17.5.2 Earth station antennas

Earth station antennas are at the earth end of satellite links. High gain is needed to receive the weak signals from the satellite, or to transmit strong signals to the satellite. The antennas can be divided into three types:

1. Large antennas required for transmit and receive on the INTEL­ SAT type global networks with gains of 60 to 65dBi (15 to 30 metres diameter).

2. Medium sized antennas for cable head (TVRO) or data receive only terminals (3-7 metres diameter).

3. Small antennas for direct broadcast reception (0.5-2 metres diameter).

Types 1 and 2 have to satisfy stringent specifications imposed by regulatory bodies. When the recommended spacing of satellites in the geostationary arc was 3 degrees, the pattern envelope was specified by 32 - 35 Iog9. This could be met with a symmetric reflector antenna. With the new spacing of 2 degrees, the pattern spec has been improved to 29 - 25 log 8, Figure 17.13. This can best be met with low sidelobe, offset reflector designs.

The minimum receivable signal level is set by inherent noise in the system. Earth stations are required to detect small signals so the control of the noise parameters is important. The noise appearing at the output terminals of an earth station used as a receiver has three components; the noise received by the main beam of the reflector; the spillover noise due to the spillover from the feed; the receiver noise. The first component may be due to natural sources or to man made interference. The natural noise emitters are the earth and sea absorption, galactic noise, isotropic background radiation, quantum noise and absorption due to the oxygen and water vapour in the Earth's atmosphere. A minimum isotropic background radiation of about 3K is always seen by any antenna. The value of the other factors depends on frequency. The spillover noise is the only com­ponent under the control of the antenna designer. Its value can be reduced by designing an antenna with very low sidelobes. The receiver noise is normally the dominant noise factor. It depends on the method of amplification and detection. Early earth stations all used cooled receivers which have low noise temperatures. Modern earth stations use uncooled receivers which are dependent on the noise performance of the front-end transistor. This was improved dramatically in recent years, especially for small DBS terminals where the economies of scale have supported considerable research to reduce the noise temperature.

The ratio of the gain to noise temperature, the G/T ratio, is a useful measure of the influence of the noise components. Typical values are 40.7dBK~' for an INTELSAT A, 30 metre diameter antenna operating at 4/6GHz (Pratt, 1986)

Large earth station antennas are expensive to construct and to maintain so that there is a premium in obtaining the maximum efficiency from the system. The axi-symmetric Cassegrain antenna (see Section 17.4.2.) is the favourite choice for a number of reasons:

1. The gain can be increased over the standard parabola- hyper­ bola combination by shaping the reflectors. Up to an extra l dB is possible.

2. Low antenna noise temperatures can be achieved by controlling spillover using a high performance corrugated horn and by using a beam waveguide feed system.

3. Beam waveguide feed systems place the low noise receivers and high power transmitters in a convenient, stationary, loca­tion on the ground.

The beam waveguide feed system (Rudge, 1986), Figure 17.14, consists of at least four reflectors, whose shape and orientation is chosen so that the transmitter and receiver can be stationary whilst the antenna is free to move in two planes. The free-space beam suffers very little loss. The dual polarised transmit and receive signals need to be separated by a beam-forming network placed behind the main feed horn. For 4/6GHz operation, this will also incorporate circular polarisers.

The narrow beam from the large antenna necessitates the incor­poration of some form of tracking into the antenna because even a geostationary satellite drifts periodically. There are a number of schemes available, including monopulse, conical scan and hill climbing. The favourite is a monopulse scheme using additional modes in the feed horn to electromagnetically abstract the tracking data.

The first generation of medium earth station antennas were axi-symmetric Cassegrain reflector antennas, sometimes shaped. How­ever the advent of tighter pattern specifications has led to the widespread use of single or dual offset reflector antennas (see Section 17.4.3). These can meet the low sidelobe specifications by removing blockage effects from the aperture. Very high efficiency designs have been produced by shaping the reflectors to optimise the use of the aperture (Bergman, 1988; Cha, 1983; Bjontagaard, 1983). For these high efficiency designs, the r.m.s. surface error on the main reflector needs to be less than 0.5mm for operation in the 1 lGHz to 14GHz band. The feed is a high performance corrugated horn. The offset reflector configuration lends itself to deployment and portable designs have been produced where the offset reflector folds for transportation.

Cost is the main driver for small earth station antennas for mass market applications. Receive only terminals in the 4/6GHz band for data or TV reception are usually symmetric prime focus paraboloid which are made by spinning an aluminium sheet. In the UGHz communication band or the 12GHz DBS band, prime focus offset reflectors made from fibreglass moulds are popular. A simple open-ended waveguide type feed is incorporated on a sturdy feed support with the first stage low noise converter incorporated directly into the feed. There is considerable interest in making flat-plate array an­tennas which can be mounted flush against buildings and incorpor­ate electronic scanning to look onto the satellite signals. The technology for electronic scanning is available from military radars but not so far at a price which is acceptable to the domestic market.