHF, collinear antennas are often commercially
sold as a complete array in a single assembly.
As we shall discuss in Chap. 8 (“Multiband and Tunable Wire Antennas”), when a
half-wave dipole is operated substantially above its design frequency, f C , it becomes a
pair of collinear arrays because the wire on each side of the center insulator is now longer
than l/4. As the operating frequency rises above f C , the maximum broadside gain
of the antenna grows relative to its value at f C because the additional wire length in the
radiating element possesses a greater length x current product. Eventually, however, current
reversal in adjacent half-wave segments begins to reduce the maximum broadside
gain of the array, which occurs when the total length of the wire is 5/4 l, corresponding
to an operating frequency near 9 MHz for an 80-m dipole. Above that frequency, the
maximum broadside gain starts to decrease with frequency, and eventually the array