Practical_Antenna_Handbook_0071639586

C h a p t e r 5 : a n t e n n a A r r a y s a n d A r r a y G a i n 159
time the radiated field from element 1
reaches 2, it has accumulated an additional
lag of 90 degrees due to the distance
it had to cover. Thus, radiation
along the axis heading in the direction of
1 to 2 is the algebraic sum (from 2 outward)
of two equal fields that are 180 degrees
out of phase, and the radiation field
is canceled in that direction.
In the opposite direction along the
axis of the two elements, heading from 2
to 1 and beyond, the radiation from the
feedpoint current in 2 (which leads the
feedpoint current in A by 90 degrees) exactly
loses the 90-degree lead as it covers
Outer circle =
3.0 dB greater
the distance from 2 to 1. Thus, from 1 outward,
radiation fields from 1 and 2 are
than single element.
perfectly in phase, and reinforced.
/4
Between these two extremes, the amplitude
of the radiation pattern at a distant
receiving site is a smoothly changing
value from maximum to minimum signal
strength, symmetrically about each halfcircle.
The resulting equation for far-field
Dipole orientation
signal strength as a function of azimuth
(compass heading) is proportional to 1 +
Figure 5.5C Cardioid pattern from two l/2 horizontal
cosq and has the shape shown in Fig.
dipoles.
5.5B. For a pair of l/4 vertical monopoles
over perfect ground, the array has a maximum
gain of 3.1 dB relative to a single identical monopole.
We say this two-element array produces a unidirectional cardioid pattern. (AM clearchannel
broadcast station WBZ, 1030 kHz, has used this pattern from their two-tower
site overlooking the Atlantic Ocean at Hull, Masssachusetts, to blanket the eastern
United States with a commanding signal for decades!) For that reason, the best antenna
elements to use for forming a cardioid are omnidirectional radiators in the azimuthal
plane—verticals, in other words—but horizontal l/2 dipoles at identical heights above
ground and oriented so that their maximum radiation is broadside to the array pattern
maximum are a possible alternative if additional side nulls and rear fishtail lobes (Fig.
5.5C) in the pattern are acceptable.
Feeding All-Driven Arrays
Thus far we have not discussed the practical issues associated with feeding all-driven
arrays. In general, feed systems for the various members of the loop and collinear families
are the easiest because the second element is driven by the first element, so there is
little or nothing for the user to do to make sure currents in the two elements are equal
or nearly so. In theory, a l/2 dipole in free space has a feedpoint impedance of 73 W
resistive. Most of us are not so lucky, however, as to be able to hang our dipoles in free
space, so virtually all MF and HF dipoles are something other than 73 W, and they often