The antenna is a constant subject of discussion, but as yet no perfect solution has been found for wireless applications. There are many varied antennas from one-dimensional (dipole or monopole), two-dimensional (inverted F and microstrip patches), and 3D (dielectric antennas). In addition, dishes and phased arrays are used for base stations and satellite based communication. In this section, only the hardware aspect will be reported while the software aspects, such as the smart antenna or software antenna, will be discussed in Chapter 6 of this report. Also, as many efforts on the antennas are quite similar, the reports are intended to provide the descriptions below only as typical examples.
NTT's multisector monopole Yagi-Uda (MS-MPYA) consists of two very low profile 12- and 6-sector units for operation at 19 GHz. The 12-sector unit has a gain of 14 dB, and the 6-sector unit has a 10 dB gain. These units are used for beam forming, as Fig. 5.22 shows.
Fig. 5.22. NTT's multi sector monopole Yagi-Uda.
NTT has developed rod-type small-sized antennas for 25 GHz applications. Microstrip antennas are printed on panels within the rod, the beam radiating out from a cylindrical disk-like radome on top of it (Fig. 5.23).
Fig. 5.23. NTT's rod type printed antenna.
At IMST, the effect of the human head on the antennas has been investigated extensively. It was found that 8 ~ 10 dB loss is caused by the human head in an experiment at 450 MHz. IMST tested the efficiency of several antennas. IMST's helix antenna has 38% efficiency while its end-inductance antenna provided 84%. A 1/4 wavelength patch has proven to be twice as efficient as the helix structure. IMST has also tried a ceramic antenna for 0.9 and 1.9 GHz and have found that the bandwidth is too narrow.
Parabolic dishes still play an important role for wireless communication, especially for satellite-based communication. NTT has developed several deployable on-board antennas (constructed of solid, wire-mesh) that are inflatable. NTT's current non-inflatable mesh model consists of 7 modules, 10 meters in diameter, and weighing about 80 kg. The target is 14-modules 14 to 15 meters in diameter and 120 kg in weight. This technology was successfully transferred to NASDA (Japanese counterpart of NASA).
1 At the recent Workshop on Single Chip Radio during the 1999 IEEE International Microwave Symposium, most panelists expressed the desirability (but extreme difficulty) in achieving this in the foreseeable future.