References:
1. Peterson, C. G., Localizer Traveling Wave Antenna Development. Report No. FAA-RD-76-129. Department of Transportation, Federal Aviation Administration. May, 1976.
2. Alford, A., French, E., and Schwartz, R., A Guide for the Selection of Antenna Characteristics for Single Frequency and Two Frequency Localizers in the Presence of Reflecting Structures. Report No. FAA-RD-75-64. Department of Transportation, Federal Aviation Administration. April, 1975.
3. U.S. Pat. No. 4,415,902
4. Disclosure Document no. 103421
The instrument Landing System Localizer is in worldwide use to provide azimuth guidance to aircraft on final approach to landing. It does so by radiating a radiofrequency carrier in the range 108-112 mhz., tone modulated with 150 hz. and 90 hz. Localizer signal is radiated so that equal tones are received on runway centerline and its extension, while 150 hz. and 90 hz. predominate right and left of centerline respectively. Within about 3 degrees either side of centerline, the difference between the amplitudes of the received tones varies linearly with angular displacement from centerline, and is used to provide an indication of the approaching aircraft's position relative to centerline. In addition to the linear region, a "clearance" region extending to .+-.35 degrees, in which "full left" and "full right" indications are received, is required by international standards.
The requirement for clearance signals means that the localizer beam is wide and may reflect from structures such as hangars or terminal buildings; such reflections being capable of producing unacceptable bends in the equal-tone surface, which is nominally a vertical plane through runway centerline (Ref. 2). To reduce their effect, "two-frequency" Localizers are used (Refs. 1 and 2). They use two separate RF carriers, separated in frequency by typically 8 khz. One carrier is radiated in a relatively narrow beam, typically within .+-.10 degrees of centerline. It is called the "course signal" and provides primary guidance near centerline. The other carrier is radiated in a wide beam, extending to at least .+-.35 degrees from centerline, and relatively weak compared to the course signal near centerline. It is called "clearance signal" and supplies full left and full right indications. The well known "capture effect" causes the receiver in the approaching aircraft to respond only to the dominant signal, i.e., to course signal near centerline and to clearance signal at angles more than about 5 degrees off centerline. Since hangars, etc., are normally in the clearance beam but not the course beam, reflections from them, received near centerline, are discriminated against by capture effect and produce very little disturbance of the equal tone surface or its neighborhood.
Localizer signals are radiated from linear arrays of antennas located at the stop end of the runway. Some two-frequency localizers use two separate arrays to radiate course and clearance, but it is advantageous from the viewpoints of cost, reliability, and maintenance to use a single array to radiate both. In order to do this, each antenna in the array must be driven by modulated RF currents at course and clearance frequencies in such a way as to generate the required course and clearance radiation patterns. Four distinct signals are involved, two generated by a course transmitter and associated with course radiation, and two generated by a clearance transmitter and associated with clearance radiation. The first of the two course signals is "course carrier", which is an RF carrier modulated equally by 90 hz. and 150 hz. Course carrier is radiated symmetrically around centerline and by itself contains no guidance information. The second course signal is "course sidebands", which is an RF carrier balanced modulated with equal 90 hz. and 150 hz. tones, the audio phase of one of these tones being reversed relative to its phase on the course carrier signal. Course sidebands are radiated antisymmetrically around runway centerline. When radiated course carrier combines with radiated course sidebands in space, a signal is produced having equal tones on centerline, a predominance of 150 hz. right of centerline, and a predominance of 90 hz. left of centerline. Two similar signals are used to radiate clearance, namely, "clearance carrier" and "clearance sidebands".
If one array is used to radiate both course and clearance, each of the four signals must drive each antenna in the array with a current of such magnitude as is determined from the required radiation pattern of that signal. A distributor is necessary to divide power of each type and to route the correct fraction of power of each type to each antenna, with the correct radiofrequency phase. Prior art distributors have disadvantages of complexity and resulting cumulative errors in power distribution. They are also generally non-repairable and inflexible in the sense that errors in power distribution discovered during or after development of the distributor are difficult and expensive to correct. The purpose of the present invention is to provide a distributor that is relatively simple, precise, economical, extremely reliable, and easily adjustable during the development process.
The novelty of the present invention is believed to be its use of a multiplicity of simple power dividers consisting of tapped quarter-wave sections of transmission line, in combination with conventional bidirectional couplers for signal combination and division. Prior art, by contrast, uses two bidirectional couplers, connected by phase-shifting transmission lines, for each power division; which technique leads to complexity and cumulative errors in power division.