area. Detailed specifications of antenna systems for the new relay stations have been adapted to take account of the results that work.

The VOA modernization program recognized that in addition to replacing transmitters the antenna systems at existing sites need to be modernized. In considering the priorities under the existing financial stringencies, attention is drawn to the fact that some of these existing stations are equipped with extremely old-fashioned antennas, such as rhombics, intended for point-to-point communications rather than broadcasting. In other cases the antenna complement appears to lack the flexibility required to adapt to changes in priority that have occurred since the stations were constructed.

The VOA is procuring new HF transmitters for its broadcasting stations, operating at 500 kW rather than 250 kW power levels. Although a doubling of station transmitter power from 250 to 500 kW achieves an increase of 3 dB in the received signal, this is only effective in the areas presently served by the (fixed) antennas available at that station. The provision of new types of antennas or the provision of improved steering capabilities for existing antennas is likely to provide larger gains in audibility than would merely replacing transmitters. It should therefore receive a correspondingly higher priority in the program. In this regard, the Committee urges the VOA to continue exploiting the best of the available antenna technology and pursuing the technological development of advanced antenna designs as a longer-term objective. Some of the possibilities available are discussed below.

ANTENNA ARRAY SYSTEM

The VOA has been acquiring relay station sites that are approximately one mile square on flat ground. Large, horizontal-dipole, curtain-array antennas are then erected, the vertical height of the arrays often being more than 400 feet. Typically, four horizontally polarized radiating elements are used in each “bay” (horizontal segment). Often, only two of the radiating elements at a time are active in a stack.

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)

h/:

height above ground, in wavelengths, of the bottom row of radiating elements.



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