These transmitters operate on the frequency range 645 KHZ to 1605 KHZ. These transmitters have a specific service area within a particular country depending on the power of the transmitters. Thus many such stations have to be set up to cover a country. These stations are used to cover a particular country and are called National Stations. The transmitters which operate on the frequency range 3 MHZ to 30 MHZ are called short wave transmitters. The transmitted waves from such stations are fed to directional antenna systems which convert into sky waves and are used to cover specific target areas lying into other countries and thus these are called international radio stations. This mode of service has been in use since long and is still in use.
The advent of satellite communication is now being used to Internationalize Radio Programmes with a very high level of quality as it operates on very high frequencies and is free of disturbances.
Prospects and Challenges:
Better prospects are now available in radio through satellites and to adopt this system is posing challenges in the field of radio communication.
The space communications equivalent of an earth-based repeater is called an active communications satellite.
A high-powered signal is transmitted from the sending station on the Earth. This line of sight microwave broadcast travels through the atmosphere and to the receiver of the active satellite. The output of this receiver is fed directly to a transmitter within the satellite and a new signal which contains the same Information as the old one is transmitted back to the Earth. Even when very high amounts of power were used by the Earth transmitter in a passive satellite system,
the signals that were received back on Earth were extremely weak. The same high amounts of power most still be used with an active communications satellite system, but since the signal is retransmitted Out in space within the satellite proper, the received transmissions back on Earth are far stronger.
The active satellite system is the one we are interested in as far as television receive-only Earth stations are concerned. The original broadcasts are transmitted from very high-powered Earth stations whose antennas are aimed directly at the satellite. These stations transmit on a frequency of around six gigahertz. Out in space, the active satellite is equipped with transponders. A transponder is simply a transmitter and receiver which are connected to one another. The detected information from the receiver is fed to the input of the transmitter and beamed back to Earth. One satellite may have a few transponders or possibly even twenty or more. When receiving satellite TV at home, each transponder serves as a separate television channel which can be selected in much the same manner as is done today with your present set.
Satellites are usually physically small devices and space must be conserved wherever possible. For this reason, a satellite with many different cahnnels may have only two antennas, one for transmit and one for receive. Each chiwmel shares these antennas by using them for small fractions of a second. To provide an easily understood example, let’s assume that a satellite has two channels which must share the same antennas. Channel A may use the antennas for J00 milliseconds and then be switched off for an equal amount of time while channel B uses the system for 1O0 milliseconds. When channel B is switched off, channel A uses the antennas again. Channel A and B will be switched on and off many times during the blink of an eye,
but you could never tell this by watching a received picture at your Earth station because of the speed with which the switching occurs. If you were watching channel A. you would never know when channel B was using the same antennas t« transmit on another frequency which you were not receTiving.
In any event, the on/off nature of satellite transponders is totally unnoticeable by human beings.
As was stated earlier, most television stations which use satellites for their broadcasts use very high-powered transmitters on the ground. They transmit at a frequency of approximately six gigahertz, but these signals do not come back to earth at the same frequency. Referring to Fig. the six gigahertz transmission leaves the earth and travels to the satellite. The orbiter’s receiver is designed to detect a six gigahertz signal. It pulls the audio and video informations from the transmissions and then feeds it directly to the satellite transmitter, which has an output at a frequency of about four gigahertz. This is the frequency which the earth-based television receive station is set up to detect.
Since we know that the satellite signal is originally transmitted on the Earth, travels into space, is retransmitted by the satellite at a different frequency and is finally received back on Earth again, it can be said that the signal that is ultimately received by an Earth station is a function of:
2.2 The signal transmitted to the satellite at six gigahertz.
2.3 Signal processing in the satellite.
2.4 Signal transmitted at four gigahertz from the satellite.
2.5 Directivity of the satellite antennas gain.
2.6 Path loss.
2.7 Gain of the receiving antenna.
2.8 Noise temperature of the antenna.
2.9 Low noise amplifier noise temperature and gain.
2.10 Cable loss to the receiver.
2.11 Receiver noise figure.
All of these terms may not be familiar to you however. This list is presented to show the difficult factors which determine how well a signal is received by your personal Earth station. There are many different satellites in orbit around the Earth, it is from these satellites that a whole new world of television enjoyment emanates. Su re, all of the signals are originally transmitted here on Earth, but with a a satellite deep in space, these transmissions can be received bv so many more persons. The, wonderful thing about receiving signals from satellites is found in the fact that we ‘end to pay very little attention to these multi-million dollar orbiters. We know they are up there and we know their positions in order to properly aim our antennas: but other than this, the erath station equipment is the part of this complex system which gets the most attention. The satellite are reliable, always present and simply do not require the attention that our personal Eart stations require. Billions upon billions of dollars of research have gone into making our satellite programme as dependable and useful as it is.