Radio Communications Engineering

Assignment - RF Propagation and AM/ FM Broadcasting Systems

Section 1 - RF Propagation

Introduction I

Electromagnetic waves are the same type of radiation as light, ultra-violet and infra-red rays, differing from them in their wavelength and frequency. Electromagnetic waves have both electric and magnetic components that are inseparable. The planes of these fields are at right angles to one another and to the direction of motion of the wave. Radio signals are a form of electromagnetic wave, and as they are the way in which radio signals travel, they have a major bearing on RF antennas themselves and RF antenna design.

According to Miller & Beasley (2002) “An antenna is a circuit element that provides a transition from a guided wave on a transmission line to a free space wave and it provides for the collection of electromagnetic energy.” In transmitting systems the RF signal is generated, amplified, modulated and applied to the antenna. In receiving systems, the antenna collects electromagnetic waves, which cuts the antenna elements and induce alternating currents and transferred to the receiver.

It is also necessary to understand the different areas of the atmosphere or other areas that affect radio propagation and radio communications signals. There are many radio propagation methods or radio wave propagation scenarios in real life. Often radio signals may travel by several means, signals travelling using one type of propagation interacting with another.

However to build up an understanding of how a signal reaches a receiver, it is necessary to have a good understanding of all the possible methods. From the above view, the interactions can be better understood and it is then possible to understand some of the reasons why mobile radio communications systems or two way radio communications systems work in the way they do.

Read more about areas of the atmosphere that affect radio propagation and using your knowledge in Electromagnetic waves, complete the following researches based on theory and precise work.

Task 1 (P2.1)

According to the introduction given above, an Antenna is the transducer which does emission and induction of EM energy, respectively in both transmission and receiving ends. By using diagrams appropriately, give a clear explanation about how electromagnetic radiation happens by exemplifying the technical concepts of how emission and induction take place at antenna elements.

Consider: - Antenna Gain, Bandwidth of and antenna, Beam-width, Polarization, and Radiation Pattern.

Task 2 (P2-2)

Do refer to proper resources and make an acceptable discussion about the appropriate mode of propagation related to different frequency bands of the radio spectrum. (The word count must not be less than 300 words)

Task 3 (P2-3)

With the aid of sketch drawings, describe the structure and nature of the ionosphere with reference to daily, seasonal and long-term changes. Appropriate discussions must be done accordingly. Furthermore, give a clear explanation about how sky waves bend towards the Earth according to atmospheric conditions. State the ratio for the refractive index and explain how the absolute refractive index for a medium is obtained from it (The word count must not be less than 400 words) (Context of the answer must consist of theoretical concepts and research base findings)

Introduction II

From the view of Kseh (2009), Radio is defined as the transmission and reception of electrical impulses or signals by means of electromagnetic waves without the use of wires. Basically, radio waves are electromagnetic radiation transmitted through free space to a receiver. Radio transmission can take place using many different frequency bands. Each band exhibits certain advantages and disadvantages.

According to Green (1985), there are performance differences between radio frequencies. Low frequencies can travel much further without losing power, but they carry much less information because the bandwidth is much lower. High frequencies offer much greater bandwidth than lower frequencies, but they are greatly affected by interference from a variety of sources. Very high frequencies suffer greatly from adverse weather conditions, particularly precipitation. There are different modes of RF propagation which happens and different bandwidth limitations are announced for each of those modes.

The way that radio signals propagate, or travel from the radio transmitter to the radio receiver is of great importance when planning a radio communications network or system. This is governed to a great degree by the regions of the atmosphere through which they pass.

Without the action of the atmosphere it would not be possible for radio communications signals to travel around the globe on the short wave bands, or travel greater than only the line of sight distance at higher frequencies. In fact the way in which the atmosphere affects radio communications is of tremendous importance for anyone associated with radio communications, whether they are for two way radio communications links, mobile radio communications, radio broadcasting, point to point radio communications or any other radio.
Based on modes for the propagation of radio waves, complete the following tasks.

Task 4 (P2-4)

By considering all those modes of propagation, accordingly with the bandwidth limitations, do a proper discussion which exemplifying all the possible situations, either advantage or disadvantage, that can be met by a radio signal through free space in between Tx and Rx.

Section 2 - Modulation

Introduction I

Amplitude modulation of a sinusoidal carrier wave is employed in radio communication systems as the means of shifting, or translating, or converting, a signal from one frequency band to another. Audio signals are transmitted and received by means of aerials but, since no kind of aerial can operate at such low frequencies, it is necessary to shift each signal to some higher frequency.

To obtain the maximum utilization of an available frequency spectrum, it is necessary for signals to be frequency translated to occupy different parts of that frequency spectrum. Frequency translation of a signal is accomplished by the signal amplitude modulating a carrier of appropriate frequency.

Task 5 (P1.1)

(a) Consider the two general expressions of the sinusoidal carrier and the modulating wave. According to that consideration, show the derivation of the expression which tells us the instantaneous voltage of the amplitude modulated signal. Make appropriate discussions about all the parameters considered and the components of the modulated signal according to the final expression obtained.

(b) A modulating signal occupying the frequency band 68 – 72 KHz amplitude modulates a 100 KHz carrier. Calculate the band width occupied by the modulated wave.

Task 6 (P1.2)

Green (1995) shows that, most of the power contained in an amplitude modulated wave is developed by the carrier component. Since this component carries no information, it may be suppressed during the modulation process, and then all the transmitted power is associated with the Upper and the Lower sidebands. Out of several sideband modulation as well as suppressing techniques, Independent Side Band (ISB) modulation and Independent Side Band Suppressed Carrier (ISBSC) modulation is more efficient in the utilization of the available frequency spectrum.

Give a brief description about ISB and ISBSC modulation and explain the advantage of ISB over SSB.

Introduction II

Furthermore, analog modulations can be done using angle modulation. The two major parts of angle modulation are Frequency modulation (FM) and Phase modulation (PM). Both FM and PM are used extensively in communications systems. Especially FM is using in radio broadcasting and for the transmission of sound signals in standard TV.

However, PM by itself and in combination with AM is used extensively in modern datacommunications systems. Angle modulation has a very important advantage over AM in its ability to provide increased immunity to noise. Angle-modulation systems typically require a larger bandwidth than AM systems, a necessary trade-off for its improved resistance to noise.

Task 7 (P1.3) M2

a) Explain the main difference between frequency and phase modulation by mainly considering the variation happens in the carrier wave components in each modulation modes. (NOTE: Appropriate equations must be derived and explanations must be according to the equations derived).

b) Explain the effect of Pre-emphasis on the out-put Signal-to-Noise Ratio (SNR) of FM transmission systems and the effect of De-emphasis on the FM receiving systems.

Section 3 - AM/ FM Sound Broadcasting Systems

Introduction I

The purpose of any radio communication system is to transmit intelligence from one point to another. The communication may be unidirectional, as in the case of radio broadcasting, or it may be bi-directional mostly with mobile radio telephony systems. At the transmitting end, the information signal must modulate a suitable carrier frequency wave to translate the signal to the allocated part of the frequency spectrum, and then be amplified to the necessary transmitted power level.

Task 8 (P3.1) D3

a) Do a research and briefly discuss about the Telecommunications Regulatory Commission of Sri Lanka (TRC) and the services offered by the above commission.

b) According to the research above (part (a)), by referring the services of TRC, discuss about the Frequency Management procedure for FM sound broadcasting systems.

Task 9 (P3.2) M3

Clearly describe about high-level and low-level amplitude modulation. Illustrate your answers with block diagrams of AM transmitters for both high and low level.

Task 10 (P3.3)

Derive a schematic system diagram for a FM sound broadcast transmitter, which is either capable for VHF or UHF bands and give proper explanations about the functions of each stage of the transmitter.

Task 11 (D1), (D2)

a) Explain an oscillator and discuss about RF oscillators considering the following points. (Introduction to RF oscillators, Hartley oscillator, Colpitts Oscillator, Clapp Oscillator, Crystal oscillator and Voltage controlled Oscillator) (D2)

b) With the aid of a block diagram, explain the operation of Frequency Synthesizer with phase lock function. (D2)

c) Discuss about the importance of Impedance matching in the case of maximum power transfer in communication systems. (D1)

Introduction II

According to Green (1995) the functions of radio receiver are: to select the wanted signal out of all the unwanted signals picked up by the receiving antenna to extract the information contained in the modulated, received signal and to produce an audio frequency out-put of sufficient power to operate the loudspeaker or the other receiving device.

According to IEEE (2012), there are many different ways a radio receiver can perform this conversion. Reginald Fessenden was the first to apply one of these methods, called the heterodyne principle, to wireless communications in 1901. Fessenden created the term heterodyne from the Greek words for "other" and "force." The heterodyne principle is based on a well-known sound phenomenon where the combination of two audio tones with frequencies A and B results in an oscillation equal to frequency A minus B. This phenomenon is exploited in the tuning of pianos.

‘Fessenden suggested that the heterodyne principle be employed in a radio receiver by mixing the incoming radio wave with a locally generated wave of slightly different frequency. The combined wave then drives the diaphragm of an earpiece at the frequency of the audio’.

Task 12 (P4.1) M1

a) With the aid of a block diagram, explain the basic operation of a Tuned Radio Frequency (TRF) receiver. Don’t forget to consider the terms within brackets, where appropriate. (Selectivity, Sensitivity, Ganging).

b) As the second stage, discuss about the disadvantages of the TRF receivers which gives us the idea about why TRF operation is not so good for modern receivers.

Task 13 (P4.2)

Derive a system diagram for a superheterodyne radio receiver and briefly explain all the stages exist in the system which altogether gives the idea of the principals of superheterodyne operation.

Consider: RF stage, Mixer stage, IF (Intermediate Frequency) amplifier, Detector stage and Audio stage.

Task 14 (P4.3)

Green (1995) shows that a radio transceiver both sends and receives radio signals. In order to be classified as a transceiver, the transmitter and the receiver must use the same set of wiring as well as the antenna or be located within the same device. In a radio transceiver, the user is able to perform a wide range of functions for both the receiver and broadcaster of signals on radio frequencies.

By using a system diagram of a VHF AM transceiver, explain the necessary arrangements for a transceiver operation.

Consider:

Transmitter and receiver parts

Aerial switch

The bunch of crystal oscillators

AGC (Automatic Gain Control)

Squelch circuit