Abstract:
A radio altimeter with at least one transmitting antenna and at least one receiving antenna in a single housing reduces coupling with antennae housings shaped to deflect electromagnetic signals, and spacing between the antennae based on the phase of the transmitting signal. Coupling of less than −40 dB is filtered by software using adaptive leakage cancelling.

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally toward phased radio antennae, and more particularly to radio altimeter systems. 
     BACKGROUND OF THE INVENTION 
     Frequency Modulated Continuous-wave (FMCW) radar is a short range measuring radar set. FMCW radar is often used as a “radar altimeter” to measure the exact distance between an aircraft and the ground during landing procedures. With the advent of modern electronics, the use of Digital Signal Processing (DSP) is used for most detection processing. The signals are passed through an Analog to Digital converter (ADC), and digital processing is performed on the result. 
     Current aircraft radio altimeter installation involves placing two separate antennae on the underside of the aircraft. Siting these antennae, including a transmitting antenna and a receiving antenna, is non-trivial. When the antennae are too far apart, low altitude performance is limited. When the antennae are too close together, the antennae suffer leakage due to poor isolation. Leakage may render the radio altimeter effectively useless if coupling between the antennae cannot be handled by software. Coupling is the transfer of energy from one circuit segment to another through some electrical process such as induction. 
     Consequently, it would be advantageous if an apparatus existed that is suitable for housing a receiving antenna and a transmitting antenna in close proximity, while limiting coupling between the antennae. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel method and apparatus for housing a receiving antenna and a transmitting antenna in close proximity, while limiting coupling between the antennae. 
     One embodiment of the present invention is a housing for a receiving antenna and transmitting antenna shaped to prevent coupling between the receiving antenna and transmitting antenna. The housing prevents coupling by deflecting electromagnetic signals that would otherwise transfer energy from one circuit segment to another. 
     In another embodiment of the present invention, the spacing between a receiving antenna and transmitting antenna in a housing, and the phase of electromagnetic signals in the system are configured to minimize coupling. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a side view of an antennae array according to at least one embodiment of the present invention; 
         FIG. 2  shows a bottom view of the embodiment shown in  FIG. 1 ; and 
         FIG. 3  shows a radio altimeter circuit block diagram according to at least one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , a radio altimeter  110  according to the present invention is shown. The radio altimeter  110  may include at least one transmitting antenna  104  and at least one receiving antenna  106  surrounded by an antennae housing  108 . The transmitting antenna  104  and receiving antenna  106  may be connected to altimeter electronics  102 . 
     The transmitting antenna  104  may be configured as a planar array, and may be further configured to transmit a modulated signal at a known frequency and phase. The receiving antenna  106  may also be configured as a planar array, and may be further configured to receive a modulated signal, originating from the transmitting antenna  104 , reflected off a surface such as the ground. The radio altimeter  110  may be installed in an aircraft fuselage  100 . 
     The antennae housing  108  may include an antennae divider  112  separating the transmitting antenna  104  from the receiving antenna  106 . The antennae divider  112  may be comprised of a radio opaque material to minimize signals from the transmitting antenna  104  reaching the receiving antenna  106  except by reflection off a surface. Furthermore, the antenna divider  112  may comprise a trapezoidal cross-section such that the surface defining the shorter of the parallel elements of the cross-section is distal to the antennae. Such shape may reflect signals originating from one antenna  104 , 106  away from the other antenna  104 , 106 , and thereby reduce coupling. One skilled in the art will appreciate that other shapes may be effective to divergently reflect signals that may otherwise cause coupling in a radio altimeter. One skilled in the art will also appreciate that while the shape of the antennae divider  112  is specifically described, the shape of the entire antennae housing  108  may employ identical principals. 
     Referring to  FIG. 2 , a bottom view of a radio altimeter  110  according to the present invention is shown. The radio altimeter  110  may comprise a transmitting antenna  104  and a receiving antenna  106  separated by some distance; that distance maybe defined by an antennae divider  112 . Each of the transmitting antenna  104  and receiving antenna  106  may comprise a phased array having multiple array elements. A phased array is an array of antenna elements in which the relative phases of the respective signals feeding the antenna elements are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. 
     Coupling due to induction caused by signals from the transmitting antenna  104  may be distance dependent such that coupling increases significantly as the transmitting antenna  104  and receiving antenna  106  are positioned in close proximity. Coupling may be reduced by coordinating phases in the array elements of the transmitting antenna  104  and array elements of the receiving antenna  106  such that ancillary signals from the transmitting antenna  104  may be out-of-phase with the receiving antenna  106  and therefore produce only a limited inductive affect. Ancillary signals are those signals from the transmitting antenna  104  that are not directed toward a surface, such as the ground, where the radio altimeter is designed to measure the distance from the transmitting antenna  104  to the surface. Rather, ancillary signals are those signals from the transmitting antenna  104  that directly interfere with the receiving antenna  106 . Spacing between the transmitting antenna  104  and receiving antenna  106  in a radio altimeter according to the present invention may be less than 32 inches. 
     One skilled in the art will appreciate that the phases of elements in a phased array such as the transmitting antenna  104  and receiving antenna  106  may be application dependent and especially dependent on the desired signal from the transmitting antenna  104 . Furthermore, the distance between the transmitting antenna  104  and receiving antenna  106  in this application may be dependent on the power of any ancillary signals which itself may be dependent on the total power of the signals sent to the transmitting antenna  104 . The actual distance between the transmitting antenna  104  and the receiving antenna  106  may therefore vary, but would be fixed in any particular application. In any application, coupling should be kept below −40 dB. Below −40 dB, coupling can be filtered through adaptive leakage cancelling. 
     Referring to  FIG. 3 , a radio altimeter circuit block diagram is shown. The circuit may include a processor  300  configured as a digital signal processor (DSP). The processor  300  may receive a signal from an analog to digital converter (ADC)  308  and send a signal to a digital to analog converter (DAC)  310 . The FMCW modulator  302  may produce a modulated waveform having a linear triangular frequency-vs-time signal. The transmitter  312  may receive a signal from the modulator  302 . The transmitter output may be sent to the transmitter antenna  304  through a delay line  320 ; power output at the transmitting antenna may be between 100 mW and 400 mW. A sample of the transmitter signal from coupler  322  may be connected to a summing amplifier  316 . The summing amplifier  316  may also receive a signal from the DAC  310 . 
     The summing amplifier  316  may produce an output signal representing the sum of the FMCW signal and a low-frequency signal from coupler  322  and the DAC  310 . The output from the summing amplifier  316  may be received by a RF mixer  314 . The RF mixer  314  may also receive a signal from a receiving antenna  306  through a delay line  318 . The output from the RF mixer  314  may be received by the ADC  308 , and the output of the ADC  308  may be sent to the processor  300 . 
     Provided that coupling between the transmitting antenna  304  and the receiving antenna  306  remains below −40 dB, the processor  300  may be configured to filter out any aberrant effects of coupling. A radio altimeter according to the present invention may be contained in single unit, easily installed in an aircraft, as opposed to prior art radio altimeters that required more than 32 inches of separation between the transmitting antenna and any receiving antennae. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.