Dipole antennas have long been used in various communications systems, including radio, television, and radiotelephone systems. It is well known that the lengths of the dipole arms on the antenna should be adapted to the wavelengths (.lambda.) of the signals transmitted and received. Typically, a plurality of arms having different lengths are used, in order to cover a predetermined range of frequencies. The sequence and spacing of these arms, and of any reflector behind them, determines various characteristics of the resulting beam or radiation field. These characteristics include vertical beam width, horizontal beam width, and front-to-back (F/B) ratio, i.e. the ratio of signal strength in front of the antenna to signal strength in back of the antenna. When a number of different arms are used, each arm makes its own contribution to the resulting field, and the overall expected result rapidly becomes difficult to calculate mathematically in advance. Therefore, considerable experimentation is often needed to achieve desired beam characteristics.
A well-known log periodic dipole antenna (LPDA) design is the "tree" configuration, in which parallel arms extend sideways from a central "trunk" or "standoff," the bottom arm near the base is the longest, and each successive arm is shorter toward the top of the antenna. Such LPDA designs typically result in a front-to-back (F/B) ratio less than 40 dB. This F/B ratio is considered insufficient for use in current PCS (Personal Communication System) cellular telephone sites, since radiation emanating out the back of the antenna tends to cause interference among adjacent sites. A horizontal beam width of 90 degrees is typical. However, in highly congested urban environments, it is preferable to have horizontal beamwidth of 65 degrees, which is obtained by using two parallel columns of dipoles, spaced 0.25 .lambda. to 0.30 .lambda. apart. The wavelength lambda (.lambda.) is the inverse of the frequency. The frequency band allotted for PCS use in the United States is between 1.85 GigaHertz and 1.99 GigaHertz, with a center frequency 1.92 GHz. The PCS band allotted in Europe has a center frequency 1.78 GHz, meaning that the wavelength is about 8% greater. Accordingly, antenna dimension examples stated for the U.S. should be scaled up about 8% for use in Europe.
My earlier LPDA design work has included an "hourglass" dipole strip configuration, in which top and bottom arms are longer than one or more middle arms. This design works well for generating a 90 degree beamwidth, but when used for generating a 65 degree beamwidth, typically results in F/B ratios in the range between 37 dB and 42 dB, better than provided by the "tree" configuration, but still insufficient. Reference is made to pending application U.S. Ser. No. 08/807,560 by myself and a colleague.