Practical_Antenna_Handbook_0071639586
C h a p t e r 4 : T r a n s m i s s i o n L i n e s a n d I m p e d a n c e M a t c h i n g 113 Rigid outer conductor Dielectric Insulators Center conductor Figure 4.1i Rigid coaxial line (hardline). Inner conductor Outer conductor Figure 4.1J Gas-filled hollow coaxial line. Dielectric (vaned) Figure 4.1K Articulated coaxial line. tric losses increase the usefulness of the cable at higher frequencies. Double-shielded coaxial cable (Fig. 4.1L) provides an extra measure of protection against radiation from the line, as well as from electromagnetic interference (EMI) from outside sources getting into the system. Stripline, also called microstripline (Fig. 4.1M), is a form of transmission line used at high UHF and microwave frequencies. The stripline consists of a critically sized conductor over a ground-plane conductor, and separated from it by a dielectric. Some striplines are sandwiched between two ground planes and are separated from each by the dielectric board. Insulating sheath Outer insulator Inner insulator Center conductor Outer shield Inner shield Figure 4.1L Double-shielded coaxial line. Stripline W Dielectric T Groundplane Figure 4.1M Stripline transmission line.
114 P a r t I I : F u n d a m e n t a l s Transmission Line Characteristic Impedance (Z 0 ) If we take a very short length of two-conductor transmission line of any kind and measure its electrical characteristics at some operating frequency with simple test equipment, we will discover four parameters: • Each of the two conductors exhibits a very small series resistance between its two ends, measured at the operating frequency. In a balanced line, the resistance of the two conductors is identical, but that is not necessarily the case for coaxial or other unbalanced lines. Regardless of the type of line, we shall define R′ as the sum of the resistive losses in the two conductors. • Similarly, each of the two conductors has a small series inductance between its two ends. All wires—even straight ones—have some inductance. Again, this inductance is the same for both conductors of perfectly balanced lines but may differ in unbalanced lines. We shall define L′ as the sum of these inductances. • At one end or the other we measure a small shunt capacitance, C′, between the two conductors. This is a measure of the capacitive coupling between two closely spaced wires; of course, it depends greatly on the dielectric material between them. • At the same end of the short length of transmission line we measure a very high shunt resistance (usually alternatively stated as a very low shunt conductance, G′, where G′ is the reciprocal of the shunt resistance) across the two conductors. This conductance is a characteristic of the dielectric material(s) filling the space between the two conductors. Sometimes the dominant part of it (lowest resistance, highest G′) is unintended—as might result from unwanted moisture in the line! In defining these four parameters, we have primed each of them to indicate that the value we are measuring is for a unit length of transmission line. Of course, the unprimed values of all four are directly proportional to the total length of the transmission line. How short is “very short”? The length of transmission line used to obtain these data must be much shorter than the wavelength of the highest frequency we intend to send through the line. If we now use these four parameters to draw a lumped element model of this short length of transmission line, we obtain the equivalent circuit of Fig. 4.2. This single-ended circuit is equally valid for balanced (parallel wire) transmission lines and unbalanced (coaxial) lines. Z s R ' L ' R ' L ' Gen C ' G ' C ' G ' Z L Source Unit length of line { Load Figure 4.2 Transmission line equivalent circuit.
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C h a p t e r 4 : T r a n s m i s s i o n L i n e s a n d I m p e d a n c e M a t c h i n g 113<br />
Rigid outer<br />
conductor<br />
Dielectric<br />
Insulators<br />
Center<br />
conductor<br />
Figure 4.1i Rigid coaxial line (hardline).<br />
Inner<br />
conductor<br />
Outer conductor<br />
Figure 4.1J Gas-filled hollow coaxial line.<br />
Dielectric<br />
(vaned)<br />
Figure 4.1K Articulated coaxial<br />
line.<br />
tric losses increase the usefulness of the cable at higher<br />
frequencies. Double-shielded coaxial cable (Fig. 4.1L) provides<br />
an extra measure of protection against radiation from the line,<br />
as well as from electromagnetic interference (EMI) from outside<br />
sources getting into the system.<br />
Stripline, also called microstripline (Fig. 4.1M), is a form of<br />
transmission line used at high UHF and microwave frequencies.<br />
The stripline consists of a critically sized conductor over<br />
a ground-plane conductor, and separated from it by a dielectric.<br />
Some striplines are sandwiched between two ground<br />
planes and are separated from each by the dielectric board.<br />
Insulating<br />
sheath<br />
Outer insulator<br />
Inner<br />
insulator<br />
Center<br />
conductor<br />
Outer<br />
shield<br />
Inner<br />
shield<br />
Figure 4.1L Double-shielded coaxial line.<br />
Stripline<br />
W<br />
Dielectric<br />
T<br />
Groundplane<br />
Figure 4.1M Stripline transmission line.
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