Practical_Antenna_Handbook_0071639586

CHAPTER 7
Large Wire Loop Antennas
Loop antennas are characterized as small or large depending on the dimensions of
the loop relative to the wavelength of the operating frequency. These two types
of loops have different characteristics, work according to different principles,
and serve different purposes. Small loops are those in which the current flowing in the
wire has approximately the same phase and amplitude at every point in the loop (which
fact implies a very short wire length, i.e., less than about 0.2l). Such loops respond to
the magnetic field component of the electromagnetic radio wave. A large loop antenna
has a wire length greater than 0.2l, with most being l/2, 1l, or 2l. The current in a large
loop varies along the length of the wire in a manner similar to other wire antennas
whose length is comparable to l/2. In particular, the direction of current flow reverses
in each l/2 section— a fact that is especially useful in the simplified analyses of loops
we will use in the following sections.
l/2 Large Loops
The radiation pattern of a large wire loop antenna is strongly dependent on its size.
Figure 7.1 shows a half-wavelength loop (i.e., one in which each of the four sides is l/8
long). Assuming S 1 is in its closed position, the real part of the feedpoint (X 1 – X 2 ) impedance
is on the order of 3 kΩ because it occurs at a voltage maximum. Unfortunately,
the imaginary, or reactive, part of the impedance is closer to 20,000 Ω! This is an antenna
that few antenna couplers can match, and one that has little to recommend it compared
to a bent dipole in the same physical configuration with a 6-in insulator inserted in the
midpoint of the wire segment opposite the feedpoint (corresponding to S 1 being open).
The fundamental difficulty with the l/2 loop is that the two ends of the wire appearing
at the terminals of S 1 , each l/4 from the feedpoint, carry high voltages that are
out of phase with each other in normal operation of a half-wave dipole. Tying the ends
together (by closing S 1 ) forces the wire into an “unnatural” mode, resulting in abnormally
high resistive and reactive components of feedpoint impedance.
Nonetheless, a simple trick can “tame” the difficult input impedance of the l/2
loop. In Fig. 7.2, an inductor (L 1 or L 2 ) is inserted into the circuit at the midpoint of each
wire segment adjacent to the driven segment. These inductors should have an inductive
reactance X L of about 370 Ω at the center of the chosen operating band. The inductance
of each coil is
8
3.6 × 10
L1 = L2
=
2πF
where L = coil inductance, in microhenrys (µH)
F = midband frequency, in hertz (Hz)
(7.1)
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