Practical_Antenna_Handbook_0071639586

C h a p t e r 7 : L a r g e W i r e L o o p A n t e n n a s 209
L 1
X L = 370
Maximum
reception
X 1
X 2
L 2
X L = 370
Figure 7.2 Inductive loading improves l/2 loop.
when making the coils, be sure to use a size that is sufficient for the power and current
levels anticipated. The 2- to 3-in-diameter B&W Air-Dux style coils are sufficient for
most amateur radio use. Smaller coils are available on the market, but their use should
be limited to low-power situations.
1l Large Loops
If space constraints are not forcing you to a l/2 loop, then a 1l loop might be just the
ticket. Such a loop has many desirable features, including a manageable feedpoint impedance,
more gain than a dipole in favored directions, and ease of analysis.
The simplest way to analyze the 1l loop of Fig. 7.3 is to treat it as two horizontal
half-wave “bent” dipoles whose outer halves are bent toward each other and connected
together electrically. As we have seen in an earlier chapter, the center half of a l/2 dipole
is responsible for most of the radiated field strength; the ends establish resonance,
minimize feedpoint reactance, and help raise the input or feedpoint impedance to a
reasonable value. Thus, bending the outer halves of the dipoles 90 degrees has limited
effect on the operation of the elements.
Only one of the two dipoles is fed directly from the transmission line, and a short
circuit is placed across the feedpoint of the other. Remember that current reverses direction
in adjacent half-wave sections of a collinear array of dipoles. Thus, after an even
number of half-wave segments, the drive current naturally wants to be in phase with
the current coming around the full periphery of the loop. At the resonant operating
frequency, the current goes to zero at what would be each end of the driven dipole and
then reverses direction in the second dipole. This results in the currents in the horizontal
sections of the two dipoles being in phase spatially—that is, they both point in the
same direction (left or right in Fig. 7.3) at all times. In free space the 1l loop is a two-Â