Chapter 11 / 11.9

11.9 REVISION QUESTIONS

1. Draw a diagram or diagrams that show the break up of radiated RF energy into the following components: ground wave, sky wave, scattered wave, space wave, surface wave, direct wave, and ground-reflected wave.

1. Relate the major propagation modes (surface, space, sky, troposcatter, ionospheric scatter, meteor-burst) to the frequency bands discussed in Chapter 2. Explain why each mode predominates in its respective band.

1. Write an equation for the Friis transmission equation. State clearly all assumptions underlying its derivation.

1. Briefly describe the need to take into account an equivalent radius of the Earth for VHF space wave communications. Explain the difference between the geometric horizon and the radio horizon.

1. List the five major sources of radio path propagation loss. Describe the source of each loss and list its relative magnitude.

1. Write equations for free-space loss (FSL), plane-earth loss (PEL) and Egli’s formula.

1. Briefly describe surface wave communications, describing the effect of ground conductivity, tilt angle, frequency of operation, type of terrain and polarization.

1. Describe the processes by which sky-wave propagation is possible. Include brief discussion on the following:

1. the formation and the structure of the ionosphere;

1. the effect of each of the layers on propagation;

1. the effect on range of the frequency, angle, and power of propagation;

1. the best frequency to use and an appropriate frequency planning process; and

1. a summary of ionospheric variations and their effect on propagation.

1. Briefly describe the three main types of scattered-wave propagation.

1. Define basic propagation loss (BPL) for a troposcatter system. Show how it differs from free-space loss and explain the approximate 30 log₁₀f frequency dependence.

1. Explain the concept of scatter angle and horizon angle. How does the sum of horizon angles affect scatter angle, scatter volume height, and total propagation loss?

1. Define aperture-to-medium coupling loss. Why does this loss increase as antenna gain and aperture size increase? Under what conditions does additional multipath cancellation occur?

1. Distinguish between short-term fading and long-term fading in troposcatter systems. State their physical causes and typical time scales.

1. Why are troposcatter systems considered survivable and relatively resistant to interception and jamming?

1. Compare ionospheric scatter with classical sky-wave propagation. Why does ionospheric scatter not require reflection from a discrete layer?

1. What frequency range is typically used for ionospheric scatter? Why are practical data rates generally limited to below about 10 kbps?

1. Explain why ionospheric scatter systems require high transmit powers and diversity techniques.

1. Explain the physical mechanism of meteor-burst communication. Distinguish between underdense and overdense trails.

1. Why are instantaneous burst data rates higher than average throughput in meteor-burst systems?

1. What factors determine meteor-burst link performance (e.g., meteor rate, path length, antenna gain, required reliability)?

1. Compare space-wave LOS microwave, troposcatter, and satellite communications for a 400 km link. Discuss infrastructure, cost, capacity, and reliability trade-offs.

1. Explain why troposcatter and meteor-burst systems remain relevant in certain military and environmental telemetry roles despite the availability of satellite systems.