b. The sizes of the coil and capacitor that resonate at about 500
megahertz are still smaller.
For example, the tuned-circuit coil in a
typical UHF communications receiver operating at 400 megahertz consists of
only one turn of wire.
Its capacitor has only two plates.
To raise the
resonant frequency of this tuned circuit, all you need is a half turn of
wire with its distributed capacity. This half turn of wire is in effect a
section of closed-end, two-wire transmission line.
4.
Transmission line sections are used as resonant circuits.
a. You know from a previous lesson that sections of transmission line
are used as resonant circuits.
We use resonant sections of transmission
line at UHF frequencies where coils and capacitors are not practical.
b. Transmission lines are similar to coils and capacitors in that they
too must be made smaller to increase their resonant frequency.
A
transmission line cut to resonate at 1000 megahertz is only a few
centimeters long and if you cut it shorter to increase its resonant
frequency, its use is awkward. However, if you take two sections, each cut
for 1000 megahertz as in Part A of Figure 72, and connect them in parallel
as in Part B of Figure 72, the length of the line is doubled.
But, the
resonant frequency of the combined sections remains the same, at 1000
megahertz.
Figure 72. Two Quarter-Wave Sections of
Transmission Line Connected in Parallel.
5. What happens
if
we
connect
more
sections
of
transmission
line
in
parallel?
a. Before
answering
this
question,
review
the
following
important
points:
103
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