Practical_Antenna_Handbook_0071639586

266 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
• Nearby thunderstorms, on the other hand, can be heard equally well regardless
of their location relative to the vertical, since the propagation mode is likely
direct line of sight from clouds in the region to the antenna.
• Man-made noise (“QRM”) is usually (but not always) nearby and unaffected by
ionospheric propagation conditions. Today most man-made QRM originates in
the high-speed microprocessors and digital circuits that infest all the appliances
and electronics gear in our homes and offices or those of our neighbors. In
general, we have little or no control over the direction of these sources from our
vertical, so we can assume that their compass headings are likely to be randomly
distributed.
Directivity and Phasing
We can improve our ability to hear and to be heard on any frequency—but especially on
the MF and lower HF bands—where we are using a vertical antenna by operating two
or more in an array. As we saw in Chap. 5, arrays can provide increased field strength
in some directions at the expense of strength in other directions. This holds for both
transmitting and receiving, so an array of vertical antennas can help overcome all but
one of the limitations itemized in the preceding bullet list. (An array will not help us
reduce atmospheric noise or QRM coming from the same direction as a weak signal we
are trying to copy.)
Most AM broadcast stations have used arrays of vertical antennas for decades. Consequently,
the best radio engineers of the past century have collectively designed and
evaluated far more combinations of verticals than any one author could do on his or her
own in a lifetime, and there is much to be learned and borrowed from the AM broadcast
band favorites. Many standard patterns for two-, three-, four-, and five-vertical arrays
dating from before World War II exist in the literature. Most of the more complicated
ones are irrelevant to anything other than the AM broadcast band, but the simpler combinations
have many virtues useful to amateurs and others.
Of course, amateurs and shortwave or broadcast band listeners usually have a requirement
that AM broadcast stations do not: Most arrays must be designed so they can
be switched or steered to more than one compass heading.
To do the subject of vertical arrays full justice would require a separate book at least
as big as this one. So this chapter will serve primarily as an overview of some simple
but effective arrays that can be formed with the proper placement and feed systems.
In general, an array of verticals involves two or more antennas in close proximity,
each fed at a specified phase angle and amplitude relative to a common reference in
order to produce the desired radiation pattern. Often the common reference is simply
the RF applied to one of the antennas in the array that has been selected to serve as the
reference point.
Two-Element Array
Figure 11.1 shows the possible patterns for a pair of vertical antennas spaced a halfwavelength
(180 degrees) apart and fed in phase or (180 degrees) out of phase. Although
other phasing relationships between the drive currents to the two antennas are possi-