Mars/Cap Coverage; Theory Of Operation - LDG IT-100 Operation Manual

MARS/CAP Coverage

The IT-100 provides continuous tuning coverage over its specified range; not just in the ham
bands. This makes it useful for MARS or CAP operation, or any other legal HF operation.

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
help in understanding what the IT-100 is doing, and how it does it.
In simple DC circuits, the wire resists 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 the principle in
1826. In RF circuits, an analogous but more complicated relationship exists.
RF circuits also resist the flow of electricity. However, the presence of capacitive and
inductive elements causes the voltage 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 a 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 a transmitter, the transmission line, and the antenna, all have a
characteristic impedance. For reasons beyond the scope of this document, the standard
impedance is nominally 50 ohms resistive, with zero capacitive and zero 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 50-ohm, non-reactive load only at its natural resonant frequencies. At other frequencies,
3
For a very complete treatment of this subject, see any edition of the ARRL Handbook for Radio Communications
(previously the Handbook For Radio Amateurs).
3
, but a little background will
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