138 Radio Engineering for Wireless Communication and Sensor Applications

nents, one perpendicular to the card and the other parallel to it. The latter component is absorbed by the card; the former component enters the output waveguide in which again its component parallel to the resistive card is absorbed. We can show that the attenuation in decibels is [1]

L = −40 log (sin u ) dB

(6.41)

where u is the angle between the electric field at the input and the plane of the resistive card in the circular section.

6.3.3 Phase Shifters

An ideal phase shifter is lossless and matched; it only shifts the phase of the output wave, or in other words, changes the phase difference between the output and input waves. Phase shifters are needed, for example, in phased antenna arrays.

An adjustable waveguide phase shifter can be realized by replacing the resistive card of the attenuator in Figure 6.20(c) with a dielectric card whose depth in the waveguide is adjustable. A structure resembling the attenuator in Figure 6.21 also operates as a phase shifter when the resistive cards are replaced with dielectric cards having proper lengths [1]. Electrically controlled phase shifters are much faster than mechanical phase shifters. They are often based on semiconductor devices such as p-i-n diodes or field effect transistors (FETs).

6.3.4 Connectors and Adapters

Connectors are needed to join different lines, devices, and circuit blocks together. An ideal connector is matched and lossless. Practical connectors cause small discontinuities. Therefore, unnecessary use of connectors should be avoided. The quality of connectors gets more and more important as the frequency gets higher.

Figure 6.22 shows some common coaxial connectors used at RF and microwave frequencies. An APC-7 connector is a precision connector that is used in measurements requiring good accuracy and repeatability. It is a sexless connector whose inner diameter of its outer conductor is 7 mm. SMA and N connectors are good enough for most cases. These connectors can be either male or female type. BNC connectors work best at frequencies below 1 GHz; at higher frequencies they may radiate. Waveguide components have flanges at their ports. Alignment pins on the flanges ensure accurate connection.

Passive Transmission Line and Waveguide Devices			139

Figure 6.22 Some common coaxial connectors.

Adapters are needed to connect components having connectors of different types or of the same sex. Figure 6.23(a) shows a transition from a coaxial line to a microstrip line. Coaxial and waveguide components can be connected using the adapter illustrated in Figure 6.23(b).

Figure 6.23 (a) Coaxial-to-microstrip transition; and (b) waveguide-to-coaxial adapter.

References

[1]Collin, R. E., Foundations for Microwave Engineering, 2nd ed., New York: IEEE Press, 2001.

[2]Pozar, D. M., Microwave Engineering, 2nd ed., New York: John Wiley & Sons, 1998.

140 Radio Engineering for Wireless Communication and Sensor Applications

[3]Rodrigue, G. P., ‘‘A Generation of Microwave Ferrite Devices,’’ Proc. IEEE, Vol. 76, No. 2, 1988, pp. 121–137.

[4]Fay, C. E., and R. L. Comstock, ‘‘Operation of the Ferrite Junction Circulator,’’ IEEE Trans. on Microwave Theory and Techniques, Vol. 13, No. 1, 1965, pp. 15–27.

[5]Lahey, J., ‘‘Junction Circulator Design,’’ Microwave Journal, Vol. 32, No. 11, 1989, pp. 26–45.