Practical_Antenna_Handbook_0071639586

C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 451
A. Operating frequency
B. Increasing frequency
C. Decreasing frequency
/ 4
a b c d
/ 4
/ 4
/ 4
a b c d
/ 4
/ 4
"a"
Dimension
Figure 20.5 Changing frequency does not affect the
analogy.
On first glance, an explanation based
on l/4 shorted stubs would seem to be
valid only at one frequency. It turns out,
however, that the analogy also holds up at
other frequencies so long as the frequency
is higher than a certain minimum cutoff frequency.
The waveguide thus acts like a
high-pass filter. Waveguides also have an
upper frequency limit; between the upper
and lower limits, waveguides support a
bandwidth of 30 to 40 percent of the cutoff
frequency. As shown in Fig. 20.5, between
segments the centerline (which represents
the conductors in the parallel line analogy)
of the waveguide becomes a “shorting
bar” that widens as the operating
frequency gets higher and the l/4 stub on
each side gets shorter or narrows as the
operating frequency becomes lower and
the l/4 stub lengthens.
At the cutoff frequency, the fraction of
the a dimension that acts as though it is a
transmission line conductor is a minimum;
in other words, the back-to-back
l/4 stubs virtually touch at the midpoint
of a. At this frequency, a = l/2. Above that
frequency, the effective or apparent width
of the conductor increases and l/2 < a.
Below the cutoff frequency, the chamber
ceases to function as a waveguide; instead,
it acts like a conventional parallel transmission
line with a pure reactance connected
across the two conductors. Thus,
the (low-frequency) cutoff frequency is defined
as the frequency at which the a dimension
is less than l/2.
Propagation Modes in Waveguides
Whether in a conventional transmission
line or a microwave waveguide, the signal
propagates as an electromagnetic wave,
not as a longitudinal current. The transmission
line supports a transverse electromagnetic
(TEM) field. As explained in
Chap. 3, the word transverse indicates that
the wave is propagating at right angles to
both the electric and the magnetic fields.
In addition to the word transverse the