Review - Chapter 4

What is an isotropic antenna?

An isotropic antenna is a point in space that radiates power in all directions equally.

What is the difference between diffraction and scattering?

Diffraction occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave. The edge in effect become a source and waves radiate in different directions from the edge, allowing a beam to bend around an obstacle. If the size of an obstacle is on the order of the wavelength of the signal or less, scattering occurs. An incoming signal is scattered into several weaker outgoing signals in unpredictable directions.

What is direct broadcast satellite (DBS)?

Direct broadcast transmission is a technique in which satellite video signals are transmitted directly to the home for continuous operation.

What factors determine antenna gain?

Effective area and wavelength.

Why are the wires twisted in twisted-pair copper wire?

The twisting of the individual pairs reduces electromagnetic interference. For example, it reduces crosstalk between wire pairs bundled into a cable.

What two functions are performed by an antenna?

The two functions of an antenna are: (1) For transmission of a signal, radio-frequency electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water); (2) for reception of a signal, electromagnetic energy impinging on the antenna is converted into radio-frequency electrical energy and fed into the receiver.

Why must a satellite have distinct uplink and downlink frequencies?

A satellite must use different uplink and downlink frequencies for continuous operation in order to avoid interference.

Show that doubling the transmission frequency or doubling the distance between transmitting antenna and receiving antenna attenuates the power by 6dB.

5. From Equation 4,2, the ratio of transmitted power to received power is Pt/P r = (4πd/λ)2 If we double the frequency, we halve λ, or if we double the distance, we double d, so the new ratio for either of these events is: Pt/Pr2 = (8πd/λ)2 Therefore: 10 log (Pr /Pr2) = 10 log (22 ) = 6 dB

What is the advantage of a parabolic reflective antenna?

A parabolic antenna creates, in theory, a parallel beam without dispersion. In practice, there will be some beam spread. Nevertheless, it produces a highly focused, directional beam.

Coaxial cable is a two-wire transmission system. What is the advantage of connecting the outer conductor to ground?

An electromagnetic wave cannot penetrate an enclosing conductor. If the outer conductor of a coaxial cable is everywhere held at ground potential, no external disturbance can reach the inner, signal-carrying, conductor.

Indicate some significant differences between broadcast radio and microwave.

Broadcast is omnidirectional, does not require dish shaped antennas, and the antennas do not have to be rigidly mounted in precise alignment.

What is the primary cause of signal loss in satellite communications?

Free space loss.

What are some major advantages and disadvantages of microwave transmission?

Point-to-point microwave transmission has a high data rate and less attenuation than twisted pair or coaxial cable. It is affected by rainfall, however, especially above 10 GHz. It is also requires line of sight and is subject to interference from other microwave transmission, which can be intense in some places.

Describe the components of optical fiber cable.

Optical fiber consists of a column of glass or plastic surrounded by an opaque outer jacket. The glass or plastic itself consists of two concentric columns. The inner column called the core has a higher index of refraction than the outer column called the cladding.

What is refraction?

Refraction is the bending of a radio beam caused by changes in the speed of propagation at a point of change in the medium.

What are some major limitations of twisted-pair wire?

Twisted pair wire is subject to interference, limited in distance, bandwidth, and data rate.

What is the difference between unshielded twisted pair and shielded twisted pair?

Unshielded twisted pair (UTP) is ordinary telephone wire, with no form of electromagnetic shielding around the wire. Shielded twisted pair (STP) surrounds the wire with a metallic braid or sheathing that reduces interference.