Engineering topics

Shafai, L.

Publications and source records attributed to Shafai, L..

3 records

NASA NTRS1995

Low profile antennas for MSAT applications

For MSAT applications, a number of different antennas have been designed and investigated. They include low gain omnidirectional antennas and medium gain to high gain directional antennas. The latter include both portable and vehicular antennas. While portable units are desirable to be low profile and low cost, the vehicular antennas have proved to be the most challenging antenna types for the mobile satellite application. The results of our efforts in design of such antennas are described briefly. Low profile designs are emphasized in most cases, and microstrip type radiators are therefore selected. The single radiator provides low gain omnidirectional patterns and is optimized for low cost applications. It provides low gains around 2-6 dBic and is useful mostly for the data transmission. Medium to high gain antennas are developed as arrays of omnidirectional elements. Again, different designs are optimized to meet the needs of different applications. For portable units, the array configuration can be flexible and is optimized for maximum broadside gains. For vehicular units, however the configurations are desirable to be low profile, or compact, and have means for scanning the antenna beam. For simplicity, fixed beam antennas with mechanical beam scan are selected. For these antennas, as well, different designs, having low profile or compact size, are selected and optimized to meet the MSAT gain and G/T requirements.

Shafai, L.↗

NASA NTRS1993

Low cost antennas for MSAT vehicular applications

For Mobile MSAT applications a number of vehicular antennas have been developed that meet the program requirements. They are, however, costly to manufacture. Two antenna candidates are described here that provide adequate gain in the coverage zone and are also lower cost. One is the mast antenna that uses three or four element arrays of aquadrifilar helices. It generates omnidirectional pattern in azimuth and its beam is scanned in elevation. The second unit is a planar spiral antenna; it generates directional beams by a summation of the azimuthal modes. A variation of this antenna uses conical spirals to fulfill the same task. In both cases beam scanning is achieved by means of electronic switches rather than phase shifters, thus resulting in simpler configurations.

Shafai, L.↗

NASA NTRS1990

MSAT vehicular antennas with self scanning array elements

A new approach for designing low profile antennas for MSAT applications is presented. It is based on stacking two microstrip antennas that operate at two adjacent modes. The beam scanning is achieved by introducing a phase shift between the stacked elements and consequently a low cost self scan array element is developed. The concept is used to investigate two different antenna types: (1) a single element unit, with its phase shifter, that provides a moderate gain of about 7 dBic; and (2) a seven element array with a peak gain of about 14 dBic. Computed and measured data for each design are presented and discussed.

Shafai, L.↗