Quantcast Section 10. Transmission Lines

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General. The type of transmission line chosen for a particular
application depends primarily upon the operating power level, characteristic
impedance, line losses, and susceptibility to RFI.  The three basic types of
transmission lines are balances lines, unbalanced lines and waveguides.  For
detailed discussions, refer to NAVELEX 0101,110.
Balanced Transmission Lines.  Balanced lines consist of two
separate conductors operated at equal and opposite potentials.  Open-wire
lines provide good balance, constant characteristic impedance, and low loss,
and they are capable of handling very high power levels.  This is particularly
true when unusually long distances between the transmitter and antenna are
necessary. For a detailed discussion of the physical and electrical
characteristics of balanced transmission lines, refer to NAVELEX 0101,104 and
NAVFAC DM-4.07, Wire Communication and Signal Systems.
Unbalanced Transmission Lines. Coaxial cables used in HF
transmitter and receiver installations operate with the center conductor at
some potential other than ground and the shield (outer conductor) at ground
potential; they are therefore called unbalanced transmission lines. Coaxial
transmission lines are used almost exclusively at all receiving installations,
as well as in most transmitting applications.  For a detailed discussion of
the physical and electrical characteristics of unbalanced transmission lines,
refer to NAVELEX 0101,104.  For cable selection, environmental considerations,
insulation and sheath, and types of wire and cable, refer to NAVELEX 0101,110.
Waveguides. A waveguide is a transmission line consisting of a
dielectric medium through which electromagnetic waves propagate. Waveguides
are generally distinguished by the frequency of the electromagnetic radiation
they transmit.
Microwave Waveguides. The three types of microwave waveguides are
rectangular, circular, and elliptical.  For detailed discussions, refer to
NAVELEX  0110,110.
Optical Waveguides (Fiber Optics). Optical fiber cables offer
several advantages over cables with metallic electrical conductors and
metallic waveguides. The optical fiber cable provides a non-conductive
dielectric media as the transmission medium; this results in electrical
isolation between the transmitter and the receiver.  Optical 'energy is
unaffected by other forms of electromagnetic radiation; therefore, optical
fiber cables can operate in a noisier electrical environment. Optical energy
is guided within the fiber; crosstalk inherent in cables with metallic
electrical conductors is extremely low in optical fibers because of the
stronger guidance of the media.  Optical fiber cables are smaller and lighter
than electrical cables of the same transmission capacity.  The advantages of
fiber optic cables over copper-based systems in general include lower
attenuation, larger bandwidth, higher security, and the absence of
electromagnetic interference, shock hazards, and short circuits.  Their
importance depends on the requirements of the system under consideration. S e e
Section 7 for safety considerations when using fiber optics.


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