Chapter 12 / 12.15

12.15 REVISION QUESTIONS

1. What is meant by an isotropic radiator? Why is it useful as a reference, even though it does not physically exist?

1. Define and distinguish between: gain, directivity, radiation resistance, and antenna efficiency.

1. What is meant by effective aperture? How is it related to antenna gain?

1. State the reciprocity theorem and explain its significance in antenna analysis.

1. Explain the difference between reactive near field, radiating near field, and far field regions. Why are most antenna parameters defined in the far field?

1. Describe how a half-wave dipole is formed from an open-circuited transmission line. Why does it radiate?

1. Why is the physical length of a resonant half-wave dipole slightly less than λ/2?

1. Explain the image principle as applied to a quarter-wave monopole over a ground plane.

1. Define radiation resistance and loss resistance. Derive the expression for antenna efficiency in terms of R_R and R_L.

1. Why does antenna length influence radiation efficiency?

1. Why must the feeder impedance match the antenna input impedance? What are the consequences of mismatch?

1. Define antenna bandwidth. How is it typically specified in practice?

1. Explain the relationship between antenna Q and fractional bandwidth.

1. How does increasing conductor diameter affect antenna bandwidth? Why?

1. Explain the principle of operation of a multiband trap dipole. How does the effective electrical length change with frequency?

1. What distinguishes a resonant antenna from a travelling-wave antenna?

1. Why are travelling-wave antennas generally broadband?

1. Compare the operational characteristics of: a long-wire antenna, a rhombic antenna, and a terminated sloping-V antenna

1. Describe the radiation pattern of a small loop antenna. Why does it exhibit ambiguity in direction finding?

1. How does a sense antenna resolve directional ambiguity in loop-based direction-finding systems?

1. How does a reflector increase the directivity of an antenna?

1. Explain the principle of beam steering in a phased-array antenna.

1. What is meant by amplitude tapering? How does it affect sidelobe levels and gain?

1. Describe the trade-offs between: beamwidth, directivity, sidelobe level, efficiency.

1. What is spatial selectivity? How does it improve system performance?

1. Explain how multibeam antennas enable spatial frequency reuse.

1. Distinguish between spatial selectivity (beamforming) and spatial multiplexing (MIMO).

1. What is beam shaping? Describe two methods by which it can be achieved.

1. Why is sidelobe suppression important in communication systems?

1. Explain why antenna height affects radiation pattern and take-off angle for ground-based systems.

1. Discuss the trade-offs involved in shortening an antenna using loading coils.