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Aircraft communications and navigation systems
2.9.6 Velocity factor The velocity of a wave in a feeder is not the same as the velocity of the wave in free space. The ratio of the two (velocity in the feeder compared with the velocity in free space) is known as the velocity factor. Obviously, velocity factor must always be less than 1 , and in typical feeders it varies from 0.6 to 0.97 (see Table 2.3).
Table 2.3 Velocity factor for various types of feeder
Figure 2.40 Coaxial connectors (from left to right: PL-259, BNC, and N-type) B mm
Type offeeder
Velocityfactor
Two-wire open line (wire with air dielectric)
0.975
Parallel tubing (air dielectric)
0.95
Coaxial line (air dielectric)
0.85
Coaxial line (solid plastic dielectric)
0.66
Two-wire line (wire with plastic dielectric)
0.68 to 0.82
Twisted-pair line (rubber dielectric)
0.56 to 0.65
2.10 Connectors Connectors provide a means of linking coaxial cables to transmitting/receiving equipment and antennas. Connectors should be reliable, easy to mate, and sealed to prevent the ingress of moisture and other fluids. They should also be designed to minimise contact resistance and, ideally, they should exhibit a constant impedance which accurately matches that of the system in which they are used (normally 50 n for aircraft applications). Coaxial connectors are available in various format (see Figure 2.40). Of these, the BNC- and N-type connectors are low-loss constant impedance types. The need for constant impedance connectors (e.g. BNC and N-type connectors) rather than cheaper non-constant impedance connectors (e.g. PL-259) becomes increasingly critical as the frequency increases. As a general rule, constant
H
(b)
ďż˝ ďż˝
(c)
(d)
(e)
Figure 2.41 Method of fitting a BNC-type connector to a coaxial cable impedance connectors should be used for applications at frequencies of above 200 MHz. Below this frequency, the loss associated with using non-constant impedance connectors is not usually significant. Figure 2.4 1 shows the method of fitting a typical BNC connector to a coaxial cable. Fitting requires careful preparation of the coaxial cable. The outer braided screen is fanned out, as shown in Figure 2.4 l (b) and Figure 2.4 l (c) and clamped in place whereas the inner conductor is usually soldered to the centre contact, as shown in Figure 2.4 l (d).
