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14 Transmission Lines
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Transmission lines are conductors intended to move current from one location to another not only without radiating, but also at a selected impedance There are two kinds of lumped RF transmission lines: unbalanced, normally in the form of coaxial cable, and balanced, such as twin-lead Waveguide, a type of transmission line, can still be found in high-powered microwave transmitters, but is normally more expensive than coaxial cable and is much harder to work with
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142 Transmission line types
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Balanced lines are characteristically 300-ohm twin-lead (Fig 146), and are distinctly different from unbalanced coaxial line, since there is no conductor in balanced line that is at a ground potential In fact, each conductor has an equal-in-amplitude but opposite-in-phase signal present on each of its two conductors Commonly operated as a feedline to a television or FM receiver antenna or, more infrequently, as a balanced feed to a dipole transmitting/receiving antenna, twin-lead has very little line losses and is able to survive high line voltages However, twin-lead is not found in the impedance required for most transmitters and receivers (50 ohms), and matching networks must be used By far the most popular line is unbalanced, which comes in the form of coaxial cable (Fig 147) and is shielded with varying degrees of copper braid (or aluminum foil) to prevent the coax from receiving or radiating any signal The inner conductor carries the RF current, while the outer shield is at ground potential Coax cable comes in many diameters, qualities, and losses per foot It is commonly the flexible type, which is covered with a protective rubber sleeve, but the semirigid type, with solid copper outer conductor, is also used Flexible coax is available, at a high cost, that can function with low losses up to frequencies as high as 50 GHz Now that coax cables can work in the microwave region, waveguide (Fig 148) has become a little less widespread Whenever possible, modern microwave designs have removed waveguides in favor of low-loss, semirigid
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Figure 146 Twin-lead transmission line
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Figure 147 Flexible coaxial cable
coax cables to transmit and receive high-frequency signals However, waveguide is still favored as the transmission line of choice in certain demanding microwave high-power applications Waveguide can be a round or a rectangular hollow metal channel made to transport microwave radiation from one point to another, with minimal signal loss, for very long distances The actual size of the waveguide itself will govern its working frequency (Fig 149), with one-quarter wavelength straight or loop probes adopted to inject or remove the microwave energy from the waveguide structure Waveguides perform as a type of high-pass filter, since they will propagate microwave radiation above their working frequency but not below their cutoff frequency However, mode shifts that arise within the waveguide structure will limit the highest frequencies they are capable of propagating, thus making a waveguide more of a very wide bandpass filter
143 Transmission line issues
With a frequency source s output and its transmission line at the same impedance, and with the transmission line also equal to the load s input impedance, no standing or reflected waves will exist on the transmission line Thus, no power will be dissipated as heat apart from that generated by the transmission line center conductor s natural resistance and the line will seem infinitely long, with no standing waves reflected back into the source, while sending the maximum power to the load The transmission line is now considered to be flat line (Fig 150) However, if there were high standing waves (high VSWR) existing on the transmission line (Fig 151), the line s dielectric and/or the wireless transmitter s final amplifier can be damaged by the reflections Generally, the larger the diameter of the coaxial cable, the higher the operating frequency and the smaller the losses This is not true at the higher microwave frequencies, where the diameter of the cable can approach a certain fraction of the signal s wavelength, causing high transverse electric mode (TEM) losses due to the coax transitioning to an undesired waveguide mode 15 S Parameters