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 161
but it is a good habit to form in any case. Even in this situation, all cables
between the common junction and individual elements should be obtained
from the same spool or production lot.
• Measure your cable lengths accurately. Even on the MF and lower HF bands,
you should strive to keep your lengths of “identical” phasing lines within a few
inches of each other.
• Make sure any matching networks between the central distribution point and
each element of the array are identical and not contributing unexpected phase
shifts or delays in the signals being sent to the elements.
Although the discussion of arrays so far has assumed they are being used for transmitting,
that isn’t always so. Some of the most exciting work in HF arrays currently
under way deals with receive-only arrays for the MF and lower HF bands, where the
cost and structural demands of transmitting arrays can be largely avoided by using
shortened elements, minimal RF ground systems, low-power components, and preamplifiers
to compensate for very low signal levels, low feedpoint impedances, and the
signal loss of very long cable runs back to the receiver. Nonetheless, all the points noted
here about the importance of precise matching of phasing lines and networks are just as
valid for receive-only arrays.
Parasitic Arrays
If a piece of metal or other conducting material with dimensions on the order of l/4 or
greater is placed in the vicinity of an antenna, it can cause “distortions” in the electromagnetic
fields radiated or received by the antenna. By deliberately positioning one or
more conductors near a simple radiating element such as dipole, we can increase the
field strength in some directions at the expense of others. The resulting configuration is
known as a parasitic array, and it is capable of producing patterns and array gains similar
to the ones created in the all-driven arrays previously discussed.
The principal value of a parasitic array is its potential for simplifying the feed system.
In general, parasitic arrays use a combination of element-to-element spacings and
element dimensions to create desired array patterns without the use of phasing lines
and multiple matching networks. Because of this, they are mechanically simpler to construct,
and at 14 MHz and above, they are today the antenna of choice because antennas
with array gains of 6 dB or more are easily rotated, allowing them to be aimed in any
direction desired.
Parasitic arrays can be vertically or horizontally polarized; they can be made of l/4
grounded elements or l/2 symmetrical elements. They can be constructed from copper
wire (typically on 160, 80, and 40 m), aluminum tubing (80 through 6 m), or solid aluminum
rod (6 m and up). Vertical arrays for 160 through 40 are often constructed with
triangular lattice tower sections—i.e., they provide their own support mechanism!
A mathematical explanation of how parasitic arrays work is beyond the scope of
this book. Instead, a short qualitative description is provided as a guide to helping the
reader develop an intuitive feel for their operation.
Picture a horizontal l/2 dipole up in the air somewhere, fed with RF energy at its
resonant frequency. With nothing around it, its radiation pattern will be the familiar