(1) You will recall that if you connect two coils of equal value in
parallel, the total inductance is equal to one-half the inductance of either
coil.
(2) Connecting two capacitors of equal value in parallel gives the
opposite result, the total capacitance is equal to twice the value of either
capacitor.
(3) Resistors are similar to coils. If you connect two resistors of
equal value in parallel, the total resistance is only one-half of either
resistor.
(4) Each quarter-wave section of the same type of transmission line
has the same amount of distributed inductance, capacitance, and resistance.
If we connect identical sections in parallel, we connect the individual
resistance, inductance, and capacitance in parallel.
b. You see then, that as we parallel identical transmission line
sections, the total inductance decreases in the same proportion that the
total capacitance increases. The resonant frequency of the combined lines,
therefore, is the same as each individual line. We can add as many lines in
parallel as we want and the resonant frequency does not change.
c. The total resistance changes though and this fact is very important.
The total resistance keeps getting smaller as we add more sections of line.
As the resistance decreases, the Q, or figure of merit rises. This means
the selectivity and efficiency of the circuit go up. If we continue to add
more sections in parallel, as shown in Figure 73, we finally get a very high
Q, flat, metal box or can, which is resonant only for an extremely narrow
frequency range. We call this metal can a resonant cavity.
Figure 73.
Development of Resonant Cavity.
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