11
Theory Of Operation
Some Basic Ideas About Impedance
The theory underlying antennas and transmission lines is fairly complex, and in fact employs a
mathematical notation called “complex numbers” that have “real” and “imaginary” parts. It is
beyond the scope of this manual to present a tutorial on this subject
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, but a little background will
help you understand what your JT-11 is doing, and how it does it.
In simple DC circuits, the wire resists the current flow, converting some of it into heat. The
relationship between voltage, current and resistance is described by the elegant and well-known
“Ohm’s Law”, named for Georg Simon Ohm of Germany, who first discovered it in 1826. In RF
circuits, an analogous but far more complicated relationship exists.
RF circuits also resist the flow of electricity. However, the presence of capacitive and inductive
elements causes the voltage in the circuit to lead or lag the current, respectively. In RF circuits
this resistance to the flow of electricity is called “impedance”, and can include all three elements:
resistive, capacitive, and inductive.
The output circuit of your transmitter consists of inductors and capacitors, usually in a
series/parallel configuration called a “pi network”. The transmission line can be thought of as a
long string of capacitors and inductors in series/parallel, and the antenna is a kind of resonant
circuit. At any given RF frequency, each of these can exhibit resistance, and impedance in the
form of capacitive or inductive “reactance”.
Transmitters, Transmission Lines, Antennas and Impedance
The output circuit of your transmitter, the transmission line, and the antenna all have a
characteristic impedance. For reasons too complicated to go into here, the standard impedance is
about 50 ohms resistive, with zero capacitive and inductive components. When all three parts of
the system have the same impedance, the system is said to be “matched”, and maximum transfer
of power from the transmitter to the antenna occurs. While the transmitter output circuit and
transmission line are of fixed, carefully designed impedance, the antenna presents a 50 ohm, non-
reactive load only at its natural resonant frequencies. At other frequencies, it will exhibit
capacitive or inductive reactance, causing it to have an impedance different from 50 ohms.
When the impedance of the antenna is different from that of the transmitter and transmission line,
a “mismatch” is said to exist. In this case, some of the RF energy from the transmitter is reflected
from the antenna back down the transmission line, and into the transmitter. If this reflected
energy is strong enough it can damage the transmitter’s output circuits.
The ratio of transmitted to reflected energy is called the “standing wave ratio”, or SWR. An SWR
of 1 (sometimes written 1:1) indicates a perfect match. As more energy is reflected, the SWR
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For a very complete treatment of this subject, see any edition of the ARRL Handbook for Radio
Communications (previously the Handbook For Radio Amateurs)
Inductive
Reactance
Capacitive
Reactance
