Abstract:
A tri-band antenna including an center conducting an inner pipe surrounding the center conductor, an outer pipe surrounding the inner pipe and Teflon provided between the inner and outer pipes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to cellular mobile antennas for vehicles. 
     2. Prior Art 
     For AM/FM B.C. and cellular mobile telephone (MT) in vehicles, more than two antennas are usually necessary. However, an AM-FM-MT tri-band antenna is desired and one with coils is used presently. 
     The tri-band antenna, as shown in FIG. 1 is usually connected to an AM/FM receiver and a mobile telephone receiver/transmitter via a branching filter. Transmission loss in the branching filter and the coaxial transmission line is about 1.5-2.0 dB in all. Therefore, it is desired that the tri-band antenna has a directive gain high enough to compensate for it&#39;s loss. In addition, the tri-band antenna is required to have a directivity broad enough to keep a sensitivity in the mobile telephone band even when the antenna inclines. The tri-band antenna referred to in this application is for U.S.A. (Transmitting frequency range of MT:825-845 MHz Receiving frequency range of MT:870-890 MHz, etc.). 
     SUMMARY OF THE INVENTION 
     The objects of the present invention are accomplished by an AM, FM and MT (Tri-band) antenna including a center, conductor, an inner pipe coaxial with and surrounding the center conductor, an outer, pipe coaxial with and surrounding the inner pipe and Teflon (polytetrafluoroethylene) provided between the inner and outer pipes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a collinear array antenna of the prior art; 
     FIG. 2 is a tri-band antenna in accordance with the teachings of the present invention; 
     FIGS. 3(a) and 3(b) show the current distribution of the antenna of FIG. 2; 
     FIG. 4 shows a model antenna used in experiments. 
     FIG. 5(a) and 5(b) respectively show the phase and amplitude the current distribution for the model antenna of FIG. 4; and 
     FIG. 6 is the radiation pattern for the model antenna of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows a double sleeve used for the tri-band antenna. This consists of two metal pipes, Teflon (polytetrafluoroethylene) and a center conductor. The lower end of the inner pipe and the center conductor are shorted, and the lower end of the outer pipe and the inner pipe are also shorted. The space between the outer pipe and the inner pipe is filled with Teflon (εγ=2.15). In this application, we call the double sleeve of which the dimensions are illustrated in FIG. 2(#1) &#34;sleeve 1&#34; and the double sleeve of which the dimensions are illustrated in FIG. 2(#2) &#34;sleeve 2&#34;. 
     FIG. 3(a) and FIG. 3(b) show a wire antenna with the sleeve(#1). The dotted line indicates the current amplitude and the signs + indicate the phase of the current distribution. λ denotes a wavelength in the mobile telephone band. The wire antenna operates as a collinear array antenna in the transmitting frequency range of MT (FIG. 3(a)). In the receiving frequency range of MT, the current amplitude in the upper linear part than the sleeve #1 is very small (FIG. 3(b)). 
     As shown in FIG. 4, when the sleeve #2 is attached to the position where a current is nearly a local maximum on an antenna, the current amplitude is very small in the upper part than the sleeve #2 in the mobile telephone band. 
     In the AM/FM B.C. bands, the double sleeves scarcely influence the current distribution of the antenna. 
     FIG. 4 shows a model antenna. La and Lb are (3/4λ, and Lc is a little longer than λ/4 (λdenotes wavelength at 825 MHz). The sleeve 1 and the sleeve 2 are denoted by #1 and #2, respectively. It would also be possible to make La and Lb each equal to (N/4) λ where N is an odd integer, i.e., either La or Lb or both could be equal to (5/4)λ. 
     FIGS. 5(a) and 5(b) respectively show the phase and amplitude of the current distribution of the model antenna shown in FIG. 4. It was measured at 825 MHz. As shown in FIG. 5(a) and FIG. 5(b), at 825 MHz, the antenna is a kind of collinear array antenna consisted electrically of the part below the sleeve #2. 
     This current distribution is almost the same in the transmitting frequency range of MT. The current amplitude is very small in the part above the sleeve 1 at 890 MHz as shown in FIG. 3(b). 
     FIG. 6 shows a radiation pattern at a center frequency 835 Mhz in the transmitting frequency range of MT for the model antenna. In the measurement, the antenna was mounted at the center of a ground plane of the diameter of 1.5 meter. The direction of the maximum radiation for the model antenna is close to the horizon. A maximum gain is about 4 dBi. 
     In the receiving frequency range of MT, the elevation angle of the peak directivity for the model antenna is from 60° to 65°. The directivity for the model antenna is similar to that for a λ/4 monopole antenna. 
     A voltage standing wave ration (VSWR) is from 1.8 to 2.4 in the transmitting frequency range of MT. In the receiving frequency range of MT, it achieves a VSWR from 2.0 to 5.5 in the measuring frequency 870-883 MHz and 2.0 or less in the measuring frequency 883-890 MHz, respectively. 
     In the FM B.C. band (88-108 MHz), the double sleeves almost never influence the current distribution. The model antenna operates as a λ/4 monopole antenna. In the FM B.C. band, the model antenna achieves a VSWR from 2.4 to 10.4. 
     Although we had not measured the characteristics of the model antenna in the AM B.C. band, we may safely assume that the model antenna will operate as a short dipole. Since the wavelength in the AM B.C. band is much longer than that in the FM B.C. band, the sleeves may never influence the characteristics. The model antenna&#39;s length is 648 mm, which is long enough for an AM/FM B.C. receiving antenna. 
     The AM-FM-Cellular mobile telephone tri-band antenna with double sleeves for vehicles has been designed and measured. The radiation patterns and the VSWR of the tri-band antenna were good in the transmitting frequency of MT. In the receiving frequency of MT, the radiation patterns are similar to those for a λ/4 monopole antenna, and the VSWR is good (2.0 or less) in the frequency 883-890 MHz. The antenna has such a length as to make the radiation efficiency enough. 
     As a sleeve can be made precisely, the antenna with sleeves is suitable for a mass production. A sleeve can be used to realize a tri-band antenna effectively.