TRANSMISSION LINES
Introduction.
A transmission line is used to carry the RF energy from the
transmitter to the antenna.
There are times when the antenna is
connected directly to the transmitter.
Normally, however, the
antenna is located some distance away from the transmitter.
The
transmission line should transfer the power with the least possible
loss.
(1) Transmission lines dissipate power in three ways:
(a) Radiation. The transmission line radiates like an antenna,
especially if its length matches the antenna.
(b) Heating.
Any current flow results in heat.
The greater
the power the more heat is produced.
To reduce skin effect, the
cross sectional area of the center conductor is increased.
(c) Reflection.
Radio energy emitted by the transmitter goes
to the antenna in what we call traveling waves. If there is no load
(antenna), the traveling waves are stopped abruptly. This causes the
waves to be reflected back to the transmitter causing loss.
(2) Types of transmission lines.
(a) Single wire line.
This is the simplest type of
transmission line - a single wire connected to the antenna with the
earth acting as the return path. Since there is only one conductor,
the line is considered to be unbalanced.
The disadvantage is that
the line radiates much like an antenna, causing high line loss,
because of no return path. The other disadvantage is that because of
no return path, it is difficult to match the line to the antenna. An
antenna tuning unit is required to match the transmitter to the line
and antenna.
However, there are times when the advantages of easy
installation far outweigh the disadvantages.
Some transmitters are
broad enough to load across many types of transmission lines and
(b) Twisted pair. Two insulated wires (WD-1) can be used as a
transmission line.
It offers easy installation, but has high loss
and should not be used above 15 MHz.
(c) Coaxial lines.
When one conductor is placed inside the
other separated by foam or plastic it transfers the RF power to the
antenna with a minimum of loss. There is some loss as the frequency
is increased. To offset this, the cross sectional area
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