Send-Receive Satellite Earth Station Planning.
To avoid any confusion between message systems and video systems, only equipment types will be discussed. Any special item pertinent to either system to aid in the installation effort will be noted.
The basic transmit/receive earth station consists of the following subsystems: antenna subsystem; low-noise amplifier subsystem; downlink subsystem; uplink subsystem; and earth station controller (optional).
Associated with the planning and installation of the transmit/receive earth station are many details, attention to which should provide safe and profitable operation of the earth station for many years. Many considerations associated with receive-only earth stations are applicable to transmit/receive earth stations. Items such as foundation centerline, antenna-motion clearance, low-noise amplifier subsystems, air-dielectric cable, power drives and video receivers are installed and operated in the same manner in both stations.
Several other items must be considered when planning and installing the transmit/receive earth station that would not normally apply to the receive-only station. This article will discuss some of the installation details of these items. Uplinks Need License
Unlike receive-only earth stations, which may be installated at the owner's risk, uplink stations require complete frequency coordination and FCC licensing because of their potential for interfering with other services. A formal, detailed request must be made to the FCC, including the following information: nature of the request and public-interest consideration; legal, technical and financial qualifications; construction proposal and schedule; environmental considerations; technical proposal; applicant's certification; and frequency-coordination and interference-analysis report.
The interference-analysis report should include the conclusions of the study, summary of the results, earth-station coordination data and certification of the applicant.
Other items to be included with the proposal are a recent financial balance sheet, description and quotation on proposed earth station, FCC Form 403 (application for radio-station license or modification thereof), list of officers and directors of the applicant and copies of the applicant's radio/TV station licenses and permits.
For obvious reasons, site selection is an important consideration in the earth station plan. Due to the physical size of the antenna, usually 10 or 11 meters, the terrain must be able to withstand the heavy construction equipment needed for the antenna erection, and soil-bearing pressure must be adequate to meet the requirement of the antenna.
Access to remote antenna locations during adverse weather conditions is a consideration that may require construction of an access road prior to site construction. Adequate area surrounding the antenna must be available for antenna motion. The equipment shelter must be located at a point where it does not hamper antenna motion and where the transmit waveguide length can be kept to a minimum.
The antenna foundation, which should be designed and approved by a local engineering firm before the data of installation, can be installed and allowed to set before the antenna parts are received. To allow adequate time for installation and curing of the foundation, Scientific-Atlanta, for example, will ship antenna anchor bolts and template, foundation drawings and foundation centerline heading information to customers. If an antenna equipment shelter is planned for the site, the shelter foundation should be poured at this time according to plans agreed upon by the customer and antenna vendor.
The AC power requirements of a transmt/receive earth station are substantially greater than the requirements of a receive-only station. Typical AC requirements for a receive-only earth station include video receivers, low-noise amplifiers, compressor-dehydrator and control. The total power required to operate this equipment is less than 10 kilowatts.
The major power consumer of a transmit/receive earth station is the high-power amplifier (HPA). Video uplink stations typically have one or more three-kilowatt HPAs, each requiring 12 kilowatts of three-phase AC power. Message earth stations typically use HPAs in the 125 to 400-watt range, requiring up to three kilowatts of single-phase power. Most transmit stations are configured for automatic protection with a hot-standby HPA protecting one or more on-line units. The three-kilowatt HPAs require 208-volt AC, three-phase, four-wire primary power with plus or minus 10 percent line-voltage regulation, and phase imbalance less than two percent.
Another large power consumer, although used seasonally, is the antenna deicing equipment. Feed/subreflector deicing requires up to three kilowatts. Half-reflector deicing requires up to 27 kilowatts of power. Full-reflector deicing deicing requires up to 51 kilowatts. In areas where severe icing or snow does not occur feed/subreflector deicing is usually sufficient.
Additional uplinks add approximately 13 kilowatts each, including power required for the HPA, exciter and additional switching equipment. Other options such as high-speed antenna drives and auto or steptracking on the antenna drives add further to the power requirements, and must be considered in planning the primary power subsystem.
Air conditioning is usually needed in a transmit station. This may require substantial power at southern sites.
In applications where prolonged service interruptions due to primary power failure cannot be tolerated, an engine-generator set is provided. These sets are usually diesel or (natural or bottle) gas-powered. Since the duration of the outage may range from minutes to days, the engine-generator must be rated to carry the entire earth station load, including air conditioning and deicing (though not necessarily simultaneously) and antenna drives. Fuel capacity must be adequate to allow continuous operation during any period when bad weather or other conditions may delay replenishment.
Applications requiring continuous operation during even brief power interruptions will require an uninterruptible power system (UPS) consisting of storage batteries, a charger and an inverter to produce the required AC power. UPS capacity is governed by electrical load and required operating time. To minimize cost, the capacity should be limited to carrying only the critical loads, such as the electronics, through a period long enough for the engine-generator to start and take the load. Five minutes is usually sufficient.
The equipment space needed for a complete transmit/receive earth station is much greater than that for a receive-only station. Each 3.35-kilowatt HPA requires shelter space that is approximately 28 inches wide by 32 inches deep by 78 inches high. For the typical earth station with redundant uplinks and downlinks and options such as motorized antenna positioning and deicing, two standard 19-inch equipment racks are normally required to accommodate the ground control equipment and control electronics. Additionally, space for waveguide runs and switching must be provided in the shelter. A typical layout is shown in Figure 3, with a typical rack installation shown in Figure 4. Be Sure Shelter Is Large Enough
The above size requirements suggest that the minimum shelter size for a redundant transmit/receive earth station is approximately 10 feet by 18 feet. Smaller sizes may be used, but they are quite cramped, especially when testing and maintenance of the earth station are in progress.
In the same manner as an equipment room layout is developed before an installation, a shelter layout must be developed and submitted to the shelter manufacturer. Manufacturers of shelters usually offer standard shelter configurations, but some customization is usually required. Some items to consider when laying out equipment in a shelter are door opening and location, cable-tray placement, power-panel location, waveguide penetration and waveguide switch location, air conditioning and venting, HPA exhaust and air intake panels, lighting type, AC outlet placement and various security features such as protection and alarm for fire and entry.
If the exciters are to be housed separately fromt he HPAs, the distance between HPA and exciter should be kept to a minimum to avoid excessive RF losses. If the distance is more than 30 feet, it may be necessary to add more amplification to the exciter output.
Other major components used in the transmit/receive earth station are the transmit waveguide and the RF switching matrix, Both are unique to transmit stations, and will be discussed separately in the following section.
The transmit waveguide is a low-loss transmission path or line connecting the output of the HPA to the transmit port of the antenna. The attenuation of the transmission path should be less than 1 dB. The 1-dB requirement specifies that overall length and number of connectors or switches be kept to a minimum.
When designed properly, the transmit waveguide system should fit together exactly with little or no air leaks. If a system is poorly desinged, it will be a plumbing nightmare. Normally, rigid waveguide is used between the HPA and RF switch matrix. Proper planning should guarantee precise connections. From the switch matrix, a short piece of twistable-flexible waveguide is used for connection to the semi-rigid elliptical waveguide run out to a point on the antenna. Another twistable-flexible waveguide section is used to connect the elliptical waveguide to the antenna port. This should provide flexibility to allow for any rotation in the polarization or antenna motion (azimuth or elevation) without damaging the waveguide.
The waveguide is firmly secured using various techniques. Inside the shelter or equipment room it is either braced to a cable tray or suspended from the ceiling using cable hangers. From the equipment area to the antenna, either a cable tray or large-diameter conduit is used. The minimum-bend radius of the elliptical waveguide is 12 inches in the E-plane and 32 inches in the H-plane. If conduit is used to house the waveguide, it should be at least four inches inside diameter and have no more than two 90-degree bends.
Due to the enormous power differences at the feed between transmit and receive ports, a transmit reject filter must be used between the receive feed port and the input to the LNA. This filter will minimize any transmit power leaking into the LNA package and cause out-of-band saturation.
The RF switching matrix is an integral part of the waveguide runs and allows protection of HPAs. By switching a backup HPA to an antenna port during a failure of the normal HPA, earth station operation is interrupted only momentarily. Existing waveguide runs to the antenna port are used since the switching occurs as close to the HPA output as possible. In some applications the backup is kept in transmit mode with the output of the HPA into a three-kilowatt dummy load. This method allows for hot-standby switching if both HPAs are tuned to the same frequency. It is sometimes useful for individual three-kilowatt dummy loads to be dedicated to each HPA through a waveguide switch for ease in testing and alignment, as well as immediate uplink capability.
Air-conditioning requirements should be tailored to the size of the earth station and the particular configuration at the site. The total number of HPAs (both in transmit and hot-standby), exciters, receivers, UPS control equipment and any other large heat-generating equipment should be included in the cooling budget. Largest Heat Source
The HPAs and associated equipment represent the largest single source of heat in a transmit station. Each HPA is vented through the rear wall of the equipment shelter. Two eight-inch-diameter vents are provided, one for fresh air (input), the other to exhaust the heated air from the equipment shelter. Each HPA has two blowers to accomplish this task. Approximately eight kilowatts of heat are removed from each HPA in this manner. This equates to approximately 27,000 BTUs per hour.
Using the guidelines specified in the above paragraphs, approximately one kilowatt of heat per HPA will remain in the shelter to be cooled by the air-conditioning system. Additionally, the air-conditioner must handle the heat released by the dummy load to which a hot-standby HPA is connected.
As a safety precaution, the shelter or equipment room where the transmitters are to be located should also contain a fresh-air system to circulate outside air into and out of the shelter in the event the normal air-conditioning system fails. This system can be fully automatic with a temperature switch to activate the air circulator, or a manual switch control.
When designing, operating and maintaining a transmit earth station, safety precautions should always be considered. Video transmit earth stations frequently exceed EIRP levels of 80 dBW. This produces a microwave radiation hazard in the area directly in front of the main reflector. Since the beam-widths of the 10-meter and 11-meter antennas are extremely narrow, the majority of the radiated microwave energy is confined to the area within an imaginary cylinder extending from the front of the main reflector. Locations with high-elevation look angles experience little or no real hazard from microwave radiation. Although US locations on domestic satellites have high-elevation angles, care should be taken in site planning to ensure that any nearby tall buildings are well out of the main beam.
The main beam should not be blocked by any object. Blocking the path will affect earth station performance (G/T) and cross-polarization rejection, as well as safety. Precautions taken to keep this area clear include enclosing at least the area surrounding the antenna and mount within a fence. This enclosure protects the antenna from vandals and from children who like to climb. Climbing on the main reflector when the station is transmitting is extremely dangerous. The area between the subreflector and main reflector has a very high microwave radiation density.
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|Date:||Jun 1, 1984|
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