Past work and recent trends in the development of antenna systems for mobile communications are reviewed. Modern antenna design is not only concerned with the antenna elements, but also involves consideration of factors related to propagation, systems, and environmental condition. Major developments in the field are described, and some prospects for the future are also introduced.

Among many diversity schemes, space and radiation pattern diversity techniques are commonly used in mobile communication systems to overcome deep fading. A novel diagram is presented that clarifies the simultaneous effect of antenna spacing and radiation pattern on the correlation coefficient (ρ_e). It is shown that space diversity is more effective than radiation pattern diversity. Next, typical examples of practical diversity antenna configurations are shown. A novel configuration for an omnidirectional base station antenna for radiation pattern diversity is explained. Some configurations of space and radiation pattern diversity schemes are shown for mobile stations. A very compact diversity antenna configuration for portable phones is also shown. Finally, for the ease of designing diversity antenna, simple formulas for determining antenna spacing are developed. In the case of base station antennas, the ρ_e value is expressed by a simple SINC function with variables of spacing and the spread angle of multipath waves. Moreover, typical spread angles are clarified experimentally for Japanese urban areas. The appropriate antenna spacing for small ρ_e values is obtained. In the case of mobile station antennas, another simple formula is derived by taking into account the mutual coupling between diversity antennas.

Since the middle of 1970's, the research and development activities on satellite communications for mobiles such as ships, aircraft and land mobiles have been continued in many countries and organizations. Typical programs are the ETS-V in Japan, MSAT-X in USA, MSAT in Canada and PROSAT in Europe. In implementing mobile satellite communications, a vehicle antenna is one of the most importent key technologies. This paper reviews system requirements, design, performances, research activities and future prospects on vehicle antennas.

A flat diversity antenna, consisting of a disk loaded monopole antenna and four notch antennas composed in the same disk, was proposed as a vehicular antenna to reduce a fading effect of urban mobile telephone. The disk loaded monopole antenna is fed by a center probe and has four matching posts to realize the same pattern as that of a monopole antenna over a wide frequency range. This paper presents a new feed circuit for the flat diversity antenna, and measurement results of its input characteristics and radiation pattern when it is mounted on a vehicle. To verify the diversity effect of this antenna, we measured urban propagation characteristics at 900MHz, and compared the results with those of a space diversity antenna. We obtained almost the same characteristics with the flat diversity antenna as with the space diversity antenna.

Theoretical and experimental studies are made in the UHF band on a quarter wavelength monopole and a half wavelength dipole located on a trunk-hood of a car. To calculate vertical radiation patterns, the theoretical model of the car body and the GTD approach are adopted. It is confirmed from a comparison between calculated and measured results that the theoretical model is very useful to analyze the trunk mount antenna performance and the effect of the body can be well examined with it. The results of pattern calculation for two type of antennas show that vertical radiation patterns, especially the pattern in the front direction of the car, depend largely on the car body. That is, many sharp lobes appear in the vertical pattern, and further, the shape of the lobe and the gain vary largely with the change of the antenna height and location. An approach of this kind can be effectively utilized as a design tool to estimate the radiation performance of mobile antennas.

In this paper, the characteristics of the card-sized paging antenna composed of plates and wires are estimated in view of its efficiency. To analyze this type of antennas using the reaction matching technique for the wire antenna, we approximate the plate parts to the wire grids. In the analysis of the antenna with a complex configuration, the numerical input impedance by the usual reaction technique assumed the local symmetry in calculating one mutual impedance, is sometimes incorrect. Therefore, we carry out the reaction matching on the condition that the dipoles are placed at their own positions throughout calculating the mutual impedances. In modeling the plate with a wire-grid, we estimate the loss effect of the plate parts by introducing the equivalent radius. Furthermore, we examine the validity of the wire-grid model for the card-sized antenna. And, we estimate the radiation efficiency as a function of the gap distance of the plates, and discuss the polarization matching and the impedance matching of the thin paging antenna, in view of the total efficiency.

A wide beamwidth and broad bandwidth microstrip antenna is presented. It consists of a rectangular patch antenna and a pair of parasitic short circuit patches placed over the radiating edges of the rectangular patch. The parasitic element not only widens the bandwidth of the antenna but also forms a narrowed pair of radiating edges, so the E-plane beamwidth becomes broader. An antenna trial model achieved a bandwidth of 16% (VSWR1.5) and a broad E-plane beamwidth of 165 degrees.

A backfire helical antenna (BHA) is numerically analyzed to form a circularly polarized conical beam. The conical beam whose radiation is null in the helical axis direction is obtained using the second mode operation of the BHA. The characteristics of monofilar, bifilar and quadrifilar BHA's are compared and discussed. It is found that a second-mode bifilar BHA radiates a circularly polarized conical beam over a frequency range of 1 : 1.4. The use of the first mode of a bifilar BHA is also investigated to form a quasi-conical beam which has the weak radiation in the helical axis direction.

Short backfire (SBF) antenna developed by H.W. Ehrenspeck is experimentally well-known to have high efficiency radiation characteristics in spite of its small size so far. Therefore this antenna is expected to be one of the candidates of single antennas and an element of array antennas for several types of communications. For example, this antenna has been proposed for shipborne antenna of INMARSAT-M system for maritime satellite communications. Although many efforts to analize and/or optimize this antenna has been made mainly experimentally, rigorous design procedure has not been almost established even now because of its complex configuration. In 1980, an approach to analyze this antenna was tried by Kawakami, H. and et al. which was the first attempt to obtain the theoretical characteristics by more rigorous process, however, from the view points of practical designing, the calculated values in their paper were insufficient because of the lack of strict numerical consideration. Upon the above circumferences, analysis of SBF antenna is performed in this paper by using method of moment with wire-grid model from the view points of establishing the design technique. This paper presents the analytical results of the radiation characteristics of this antenna and also the comparison with the experimental results. From this study, it can be noticed that the radiation characteristics of this antenna can be almost exactly analyzed and also there exists the good agreements between the theoretical and experimental results.

This paper presents a newly developed two-layer self-diplexing antenna for mobile satellite communications with mutual coupling suppression between transmitting and receiving antenna. The self-diplexing antenna is composed of circular microstrip and ring patches. Numerical results based on an electromotive force method using boundary admittance are compared with experimental results. The numerical results significantly agree with the experimental results. The experiments show that the total isolation between transmitting and receiving antennas is more than 35 dB.

The authors propose a new circular polarization method for a slot-coupled microstrip antenna. The simple configuration antenna in which a parasitically excited slot exists in addition to a directly excited slot generates circular polarization, by the effect of higher order modes. Experimental results and numerical results using the Spatial Network Method are described to verify the performance and discuss the circular polarization generating mechanism.

A new circularly polarized microstrip antenna fed by a cross slot using an aperture coupled feed is proposed. The proposed antenna has no 90 degree hybrid coupler for circular polarization and allows easy connection to microwave devices for electronical beam scanning. In this paper, the conditions for circular polarization in this new antenna are derived, and the measured results are also described to verify the authors' proposed conditions for circular polarization. Good return loss and axial ratio characteristics have been obtained.

The radiation pattern of a car antenna is evaluated with the power received by its antenna in mobile communication environments. In the evaluation, the horizontal radiation pattern is assumed to be omnidirectional; the vertical one is expressed in terms of the main beam direction θ₁ and the half-power beamwidth θ_BW. The received power is calculated from such a radiation pattern and an angular probability density distribution of wave arrival. The distribution is estimated using a propagation model to simulate radio propagation paths and surroundings of them in urban areas. The tendency of the estimated distribution is in good agreement with that of the measured one. It is clarified from the estimated results that the angular probability density distribution of wave arrival strongly depends on the street angle ψ. Since the received power calculated from the distribution also depends on ψ, the minimum value (minimum received power) in the received power for ψ=5, 10, 15, , 85 is used to evaluate the radiation pattern. From the view that the radiation pattern for which the minimum received power is highest is suitable for mobile communications, it is revealed that θ₁ and θ_BW in the suitable radiation pattern of the car antenna are roughly 20 and 15, respectively.

In maritime satellite communications particularly for low G/T earth stations, multipath fading becomes a serious problem, especially at low elevation angles. In this paper, we show a new fading reduction scheme for sea surface reflection applicable to INMARSAT-C system. This method using open-loop-controlled diversity combined with an FEC and a interleaver overcomes the disadvantages generally associated with a conventional diversity system. This method has a respectable fading reduction effect, moreover, it can shorten the interleave length. We evaluate these effects of the system by a computer simulation and discuss its performance. The applicability of this fading reduction technique is not only to the INMARSAT-C but also to any kind of digital communication systems where interleave, FEC, and diversity can be used.