Antennas, Satellite Broadcasting, and Emergency Preparedness for the Voice of America (1988) / Chapter Skim
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Antenna, Propagation, , and Monitoring Considerations for High-Frequency Broadcasting Systems
Antenna, Propagation, , and Monitoring Considerations for High-Frequency Broadcasting Systems
Pages 8-23
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From page 8... ...
This quality of service will be characterized by certain requirements, such as signal strength, resistance to propagation disturbances, and resistance to jamming. There will also be certain constraints, such as frequency allocations, available transmitting sites, and cost.
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From page 9... ...
Figures 2-1 and 2-2 show the arrangement of dipole radiating elements and the associated feeder system in a multiband, horizontal-dipole, curtain antenna, as used for high power operation in short-wave broadcasting. The nomenclature conventionally used to describe horizontal-dipole, curtain antennas has been standardized internationally, using the form HR m/n/h, with the following meaning (International Telecommunication Union, 1984~: HR: horizontal dipole curtain antenna with reflector curtain m/: number of dipole elements in each row n/: number of dipole elements in each stack (one above the other)
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From page 10... ...
10 n an'' By., t1 , ,~ Of I FIGURE 2-1 Connection schematic for curtain array antenna I, .
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From page 11... ...
] ~ FIGURE 2-2 Curtain array connections with reflector curtain and antenna mechanical assembly.
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From page 12... ...
Switching, which may well be quite complicated, is usually introduced in the feeders at ground level to slew the beam azimuthally or to change the radiation characteristics in the elevation plane by modifying the phase relationships between the various feed points. Since switching feeder line lengths in the switchyard provides control of the phase of feed to each array element, these arrays may be thought of as "phased" or "scanned." Some typical examples of azimuth and elevation beamwidths based on the radiation patterns of horizontal dipole antennas over average ground are shown in Tables 2-1 and 2-2.
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From page 13... ...
An alternative approach is to use a distributed antenna array in which a separate power amplifier is used to drive each row or column of radiating elements or even each radiating element, thus coupling a "distributed amplifier" with the distributed antenna array. Such arrays allow beam steering in azimuth and elevation to be accomplished electronically at low power levels, allow beamwidth changes, provide graceful degradation, permit on-line power-amplifier maintenance, and avoid losses and other problems of high-power switchyards.
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From page 14... ...
For valid cost comparisons of systems using distributed versus monolithic amplifiers it will be important to ensure' that the multiple applications and the wide frequency range and steering capabilities of distributed-amplifier arrays be compared with those of an equivalent conventional antenna fed from a single transmitter. The comparison should include the cost of associated feeders and feeder and beam-control switching.
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From page 15... ...
Pattern sidelobe levels provide a sensitive indication of errors in the current distribution. In the event that the measured patterns do not agree with the design values, patterns calculated from the measured current distributions should be compared with the measured patterns.
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From page 16... ...
The Committee believes that such tests should be conducted using airborne receiving equipment to measure the radiation from the powered antenna.- Airborne measurements have the unique advantage that they are the only method of deter-~ining the eventual radiation pattern. The radiation pattern of an HE antenna is formed as a result of reflection by the ground, and it may also be modified by currents flowing in the support structure.
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From page 17... ...
VALSUM (acronym for VALidation SUMmary) takes results of IONSUM for a particular audience area, ionospheric conditions, and sunspot conditions and calculates required power gain, transmitting antenna gain, and beam illumination requirements for specified signal-to-noise ratios and fading margins.
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From page 18... ...
Conventional signal strength monitors have relatively broad bandwidths, which makes them susceptible to noise and interference at frequencies near the frequency of the VOA transmission being monitored. To avoid this out-of-band contamination narrow-band monitoring of the RF carrier should be implemented to determine the signal strength of the desired signal alone.
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From page 19... ...
After suitable processing the filter output can be combined with the signal-strength data to provide the required signal strength and signal-to-noise ratio information. Two possible techniques for estimating signal quality automatically are suggested.
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From page 20... ...
Monitoring of this type can be quite powerful since it provides near real-time information on both the spatial extent of the footprint and the azimuth and elevation angles for optimum transmission. This information can be used to increase the signal strength in the audience area as well as to improve VOA ionospheric prediction models.
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From page 21... ...
The monitoring facility could be designed to utilize the transmissions from the local VOA relay facility or it could utilize its own signal source. In the former case, some additional modulation would be required on the VOA signal, or some sophisticated pattern recognition software or hardware would be required at the monitoring site in order to provide range information on the backscatter source.
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From page 22... ...
c. Implement a test program to evaluate VOA performance enhancements that may be achieved through the utilization of backscatter monitoring data.
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From page 23... ...
1985. VOA Worldwide Monitoring Analysis: Task 2 -- Geographic Offset Monitoring.
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