30° and Z = a close match for the impedance (292.5 (2) of a very 400 S / - 10° What is the characteristic impedance popular antenna (folded dipole antenna) which is (Zo) of the line at 1.6 MHz? used extensively for VHF-FM reception. The com- ments relating to the different types of 50 L lines Given: f = 1.6 MHz, also apply to 300 S lines. Zoc = 900 $ / - 30° Zsc = 400 2 / 10° 600 0 - This type of cable is used extensively by the tele- Required: Z at 1.6 MHz. phone industry and many of their instruments are matched to this impedance. The comments relating to Solution the different types of 50 O lines also apply to 600 L lines. Using Equation 2.2.21 2.2.8 Signal propagation Z = VZscZoc = 900 400 of - 10° on transmission lines = 600 2/ - 20° 2.2.8.1 Pulse propagation on an infinitely long or matched transmission line 2.2.7 Typical commercial cable We are now going to use some of the ideas introduced in impedances the previous sections, particularly Section 2.2.2.2, to describe qualitatively the propagation of signals along an Manufacturers tend to make cables with the following infinitely long transmission line. In this description we characteristic impedances (Zo). These are: will only make two assumptions: 50 S - This type of cable finds favour in measure- the transmission line is perfectly uniform, that is its ment systems and most radio instruments are electrical properties are identical all along its length; matched for this impedance. It is also used exten- the line extends infinitely in one direction or is per- sively in amateur radio links. fectly terminated. Most cable manufacturers make more than one To keep the explanation simple, we will initially only type of 50 O line. For example, you can buy 50 L consider the propagation of a single electrical pulse along rigid lines (solid outer connector), 50 S low loss lines the line9 shown in Figure 2.2-11. At the beginning of the (helical and air dielectrics), 50 S high frequency lines for use up to 50 GHz with minimal loss. The reason Plane 1 Plane 2 Plane 3 fo