Practical_Antenna_Handbook_0071639586

470 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
Coaxial cable
Loop
Loop
Possible location
for loop
H-lines
Waveguide
H-field
Figure 20.18 Loop (inductive) coupling.
Inductive, or loop coupling, is shown in Fig.
20.18. A small loop of wire (or other conductor) is
placed such that the number of magnetic flux lines
it cuts is maximized. This form of coupling is popular
on microwave receiver antennas as a way of
making a waveguide-to-coaxial cable transition. In
some cases, the loop is formed by the pigtail lead
of a detector diode that, when combined with a
local oscillator, downconverts the microwave signal
to an intermediate frequency (IF) in the 30- to 300-
MHz region.
Aperture, or slot, coupling is shown in Fig.
20.19. This type of coupling is used to couple two
sections of waveguide, as on an antenna feed system.
Slots can be designed to couple electric,
magnetic, or electromagnetic fields. In Fig. 20.19,
slot A is placed at a point where the E-field peaks,
so it allows electrical field coupling. Similarly,
slot B is at a point where the H-field peaks, so it
allows magnetic field coupling. Finally, we see
slot C, which allows electromagnetic field coupling.
Slots can also be characterized according to whether they are radiating or nonradiating.
A nonradiating slot is cut at a point that does not interrupt the flow of currents in
the waveguide walls. The radiating slot, on the other hand, does interrupt currents
flowing in the walls. A radiating slot is the basis for several forms of antenna, which are
discussed at the end of this chapter.
0111057 FIG 18-18
A
C
B
Figure 20.19 Slot coupling.