Practical_Antenna_Handbook_0071639586

346 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
Figure 14.7 Simple loop antenna.
spective cross-sectional areas (πr 2 for circular loops and A 2 for square loops) are less
than l 2 /100.
The reason a small loop has a null when its broadest aspect is facing the signal is
simple, even though it seems counterintuitive at first blush. In Fig. 14.8 we have two
identical small loop antennas at right angles to each other. Antenna A is in line with the
advancing radio wave, whereas antenna B is broadside to the wave. Each line in the
wave represents a line where the signal strength is the same, i.e., an isopotential line.
When the loop is in line with the signal (antenna A), a difference of potential exists from
one end of the loop to the other and current can be induced in the wires. When the loop
is turned broadside, however, all points on the loop are on the same potential line, so
there is no difference of potential between segments of the conductor. Thus, little signal
is picked up (and the antenna therefore sees a null).
The actual voltage across the output terminals of an untuned loop is a function of
the angle of arrival of the signal a (Fig. 14.9), as well as the strength of the signal and the
design of the loop. The voltage V o is given by
V
o
2πANE
f
cos ( α)
=
λ
(14.3)
where V o = output voltage of loop
A = area of loop, in square meters (m 2 )
N = number of turns of wire in loop
E f = strength of signal, in volts per meter (V/m)
a = angle of arrival of signal
l = wavelength of arriving signal