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Small Antennas and Technical Advances Make the Promise of DBS a Reality.

Television receive-only (TVRO) equipment for home use has, until the advent of direct-broadcast satellite (DBS) and Ku-band applications, been a derivative of professional level equipment and some minor scaling down of products intended for cable television (CATV).

The range of equipment for direct 4 GHz home reception has been for those not weak of heart, living alone or with an estate capable of hiding 3 to 4.5-meter antenna. While it is estimated that some half-million of these 4-GHz systems will have been sold by the end of 1984, this is fraction of the market.

The rather severe limitations of transponder power and practical receiving equipment have left the home TVRO market very vertical and relatively small when compared to estimates of the future DBS consumer market.

Late in the 1970s, Satellite Television Corporation, a subsidiary of Comsat, began envisioning the use of 12 GHz for home satellite TVRO equipment. The use of 12-GHz satellites with high-power transponders would allow the use of small, economical antennas and rather evolutionary home terminals capable of receiving multiple channels, special services, and the promise of nationwide television services unrestricted by capital considerations on a local level.

STC, recognizing the impracticality of cable TV services in rural areas due to the low density of homes passed and a similar impractical capital restraint on terrestrial VHF and UHF services, discovered through research that there were more than 29 million homes either unserviced by cable or presently receiving three or less commercial television channels. This base of underserviced television homes represented an excellent subscriber universe. By 1981 action had been filed to create a DBS service in the 12.2 to 12.7-GHz band.

The antenna for the DBS home system is perhaps the one system component most influenced, if not restricted, by appearance. The necessity for a greater than one-meter antenna would place DBS no further in a general market than 4-GHz technology. The promise of serving rural as well as urban communities predicated the need for small, efficient dishes that could be easily installed and not present a liability to the structure upon which the antennas mount, including the roofs of residential homes.

Use of Smaller Dishes

Satellite link budget calculations quickly demonstrate that a 200-watt transponder at 12 GHz allows the use of 0.6 to 1-meter dishes. The less than 1-meter antennas are quite reasonable for consumer use and in fact often represent less mass than fringe area VHF TV yagi arrays.

By employing offset fed paraboloid geometry, the dishes gain another significant advantage, the ability to attain a more vertical position thereby becoming less susceptible to now and ice accumulation. Use of the offset design further reduces the satellite signal blockage by the LNC itself, that otherwise would contribute a significant percentage of inefficiency in a small parabola. The ability to have a low blockage factor has a positive effect on sidelobe performance that is quite critical to a small reflector.

Control of LNC local oscillator radiation as well as eventual DBS orbital spacing of two degrees demands sidelobe performance well in excess of most 4-GHz installations.

Other degradations to performance of less concern with larger parabola type antenna become critical to DBS. Hysterisis, creep, delamination and rusting can become cumulative losses that mean the difference between excellent reception and marginal reception. Tolerances of all types are more critical to DBS at 12 GHz that experiences incremental losses from weather that are of little or no concern at lower frequencies.

The mounts for an antenna to hold accuracies of 0.5 degrees are a critical concern. Two satisfactory approaches have been isolated through wind tunnel and structural analysis: a tripod mount with movable legs adaptable to sloped roofs, flat roofs or side mounting, and a single pipe mount that depends on anchoring to roof braces beneath the roof itself. Both mounting techniques will hold the pointing specifications required although the tripod approach does require more holes to be drilled in the roof than does the single pipe technique.

The combination of a press-formed pre-painted rigid dish with a substantial backplate and sturdy mount results in confidence of placing up to 1-meter dishes on a pipe-type roof mount. Special paint surfaces that behave much like Teflon reject the accumulation of moisture and dirt that could degrade performance.

Aluminum dishes available for DBS use now include 0.6, 0.75, 0.9, 1 and 1.2-meter antenna all exceeding 70 percent efficiency. Low power DBS, satellites with much less than 200 watts, generally use a range of dishes from 1.2 to 1.8-meters. Only the 1.2-meter antenna is available in offset design and is the maximum size recommended for roof mounting. The other rather critical component of the antenna is the feedhorn.

A feedhorn has been developed that mates with the LNC's circular waveguide and operates over the entire 11.7 to 12.7-GHz Ku-band allocations. The feedhorn is linear in polarization but is easily converted to circular by inserting a square dilectric component into the signal path; even without the dilectric component, a circular polarized signal can be received with only a 3 dB loss that is reasonable using high-power transponders.

Techniques for handling linear changes of horizontal and vertical planes as well as left and right-hand circular polarization are under development as this element of transmission has not as yet been standardized within the industry. It is expected that eventually the polarization changes will be performed electronically in the LNC itself rather than mechanical means commonly used today.

Who might have conceived ten years ago that microwave technology would be a part of everyday life? Who would have conceived that prices of highly sophisticated microwave receiver components would have dropped some 97 percent for similar, and in some ways superior, performance?

DBS is an exciting and challenging business that has and will continue to have dramatic impact on all elements of the microwave, semiconductor and computer business.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Fraser, R.D.
Publication:Communications News
Date:Jun 1, 1985
Words:988
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