Peterson, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 III.C.1 FDMAĮach earth station accessing the satellite with FDMA is assigned its own exclusive carrier frequency and bandwidth within a particular transponder channel on both the up-link and down-link along with the carriers of several other stations. The technology for electronic scanning is available from military radars but not so far at a price which is acceptable to the domestic market. 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. 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. In the 11GHz communication band or the 12GHz DBS band, prime focus offset reflectors made from fibreglass moulds are popular. 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. The offset reflector configuration lends itself to deployment and portable designs have been produced where the offset reflector folds for transportation.Ĭost is the main driver for small earth station antennas for mass market applications. The feed is a high performance corrugated horn. surface error on the main reflector needs to be less than 0.5mm for operation in the 11GHz to 14GHz band. For these high efficiency designs, the r.m.s. Very high efficiency designs have been produced by shaping the reflectors to optimise the use of the aperture ( Bergman, 1988 Cha, 1983 Bjontagaard, 1983). These can meet the low sidelobe specifications by removing blockage effects from the aperture. 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). The first generation of medium earth station antennas were axi-symmetric Cassegrain reflector antennas, sometimes shaped. The favourite is a monopulse scheme using additional modes in the feed horn to electromagnetically abstract the tracking data. There are a number of schemes available, including monopulse, conical scan and hill climbing. 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. Beam waveguide feed system for large earth stations Typical values are 40.7dBK −1 for an INTELSAT A, 30 metre diameter antenna operating at 4/6GHz ( Pratt, 1986)įigure 17.14. The ratio of the gain to noise temperature, the G/T ratio, is a useful measure of the influence of the noise components. 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. Modern earth stations use uncooled receivers which are dependent on the noise performance of the front-end transistor. ![]() Early earth stations all used cooled receivers which have low noise temperatures. It depends on the method of amplification and detection. The receiver noise is normally the dominant noise factor. ![]() Its value can be reduced by designing an antenna with very low sidelobes. The spillover noise is the only component under the control of the antenna designer. The value of the other factors depends on frequency. ![]() A minimum isotropic background radiation of about 3K is always seen by any antenna. 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. The first component may be due to natural sources or to man made interference. 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. Earth stations are required to detect small signals so the control of the noise parameters is important. The minimum receivable signal level is set by inherent noise in the system. FCC earth station antenna pattern specification
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