Practical_Antenna_Handbook_0071639586

162 p a r t I I : F u n d a m e n t a l s
doughnut shape of Fig. 3.7. Now suppose we place another thin metallic object, of approximately
the same dimensions, near the dipole (typically within a wavelength or
less) and in the dipole’s broadside radiation lobe. Electromagnetic waves from the dipole
will impinge upon this neighboring object, inducing currents and voltages of the
same frequency along the object’s surfaces. The new electromagnetic fields resulting
from those currents also radiate into space in all directions, very much like the fields
from the dipole itself. Thus, a distant receiving antenna will pick up a signal that is the
linear combination (superposition) of the original signal from the dipole and the induced
signal from the metallic object. But the latter signal arrives at a distant receiving point
with amplitude and phase that are, in general, different from those of the dipole. Depending
on the different paths traveled by the two waves and the degree to which the
radiated fields from the added conductor differ in phase from those of the dipole, the
signals may add or subtract at the receiving antenna.
The superposition at a distant receiving point of waves from the two radiating conductors
is identical to the process we saw previously with all-driven arrays, so we can
conclude that we have—even if only by accident—created an array despite the fact that
only one of its elements is driven. We call that element the driven element, of course, and
the other conductor is called a parasitic element.
With a driven array, the pattern and array gain are controlled by judicious choice
of element spacing and by individually setting or forcing the feedpoint current amplitude
and phase for each element of the array. With a parasitic array, however, the
only control we have is the ability to adjust element dimensions and interelement
spacings.
As you might suspect, if the parasitic element is identical to the dipole—that is, if it
is resonant at the operating frequency—it is likely to be “receptive” to the generation of
induced currents and voltages from the dipole’s fields. However, reradiation by a perfectly
self-resonant parasitic element does not generally lead to array patterns that are
of much use.
But now suppose we slightly lengthen the parasitic element. This does not materially
reduce its ability to pick up and reradiate a portion of the passing field from the
dipole, but it does substantially alter the phase of the currents induced in it. For typical
interelement spacings, the resulting field from the parasitic element opposes or partially
cancels the dipole field in directions beyond the added element, such that the overall
field seen by a distant receiving antenna is significantly reduced.
Similarly, if we now make the same element somewhat shorter than its self-resonant
length at the operating frequency, the reradiated fields are again of a different phase
from the dipole’s fields, but this time the effect is usually to add to the dipole’s fields at
a distant receiving point beyond the added element.
In the first case we call the parasitic element a reflector, and in the second we call it a
director. Depending on the specific design objectives for a parasitic array, there can be
multiple reflectors and/or multiple directors. Compared to all-driven arrays, interelement
spacings in parasitic arrays are often smaller; 0.1l to 0.3l is typical.
More than 85 years after its invention, arguably the most famous parasitic array is
the Yagi-Uda beam antenna—often called Yagi, for short. It has attained such a level of
popularity that an entire chapter (Chap. 12) of this book is devoted to it. But other kinds
have existed for at least as long, including many wire arrays and combinations of
grounded verticals. We will explore these in more detail in later chapters specifically
dedicated to the different families of arrays.