This invention relates to signal coupling apparatus and more particularly to antenna signal coupling apparatus which is shielded to confine a predetermined signal and to exclude all other signals.
In general, a complete radio system includes apparatus for transmitting an output signal in the form of electromagnetic energy, and receiving apparatus for capturing a portion of the transmitted electromagnetic energy and processing the energy to reproduce the original signal. A structural member that is found in both the transmitter and receiver of the system is a transmitting antenna for emitting electromagnetic energy, and a receiving antenna that is suitably matched to the characteristics of the radiated electromagnetic energy to capture a usable portion thereof.
An inevitable requirement of the radio system is eventual repair and adjustment. Moreover, the system requires periodic tests to determine if its performance ratings are achieved. The significance of maintenance and performance tests is of considerable importance in applications where the radio system is used as a navigation aid. It is even more important to be aware of the operating characteristics of the radio system if it is used primarily in emergency applications. In this regard, the transmitter may be an Emergency Locater Transmitter (ELT) of a type used aboard aircraft. Since the ELT is relied upon to transmit a distress signal for a downed aircraft, it is imperative that the quantitative and qualitative characteristics of the signal be known. Similarly, in the case of radio receiving apparatus used in search and rescue operations, the receiving characteristics of the equipment must be known if search patterns are to be conducted effectively.
Known VHF receivers in search and rescue aircraft, operating on 121.5 MHz., can detect and home in on an ELT at close to line-of-sight limits. In view of the normal operational altitudes of such aircraft, these limits can be well in excess of 100 nautical miles. It is apparent, therefore, that a most important consideration in maintaining optimum conditions in search and rescue operations resides in providing that all components of the VHF homing system are functioning properly. In particular, it must be ascertained that the receiver meets its sensitivity specification, that the amplifier, detector, indicator and phasing network that comprise a switched-cardioid homing system are performing as they should, and that no faults exist in the antenna elements and associated feedlines.
During periodic maintenance and performance tests, the equipment is removed from the aircraft and is bench-tested in a workshop environment. Such testing is very time consuming since it involves removal and reinstallation of the equipment and furthermore does not cover all components of the system and their interfacing elements. Accordingly, a serious flaw occurs in the bench-testing procedure, i.e., that while the tested components may work satisfactorily in the workshop, there is no guarantee that the system will perform according to rated specifications after reinstallation. Moreover, it is possible that malfunctions may even be introduced after the testing procedure because of a faulty reinstallation. A search crew may therefore engage in a mission under the false apprehension that their electronic gear capability is neither reduced nor destroyed by defective equipment.
Since the object of search and rescue operations is to save human lives, which object may be defeated by the known deficiencies in pre-flight testing of radio gear, there is an obvious requirement for a simple pre-flight diagnostic procedure and apparatus to rapidly check the overall aircraft homing system in situ while the aircraft is in the hangar or on the tarmac. If a fault is found, repairs or replacment can be made immediately and the equipment immediately retested or an alternate aircraft can be put into service. Moreover, since in search and rescue time is frequently of the essence, if only moderate degradation of system performance is found, it is possible that the aircraft would still be flown but with modified search tactics, such as reduced track spacing, in order to compensate for the reduced capabilities of the homing system.