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 267
ble, we’ll stick with these because of the simplicity of the networks required to feed
them.
When the two verticals (A and B) are fed in phase with equal currents, the radiation
pattern of Fig. 11.1A (idealized here) results. It is a bidirectional figure eight that is
maximum broadside to a line drawn between the two antennas. If there are no nearby
obstructions and the ground around the two verticals is reasonably flat and featureless,
and if the drive currents to the two antennas are well matched in both amplitude and
phase, a deep null is formed along the axis of the array—i.e., along the line drawn between
the elements A and B.
If the phase of the drive current to one of the elements is shifted by 180 degrees, the
pattern rotates 90 degrees (a quarter of the way around the compass) and now exhibits
directivity along the line of the centers (A-B), as depicted in Fig. 11.1B. This is often
called an end-fire pattern or an end-fire array.
Note that the forward gains and the exact shapes of the broadside and end-fire patterns
are seldom the same.
The simplest way to make sure the amplitudes of the feed currents to the two array
elements are identical is to bring a common feedline from the transmitter to a point
midway between the two elements and then feed each element from there. Figure 11.2A
shows the feedline configuration for driving the array elements in phase; note, in particular,
that lengths L 1 and L 2 must be the same, and ideally they should be cut from the
same roll of coaxial cable or hardline so that their impedances and velocities of propagation
are as closely matched as possible.
To switch the two-element array to its end-fire mode of Fig. 11.1B an extra 180-
degree phase shift must be added to the drive signal delivered to one or the other (but
not both) of the two elements. Although this
phase shift can be provided with a discrete
component LC network, a simple solution is
to add an extra l/2 section of the same cable
in the line to one element. When doing so, it’s
important to include the velocity factor (v F ) of
the cable in the calculation. v F is a decimal
fraction on the order of 0.66 to 0.90, depending
upon the specific transmission line used.
A
B
A
B
Figure 11.1A Pattern of two radiators fed in phase,
spaced a half-wavelength apart.
Figure 11.1B Pattern of two radiators fed out of
phase, spaced a half-wavelength apart.