Practical_Antenna_Handbook_0071639586

CHAPTER 4
Transmission Lines and
Impedance Matching
Transmission lines and waveguides are conduits for transporting RF energy between
elements of a radio system. For example, in a typical station with a transmitter
(or exciter) and a power amplifier, one transmission line carries exciter
output to the amplifier input and a second line delivers transmitter output energy to the
antenna. A third line may carry the incoming RF energy from a transmit/receive switch
or separate receiving antenna to the station receiver. Still other lines may switch filters
in and out, or pass transmitter RF through signal monitors, power meters, and other
station accessories.
A good analogy for visualizing how transmission lines do their job is a forced hot
water heating system for your home or office. In such a system, copper pipes carry hot
water from a central furnace or boiler to distant radiating units that extract a large percentage
of the heat from the water and deliver it to the air or the floors in your building.
In both radio and heating systems:
• The objective is to transfer as much energy as possible to the radiating unit(s)
and minimize the unintentional loss of energy in the transmission line or copper
pipes.
• A return path for the delivery medium back to the energy source must be
provided.
• The longer the distance between the source and the radiating unit(s), the greater
the potential for loss of energy in the transmission (distribution) system.
Because of the requirement for a return path, virtually all transmission lines below
microwave frequencies (where waveguides are an important exception to this statement)
have at least two conductors: At the same time RF current is flowing toward the
radiating unit (antenna) in one conductor it is flowing away from the antenna and back
to the source in the other.
Similarly, to minimize the unintentional loss of energy from the transmission line,
virtually all lines in use today keep the conductors very close together for the entire
distance between the source and the antenna. (This is where our analogy with the heating
system breaks down because the return lines in many, if not most, heating systems
travel completely different paths back to the furnace.)
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