Abstract:
An antenna switch is provided, comprising: an antenna cable connector that connects to a radio RF output port that both receives an RF signal at a receive frequency, and transmits an RF signal at a transmit frequency that differs from the receive frequency; an RF switch having a common contact that is switchable between a receive antenna contact and a transmit antenna contact; a level comparator comprising an input connected to the antenna cable that: when a voltage at the input is below a predefined threshold value, causes the RF switch to contact the receive antenna; and when the voltage at the input is above a predefined threshold value, causes the RF switch to contact the transmit antenna.

Description:
BACKGROUND 
       [0001]    Disclosed herein is an automatic self-powered antenna switch which automatically connects a common antenna feed line to an antenna receive or transmit element as appropriate. 
         [0002]    Many radio systems utilize different frequencies for transmitting and for receiving signals. Ideally, antennas can be provided which will work efficiently over both frequency bands, allowing the use of single elements for the total system. In practice, due to various constraints, it is not always possible to design such antennas. This requires the use of separate antenna elements for transmitting and for receiving. Additional system complexity and cost is incurred by the need for multiple feed lines or separate control lines to operate antenna switches. 
         [0003]    The simplest way of connecting to multiple antenna elements is with multiple feed lines. In one implementation currently in use, the radio transmitter connects to its dedicated antenna through one coaxial cable and the radio receiver connects to its dedicated antenna through a different coaxial cable. This requires that the radio be designed with separate RF connections for the receiver and the transmitter, plus one must incur the additional cost of two lengths of coaxial cable and their associated connectors and supports. 
         [0004]      FIG. 1  schematically shows an alternative that include an RF antenna relay RFSW 1  in the antenna housing. This allows the use of radios with single RF connections and single lengths of coaxial cable. In one implementation currently in use, a separate cable CC 1 , containing one or more conductors, is used to control the antenna relay RFSW 1 . 
         [0005]    An antenna assembly AA 1  contains discrete receive RA 1  and transmit TA 1  elements. Either may be connected to the antenna cable AC 1  and radio RAD 1  through appropriate contacts in the RF switch RFSW 1 . This switch RFSW 1  is controlled by a control CONT 1 . Quiescently, the switch RFSW 1  causes the radio RAD 1  to be connected to receive antenna RA 1 , because this is usually the dominant mode of operation. At such time as it is desired to initiate a transmission, the controller CONT 1  first generates a signal to cause the switch RFSW 1  to move to its normally open NO set of contacts, connecting the transmit antenna TA 1  to the radio RAD 1 . The control CONT 1  then instructs the radio RAD 1  that it may transmit. At the conclusion of the transmission, the radio RAD 1  informs the control CONT 1  of this status and the control CONT 1  removes the actuating signal from the switch RFSW 1 , causing the switch RFSW 1  to return to its normally closed NC contacts, again connecting the receive antenna RA 1  to the radio RAD 1 . The ground return is best effected through a second conductor within CC 1 , but may also use the shield of the antenna cable AC 1 . The ground return is omitted from the figure for clarity. 
         [0006]    When the antenna assembly AA 1  is distant from the radio RAD 1 , the cost of the control cable CC 1  can be significant. 
         [0007]      FIG. 2  shows an alternative currently in use that uses the RF coaxial cable for powering the antenna as well. This is commonly called “phantom powering”. 
         [0008]    Here again, the antenna assembly AA 2  is made up of receive RA 1  and transmit TA 1  antenna elements. They are connected to contacts of the RF switch RFSW 1  like in  FIG. 1 . However, in this implementation, there is no need for a separate control cable CC 1 . Power to operate the antenna switch RFSW 1  is passed along the same physical antenna cable AC 2  as the RF signal. 
         [0009]    First and second capacitors C 1  and C 2  are sized to be of insignificant impedance at the frequencies of interest; first and second inductors L 1  and L 2  are sized to be of effectively infinite impedance at the frequencies of interest. Thus, the RF path through the first capacitor C 1 , the antenna cable AC 2 , and the second capacitor C 2 , and the DC power path through the first inductor L 1 , the antenna cable AC 2 , and the second inductor L 2  can coexist within the same physical antenna cable AC 2  without interfering with each other. The ground return, through the shield of the antenna cable AC 2  is not shown for clarity. 
         [0010]    Although this solves the problem of multiple cables going from the radio to the antennas, it still requires circuitry added at both ends of the cable, plus an explicit control signal to determine when to switch the antenna relay to the transmit or receive element. 
         [0011]      FIG. 3  is a circuit diagram that illustrates a design that can be used when the receive and transmit frequencies are sufficiently far apart. In this design, it is possible to insert a frequency selective network which electrically connects the single coaxial cable to the appropriate antenna depending on the frequency of signals it is carrying. 
         [0012]    Here the two antenna elements RA 1  and TA 1  are as in the previous figures. The radio RAD 1  is as before. However, there is no need for a separate controller or control signal. Instead of an RF switch, this implementation introduces a frequency dependent splitter FS 1 , which may contain passive or active components or both. Depending on the frequency present, it will steer the RF energy accordingly. For instance if the transmit frequency is above the receive frequency, a high-pass  filter might connect the antenna cable AC 2  to the transmit antenna TA 1  and a low-pass filter might connect the antenna cable AC 2  to the receive antenna RA 1 . 
         [0013]    However, there are many cases where the two frequencies are not far enough apart to allow realization of a frequency selectable element as shown here. These configurations require an antenna relay as described above ( FIGS. 1 and 2 ). In addition, the frequency splitter requires low loss components on the high power (transmit) side, which may be difficult and/or expensive to implement. 
       SUMMARY 
       [0014]    Accordingly, an antenna switch is provided, comprising: an antenna cable connector that connects to a radio RF output port that both receives an RF signal at a receive frequency, and transmits an RF signal at a transmit frequency that differs from the receive frequency; an RF switch having a common contact that is switchable between a receive antenna contact and a transmit antenna contact; a level comparator comprising an input connected to the antenna cable that: when a voltage at the input is below a predefined threshold value, causes the RF switch to contact the receive antenna; and when the voltage at the input is above a predefined threshold value, causes the RF switch to contact the transmit antenna. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0015]    Various embodiments of the invention are illustrated in the drawings and detailed description below: 
           [0016]      FIG. 1  is a schematic diagram illustrating a known design for connecting a radio to transmit and receive antennas using a switch controlled by a control; 
           [0017]      FIG. 2  is a schematic diagram illustrating a known design similar to that of  FIG. 1 , but using the RF coaxial cable for powering the antenna as well; 
           [0018]      FIG. 3  is a schematic diagram illustrating a known design that can be used when the receive and transmit frequencies are sufficiently far apart; 
           [0019]      FIG. 4  is a schematic diagram illustrating an embodiment of the inventive design for connecting a radio to transmit and receive antennas using a switch controlled by a control; and 
           [0020]      FIG. 5  is a schematic diagram illustrating an embodiment for powering the active devices. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    An automatic antenna switch as described in the following paragraphs may be used for those applications where: 1) the transmit power is significantly higher than the receive power; and 2) the baud rate is slow as compared to the switch speed. 
         [0022]    Referring to  FIG. 4 , the radio contains a single RF output port which internally connects to both the receiver and the transmitter. In one implementation using a system of low earth satellite repeaters, the transmitter output is 5 W into 50 ohms (37 dBm), while the receive signal strength may be as much as −80 dBm. The first condition stated above (transmit power much greater than received power) is met by this system. Converting from dBm to peak sinusoidal voltage, when transmitting, there is 22.3 v peak on the transmission line, while when receiving there is 31.6 μv peak on the line. A level comparator COMP 1  is used to sense this difference and moves the antenna switch RFSW 1  when the signal is large (i.e., transmitter is active).A diode D 1  and capacitor C 1  form a peak detection circuit PDC which charges the capacitor C 1  to a diode drop voltage below the peak voltage of the transmission. A resistor R 1  allows for a deterministic decay of the voltage so the system switches to a receive mode a predetermined time after the transmission has ended. 
         [0023]    For the first several hundreds of nanoseconds, before the antenna switch RFSW 1  has connected the transmitting antenna TA 1  element to the transmission line AC 1 , the receive antenna RA 1  will still be connected. Because this receive antenna RA 1  is not tuned to the transmitter frequency FT, it will present an impedance mismatch to the transmission line AC 1 , potentially damaging the transmitter or generating spurious radiations. To counter this, an optional matching network CN 2  is included. 
         [0024]    Working in conjunction with the receive antenna RA 1 , this presents an impedance match close to, e.g., 50 ohms for the transmission line AC 1 . This is separated from the transmission line AC 1  by a pair of diodes D 2  and D 3 , which appear as open circuits to signals as small as the receive signals, but conduct for signals as large as the transmission signals. Thus, a matching network is implemented which only is connected for signals above a diode drop in magnitude 0.6 v. Therefore, the bulk of the transmission signal will see a proper match before the antenna switch RFSW 1  connects the proper transmission antenna TA 1 . The receive signal, having a voltage so much smaller than the voltage necessary to bring a diode into conduction, will not be affected by this matching network CN 2 . 
         [0025]    A switch that might be used in this implementation is RFSW8000 from RF Micro Devices, Inc., the specification sheet being incorporated herein by reference. In the system according to the embodiment described above, an uplink data communications baud rate is 2400 baud (416 μsec/bit). The switch changes state in 100-300 nsec, far faster than the bit rate, so the second condition stated above is met (this is true for bit rates over 3.3 Mbaud, with a 300 nsec switch, and three times that for a 100 nsec switch).The small fraction of the first portion of the transmission signal which is used to charge up C 1  is insignificant as compared to the full bit width and does not interfere with the decoding of the data. 
         [0026]    Switches such as the RFSW8000 require a pair of drive signals in quadrature, i.e., one must be at a high voltage and other at a low voltage for the switch to be in one state, and the converse must be true to move the switch to the other state. This may be implemented with a pair of CMOS logic inverters, such as CD4049, available from Texas Instruments and others. 
         [0027]    Several ways are proposed for powering active devices that are associated with the switch (including the switch itself and/or logic components, such as the inverter stage noted with respect to the CD4049 device, or other components). The first of these is to include a small primary battery in the antenna housing. The switch RFSW 1  described above RFSW8000requires a bias current of 5 μa, so a small lithium battery will last for several years, and provisions should be made to replace this battery before it has run down. The RF switch RFSW 1  may be designed to be quiescently connected to the receive antenna, so the system is ready at all times to receive signals. 
         [0028]    A second way of powering the active devices is shown in the second power option PWR of  FIG. 5 . Here a fraction of the transmission power is used to energize the switching system. A rectifier and filter network, made from a fourth diode D 4  and second capacitor C 2  gleans a DC charge from the transmission waveform, storing enough energy on the large value capacitor C 2  to bias the system until the next transmission occurs to charge it up again. 
         [0029]    In one implementation, the system is configured to transmit at least once per day, which also serves to re-charge the capacitor. A second resistor R 2  may be inserted to lessen the peak load on the transmission signal when the capacitor C 2  voltage is well below peak and would appear as a temporary short circuit across the transmission signal. With properly sized components, the loss to the leading edge of the transmission signal will be negligible. 
         [0030]    Note that the battery powered system described does not require any transmissions to properly bias the antenna relay, while the self-powered system PWR does. If transmissions do not occur in time to recharge the capacitor C 2  to a necessary level to power the system, the RF switch RFSW 1  may move to a state wherein the neither antenna element RA 1 , TA 1  is connected. It will resume proper operation directly after a transmission occurs. 
         [0031]    The power supply can utilize any form of active or self-powered implementation or power storage mechanisms. For example, the following could be used in the power supply: solar power, wind-based power, water-based power, chemical reaction devices, such as various types of batteries and fuel cells, and the like. 
         [0032]    All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated as incorporated by reference and were set forth in its entirety herein. 
         [0033]    For the purposes of promoting an understanding of the principles of the invention, reference has been made to the preferred embodiments illustrated in the drawings, and specific language has been used to describe these embodiments. However, no limitation of the scope of the invention is intended by this specific language, and the invention should be construed to encompass all embodiments that would normally occur to one of ordinary skill in the art. 
         [0034]    The embodiments may be described in terms of functional block components and various processing steps. Such functional blocks may be realized by any number of components that perform the specified functions. 
         [0035]    The particular implementations shown and described herein are illustrative examples of the invention and are not intended to otherwise limit the scope of the invention in any way. For the sake of brevity, conventional aspects of the systems (and components of the individual operating components of the systems) may not be described in detail. Furthermore, the connecting lines, or connectors shown in the various figures presented are intended to represent exemplary functional relationships and/or physical or logical couplings between the various elements. It should be noted that many alternative or additional functional relationships, physical connections or logical connections may be present in a practical device. Moreover, no item or component is essential to the practice of the invention unless the element is specifically described as “essential” or “critical”. 
         [0036]    The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
         [0037]    The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) should be construed to cover both the singular and the plural. Furthermore, recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. Finally, the steps of all methods described herein are performable in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. 
         [0038]    The words “mechanism” and “element” are used herein generally and are not limited solely to mechanical embodiments. Numerous modifications and adaptations will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention. 
       TABLE OF REFERENCE CHARACTERS 
       [0000]    
       
         AA 1 , antenna assembly 
         AA 2   
         AC 1 , antenna cable 
         AC 2   
         C 1 , C 2  first and second capacitors 
         CC 1  control cable 
         CCON common contact 
         CN 2  optional matching network 
         COMP 1  level comparator 
         CONT 1  control block 
         D 1 , D 2 , first through fourth diodes 
         D 3 , D 4   
         FR receive frequency 
         FS 1  frequency splitter 
         FT transmit frequency 
         IN 1  comparator input 
         L 1 , L 2  first and second inductors 
         PDC peak detection circuit 
         PWR second power option 
         R 1 , R 2  first and second resistors 
         RA 1  receive element 
         RAD 1  radio 
         REF 1  comparator reference 
         RFSW 1  RF switch 
         TA 1  transmit element