Practical_Antenna_Handbook_0071639586

) ) )
) ) )
158 p a r t I I : F u n d a m e n t a l s
Anywhere to
left of element 1:
I 1
= I 2
, so
I 1
lags I 2
by 90˚.
Anywhere to
left of element 2:
I 1
= -I 2
, so
I total
= 2I
I 1
) ) )
I 2
) ) )
I total
= I 1
= -I 2
= 0.
4
Figure 5.5A Two-element quadrature or cardioid array.
be broken and fed anywhere on that path. If a vertically suspended loop is fed in the
middle of a side, it becomes predominantly vertically polarized and may well exhibit
better ability to work or hear long-haul DX. If a horizontally suspended loop is fed from
the “next” side, the azimuthal orientation of maximum performance will rotate 90 degrees.
Unfortunately for the HF loop, low dipoles and arrays of low dipoles are (usually
adversely) affected by the ground beneath them. The effect of the ground, in fact, is to
create yet another array with its own array factor, so proper analysis of the operation of
a loop antenna or array must take that into account. (Array factors multiply each other.)
Ground reflection effects are discussed later in this chapter. Chapter 13 goes into loops
and quads in much more detail.
Thus far we have limited the discussion
of arrays to those with feedpoint
currents that are either in phase or
out of phase. Actually, any phase relationship
between feedpoint currents
can be used, but as a rule the math gets
more complicated, the patterns less intuitive,
and the physical implementations
more difficult when the phase
difference between fed elements is
other than 0 or 180 degrees.
One popular array that employs
quadrature phasing of element currents
is the two-element cardioid array (Fig.
5.5A). Here two identical omnidirectional
radiators (vertical monopoles,
for instance) spaced l/4 apart are fed
Outer circle =
3.1 dB
greater than
single element.
Figure 5.5B Cardioid pattern using omnidirectional
radiating elements.
with currents of identical magnitude
phased 90 degrees apart. Visualizing
how the array pattern is formed is not
that much more difficult than for the
broadside and in-line arrays: Suppose
the current in element 1 lags the current
in element 2 by 90 degrees. By the