Practical_Antenna_Handbook_0071639586

342 p a r t V : h i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
has been around for years. The second is a whole new class of small, single-turn loops
that emulate the directivity of large transmitting loops while exploiting size reduction
design techniques uniquely available to receiving antennas that do not need to exhibit
RF efficiency.
Small Loop Receiving Antennas
Radio direction finders and people who listen to the AM broadcasting bands, VLF,
Âmedium-wave, or the so-called low-frequency tropical bands are all candidates for a
small loop antenna. These antennas are fundamentally different from the large (transmitting)
loops of Chap. 7 and the multielement versions of Chap. 13. Large loop antennas
typically have a total wire length of 0.5l or greater. Small loop antennas, on the
other hand, have an overall total conductor length that is less than about 0.1l.
Earlier in this book we talked about the importance of proper physical configuration
to efficient performance as a radiating antenna by a conductor. In particular, we
pointed out that a half-wavelength of copper wire that is wound into a tight coil is a
poor antenna and is best treated as a lumped-circuit inductor. Now we go a step further
and note that a small loop antenna responds to the magnetic field component of the
electromagnetic wave instead of the electrical field component. (Remember from high
school science class that passing a bar magnet back and forth rapidly through the interior
of a multiturn coil produces temporary voltages across the terminals of the coil.
Consider the time-varying magnetic field in a transmitted wave as analogous to the
movement of the bar magnet.)
Thus we see a principal difference between a large loop (total conductor length
greater than 0.1l) and a small loop when examining the RF currents induced in either
loop when a signal intercepts it. In a large loop, the current is a standing wave that varies
(sinusoidally) from one point in the conductor to another, much like a dipole, and
we analyze its operation in much the same way we have examined dipoles; in the small
loop antenna, the current is the same throughout the entire loop.
Since the electric and magnetic fields of a radiated wave in free space are at right
angles to each other and to the direction the wave propagates through space, and since
we are primarily listening for vertically polarized signals with loops, the magnetic field
is oriented horizontally, or nearly so.
The differences between small loops and large loops show up in some interesting
ways, but perhaps the most striking difference is found in the directions of maximum
response—the main lobes—and the directions of the nulls. Both types of loops produce
figure eight patterns but at right angles with respect to each other. The large loop
antenna produces main lobes that are orthogonal—i.e., at right angles or “broadside”—
to the plane of the loop. Nulls are off the sides of the loop. The small loop, however, is
exactly the opposite: the main lobes are off the sides of the loop (in the plane of the
loop), and the nulls are broadside to the loop plane (Fig. 14.6A). Do not confuse small
loop behavior with the behavior of the loopstick antenna. Loopstick antennas are
made of coils of wire wound on a ferrite or powdered-iron rod. The direction of maximum
response for the loopstick antenna is broadside to the rod, with deep nulls off the
ends (Fig. 14.6B). Both loopsticks and small wire loops are used for radio directionfinding
and for shortwave, low-frequency medium-wave, AM broadcast band, and
VLF listening.
The nulls of a loop antenna are very sharp and very deep. If you point a loop antenna
so that its null is aimed at a strong station, the signal strength of the station ap-