FREQUENCIES FOR SATELLITE COMMUNICATIONS

Whenever we listen to radio broadcasts, watch TV, use a cellular phone or talk on a CB radio, invisible waves of electro-magnetic energy are bringing us messages from distant locations. These invisible waves continually bombard us as we walk down the street, play sports or putter about the garden. We only become aware of the electro-magnetic soup that surrounds us if we have the right antenna and receiver for tuning in to these signals.

Back at the turn of the Century, Marconi discovered that it was possible to combine messages with invisible waves of electro-magnetic energy that radiate through space at the speed of light. For the first time ever, mankind was able to communicate over vast distances almost instantaneously. By the late-1920s, millions of people world-wide were tuning in to AM radio stations: the world's first electronically generated virtual realities. Back then, these transmissions were simply called radio waves. As time went on, however, it became apparent that waves of electro-magnetic energy could be used to transmit all sorts of information, including TV pictures.

In many respects, electro-magnetic waves are similar to waves on the ocean. In one complete cycle of a wave, water rises from sea level until it swells to reach the crest of the wave, then plummets downward into the wave trough before rising again to sea level. A communication signal is the electro-magnetic equivalent of a message in a bottle; it rides a never ending succession of waves before arriving at its final destination. The frequency of any communications signal is the number of cycles per second at which the radio wave vibrates or cycles. Electro-magnetic waves cycle at phenomenal rates: one thousand cycles per second is called a kilohertz (kHz), one million cycles per second a megahertz (MHz), and one billion cycles per second a gigahertz (GHz). Today we refer to the continuum of frequencies used to propagate communications signals, 100 kHz to 100 GHz and beyond, as the electro-magnetic spectrum.

The distance that each wave travels during a single cycle is called its wave length. There is an inverse relationship between frequency and wave length: the higher the frequency, the shorter the wave length.

Each sub-set or band of frequencies within the electromagnetic spectrum has unique properties that are the result of changes in wave length. For example, Medium Wave signals (500 kHz to about 3 MHz) radiate along the Earth's surface over hundreds of miles, perfect for relaying AM radio stations throughout a region.

International radio stations use the Short Wave bands (3 to 30 MHz) to span distances of thousands of miles; the ionosphere, upper layers of the Earth's atmosphere that are electrically charged by the sun, reflects these short waves back down to Earth, much as a mirror or any other shiny metal object can reflect beams of light.

TV and FM radio broadcasters use the Very High Frequency (VHF) and Ultra High Frequency (UHF) bands located from 30 to 300 MHz and 300 to 900 MHz because these signals only cover short distances; they cannot travel very far along the Earth's surface or skip off the ionosphere. The advantage to using these frequency bands for local communications is that dozens of TV and FM radio stations can use identical frequencies within any one country or region without causing interference.

The International Telecommunication Union, an agency of the United Nations, has set aside space in the super high frequency (SHF) bands located between 2.5 and 22 GHz for satellite transmissions. At these frequencies, the wave length of each cycle is so short that the signals are called microwaves. These microwaves have many characteristics of visible light: they travel directly along the line of sight from any satellite to its primary coverage area and are not impeded by the Earth's ionosphere.

The scientists who developed the first microwave radar systems during World War II assigned a letter designation to each microwave frequency band. For example, the 800 MHz to 2 GHz frequency range was called the L band, 2 to 3 GHz: the S band; 3 to 6 GHz: the C band; 7 to 9 GHz: the X band; 10 to 17 GHz: the Ku band; and 18 to 22 GHz: the Ka band. At the dawn of the Satellite Age during the mid-1960s, microwave engineers decided to carry forward the existing radar terminology and apply it to the communications satellite bands as well.

The world's first commercial satellite systems used the C band frequency range of 3.7 to 4.2 GHz. By the late 1960s, many telephone companies around the world had numerous terrestrial microwave relay stations operating within the 3.7 to 4.2 GHz frequency range. The amount of power that any C-band satellite could transmit had to be limited to a level that would not cause interference to terrestrial microwave links.

The first commercial Ku band satellites made their appearance in the late 1970s and early 1980s. Relatively few terrestrial communications networks were assigned to use this frequency band; Ku-band satellites could therefore transmit higher-powered signals than their C-band counterparts without causing interference problems down on the ground.

Ku-band satellite antennas have a much narrower beam width, the corridor through which the dish looks up at the sky, than C-band parabolic antennas of a given diameter. There is a direct relationship between wavelength and antenna beam width: the shorter the wavelength, the narrower the beam width.

From the graph above, we can see that a 60cm C-band antenna could potentially receive three satellites within its main beam if the satellites are separated by 3 degrees in longitude. In North America, where 2 degree spacing has become the norm, a 60cm C-band antenna could potentially have signals from four satellites falling within its main beam.

References to any satellite's orbital location, as well as to the intervals between adjacent satellites, are made in degrees of longitude. Keep in mind, however, that the geostationary orbit is a circle and the reference point for the calculation of degrees longitude is the Earth's center. Since we all live on the surface of the Earth, the apparent spacing between two satellites will be greater than the actual spacing in degrees of longitude. The precise amount of variance between actual and apparent spacing is a function of site latitude and the difference between site longitude and satellite longitude. For the purpose of calculating the charts which appear in this article I have assumed an apparent spacing of 3.4 degrees between satellites which are separated by three degrees in longitude.

The next graph shows the equivalent performance of the various antenna apertures when receiving Ku-band satellite signals. The 30 to 60cm dish becomes possible because of the dramatic reduction in parabolic antenna beam width that takes place when we use the higher satellite frequency bands.

Satellite signal strength is expressed in decibels referenced to one Watt of power (dBW). An increase of 3 dBW represents a doubling of power; 10 dBW represents a ten-fold increase; and 20W dBW a one-hundred-fold increase.

C-band satellites typically transmit signal levels ranging from 33 to 38 dBW. The strongest signals fall within the center of each satellite's coverage beam, with signal intensity decreasing outward from there. Depending on the location of the receiving site within the satellite's primary coverage beam, the antenna apertures required to receive crystal clear TV pictures typically range from 2.0 to 3.7m in diameter. Today's Ku-band satellites transmit nominal signal levels ranging from 47 to 52 dBW, a 14 dBW increase in power over what most C-band satellites can deliver. Receiving antennas as small as 30cm in diameter can therefore be used to receive Ku-band satellite signals. This significant reduction in antenna size lowers the cost of the receiving equipment and simplifies the system installation requirements.

There is one major drawback to satellites downlinking signals at frequencies greater than 10 gigahertz: the length of these microwaves is so short that rain, snow or even rain-filled clouds passing overhead can reduce the intensity of the incoming signals. At these higher frequencies, the length of the falling rain droplets are close to a resonant sub-multiple of the signal's wave length; the droplets therefore are able to absorb and de-polarize the microwaves passing through the Earth's atmosphere.

In places such as Southeast Asia or the Caribbean, torrential downpours can lower the level of the incoming Ku-band satellite signal by 20 dB or more; this may severely degrade the quality of the signals or even interrupt reception entirely. The duration of rain outages, however, is usually very short and typically occurs in the afternoons or early evenings rather than during the prime time evening viewing hours. For most Ku-band satellite TV viewers, these service interruptions will only amount to the loss of a few hours of viewing time over the course of any year.

To help counteract the effects of rain fade, Ku-band system designers typically use a larger antenna than what would be required under clear sky conditions. This increase in antenna aperture gives the system several dB of margin so that the receiving system will continue to function in light to moderate rain storms. Satellite TV viewers in arid regions such as central Australia or the Middle East will rarely experience rain outages. In the Middle East, however, satellite dish owners may experience outages caused by intense sand storms. The presence of any atmospheric particulate, even sand, can have an adverse effect on satellite TV reception.

Although the International Telecommunication Union has assigned S-band frequency spectrum for direct-to-home TV transmissions, few organizations have so far elected to use this spectrum for satellite broadcasting. One limiting factor has been the bandwidth available: just 100 MHz of spectrum from 2.5 to 2.6 gigahertz. India and Arabsat have included S-band transponders on their C-band satellites to make it cost effective to launch S-band payloads into geostationary orbit. In 1997, Indonesia will launch Indostar 1, the world's first dedicated S-band satellite (carrying one transponder transmitting 70 Watts of power) to an orbital assignment of 107.7 degrees east longitude. Digital video compression will make it technically and economically feasible for Indostar to broadcast a multi-channel TV package to subscribers in Indonesia.

Indostar intends to use antennas ranging from 70cm to 1m in diameter for DTH reception. As Figure 9 illustrates, DTH systems such as Indostar will only work if there are no other adjacent satellites using the same frequency spectrum. Even a 1.2m antenna would have problems in a 3 degree spacing environment. S-band does offer the advantage of minimal rain fade problems, a significant consideration for satellite broadcasters in the world's highest rain rate region. But in terms of fulfilling global satellite communications needs in the next Century, S-band's contribution will be minimal.

Sixteen years ago, the number of commercial Ku-band communications satellites orbiting the earth could be counted off using the fingers of one hand; today there are more than seventy-five Ku-band satellites in operation world-wide. Within the past few years, satellite operators have begun exploring the brave new world at 20 GHz. Only a few Ka-band satellites are currently in orbit: ACTS (USA); Superbird and N-STAR (Japan), DFS Kopernikus (Germany), and Italsat (Italy). However, expect the use of this higher frequency band to increase dramatically during the first decade of the 21st century.