Applications

308 Radio Engineering for Wireless Communication and Sensor Applications

Table 12.1

Frequency Ranges for Broadcasting in Region 1

several advantages: The radio spectrum is used more effectively than in analog systems, the quality of the sound and picture is excellent even if the receiver is moving, and many kinds of services can be attached to the signal.

Digital Audio Broadcasting (DAB) is a European standard for digital radio transmissions. DAB uses a coded orthogonal frequency division multiplex

(COFDM), in which information is coded effectively to reduce the number of errors. Bits are then distributed to several orthogonal carriers, which are modulated using a differential QPSK. A signal with 1,536 carriers has a bandwidth of about 1.5 MHz and can carry, for example, 6 stereo programs or 18 speech programs. Digital Video Broadcasting (DVB) is a standard for broadcasting digital TV. It is also based on COFDM. The maximum number of carriers is 6,817, and alternative modulation methods are QPSK, 16QAM, and 64QAM. Four or five TV programs fit to an 8 MHz wide channel. Because there are many carriers in COFDM, in each carrier the symbol rate is low and the symbol duration long. This makes COFDM insensitive to the adverse effects of multipath propagation and allows a use of singlefrequency networks where all synchronized transmitters use the same frequency.

12.1.1 Broadcasting in Finland

In Finland, radio and television programs are delivered by the Finnish Broadcasting Company, Yleisradio (YLE) and some commercial companies. The network includes about 40 large transmission stations, 160 substations, and link stations. There are also two MF stations and one HF station. DAB test transmissions were started in 1994 and regular transmissions in the

VHF III range (174–240 MHz) began in 1998. Test transmissions of digital TV were started in 1998 and regular transmissions in the UHF range began in 2001.

The antenna masts for FM, DAB, and TV broadcasting are usually 100m to 300m high. Several antennas are placed on a mast so that the antennas of the highest frequency range are placed on the top. The directional pattern should usually be omnidirectional in the horizontal plane, whereas in the vertical plane the beam should be narrow and shaped to give equal field strength at different distances. The mast disturbs antennas, except the one on the top. The antennas lower in the mast have to be made of several elements. There are usually three or four elements in the horizontal plane and several elements in the vertical plane. The vertical pattern is realized by adjusting the phase and amplitude of the elements. The antenna elements are often dipoles with a reflector. Slot antennas are also used at UHF. Tetrodes and parallel-connected transistor amplifiers are used as the output stages of the VHF transmitters. At UHF, klystrons work as output stages.

Because frequencies used for the FM, DAB, and TV need an almost free LOS path, one transmitter can cover typically an area with a radius of about 70 km. Therefore, about 40 stations are needed to cover all of Finland, which has a land area of 338,000 km2 as well as scattered islands. Figure 12.1 shows the locations of FM stations and their coverage areas. At the outer limit of the coverage area, the field strength is 0.5 mV/m (54 dBmV/m). Reception is often possible at distances 20 km to 30 km greater than these limits. The equivalent isotropic radiated power (EIRP), or the transmitted power that would be needed in the case of an isotropic antenna to give the same field strength as the actual antenna, is usually a few tens of kilowatts for FM transmissions. Total real radiated power is much less than EIRP. The polarization of FM signals is horizontal. In Lapland, circular polarization is also used. DAB transmissions are vertically polarized.

EIRP of TV transmissions can be as high as 1,000 kW. The polarization of TV signals is mainly horizontal; in the VHF I range vertical polarization is also used. In addition to the main transmission stations, many substations are needed to cover the whole country. A substation receives the signal from a main station, converts the frequency of the signal, and transmits it to those viewers who are unable to receive the signal from any main station. The coverage of the TV-1 and TV-2 networks is more than 99.9% of the population.

MF stations are in Helsinki (558 kHz) and Pori (963 kHz). The antennas are short (compared to wavelength) monopoles and the output stages are tetrodes. The HF station in Pori operates in several bands. The

310 Radio Engineering for Wireless Communication and Sensor Applications

Figure 12.1 FM radio stations and their coverage areas in Finland (338,000 km2 ).

antennas are horizontal dipole arrays. Reception of these HF signals is possible even in Australia.

In addition to YLE, many local FM radio stations operate in Finland. They use vertical polarization. In urban areas cable network companies distribute TV programs received from satellites.

12.1.2 Broadcasting Satellites

A satellite can be used as a platform for a broadcasting station. One single transmitter onboard a satellite can cover a whole country or continent.

Broadcasting and most other communication satellites are placed on geosynchronous Earth orbit (GEO) at the height of 35,800 km above the equator. At that height the orbiting time is equal to the rotation time of the Earth, 23 hours and 56 minutes. Then the satellites seem to be stationary and the antennas on Earth can be fixed. Positions of the satellites have to be corrected every now and then due to disturbances in the gravitational field. The amount of the steering gas limits the lifetime of satellites to about 10 years.

Broadcasting satellite service (BSS) satellites transmit (downlink) at frequencies from 11.7 GHz to 12.5 GHz into Europe. Fixed satellite service (FSS) satellites transmitting in the bands 10.7 to 11.7 GHz and 12.5 to 12.75 GHz also send radio and TV programs. The band 11.7 to 12.5 GHz is divided into 40 channels having a separation of 19.18 MHz. The width of the channels is 27 MHz and they are interleaved in orthogonal polarizations. BBS satellites send circularly polarized waves and FSS satellites linearly polarized waves. Analog TV programs are frequency modulated. Several digitally modulated TV programs fit in one 27-MHz band. The programs are sent to broadcasting satellites (uplink) in the band 17.3 to 18.1 GHz and to FSS satellites in the band 14.0 to 14.5 GHz. Due to crowding of the 12-GHz band, 20-GHz (downlink) and 30-GHz (uplink) bands will be introduced for the broadcasting service.

The microwave unit of a broadcasting satellite includes antennas and transponders. A transponder consists of a low-noise receiver, mixers and oscillators for frequency conversion, and power amplifiers. To improve reliability, transponders often have parallel units; for example, a broken power amplifier can be replaced with another unit. The power transmitted is typically about 100W so that an antenna having a diameter of 0.5m can be used for reception. The beam of the satellite antenna is shaped according to the service area. Figure 12.2 shows a typical footprint of a TV satellite.

In Finland, TV programs can be received from more than 20 satellites. The maximum elevation angle of geostationary satellites is only 21° in the most southern part of Finland. Because transmissions are usually directed to Central Europe, especially in the northern Finland, a large receiving antenna may be necessary.

Example 12.1

A geostationary satellite transmits with a power of Pt = 100W. The width of the circular antenna beam is 3°. The beam can be assumed ideal so that the normalized pattern level is 1 within the beam and 0 outside the beam. Find the power density at a distance of r = 40,000 km. What is the power received with a 0.5-m dish?