Abstract:
An apparatus directs two high-power UHF transmitter signals to one or the other or a combination of output destinations as determined by the setting of control components. Redirection between outputs can be performed continuously under full power. Using the apparatus, synchronous amplifiers directed to the same output produce a signal with all of the power of both amplifiers. The signals can be shifted to the station load without shutting down the amplifiers. After a failure, the remaining amplifier can be redirected to provide a clean signal.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to broadcast radio frequency (RF) transmission apparatus. More particularly, the present invention relates to switching and combining systems for high-power broadcast transmitters.  
         BACKGROUND OF THE INVENTION  
         [0002]    RF broadcasting transmission apparatus for connecting high power transmitters to their antennas uses either coaxial line or waveguide as determined by factors such as frequency, power level, distance between transmitter and antenna, height of antenna tower, number of channels to be transmitted, and the like. For Very High Frequency (VHF) television, as for FM radio broadcasting and the various business and other bands embedded within the VHF range, the frequencies are low enough—which means the wavelengths are long enough and the structures must be large enough—to make waveguide-based transmission lines and signal manipulation largely infeasible. For the Ultra-High Frequency (UHF) television band, as for business broadcast channels with comparably high frequencies and frequencies higher still, up into the so-called microwave bands, waveguide may have utility comparable to or superior to that of coaxial line for many purposes.  
           [0003]    Functions commonly performed at lower frequencies with discrete passive elements such as resistors, inductors, transformers, capacitors, transmission line sections, and the like can be replaced in waveguide systems by tuned cavities, dimension changes, resonant pins, blocks of solid dielectric material, and other apparatus to achieve comparable effects to the conventional components while working well at the power levels called for in RF transmission systems. An example of this, termed a waveguide-based switchless combiner, can accept two inputs, each of which is a broadcast signal from a transmitter. If the two signals come from transmitters that are synchronized, such as by accepting synchronous excitation and being well matched dimensionally, and if the frequency range for the switchless combiner includes the full channel width of the signals, then the switchless combiner can split each signal into two orthogonal parts, pass them through two waveguide sections, and join them into a single signal that can deliver virtually the full energy of the two transmitters to the output waveguide or coaxial line that carries it to the antenna, effectively adding the signal strength of two lower-power transmitters.  
           [0004]    A transmitter system including a combiner device commonly requires one or more mechanical switches to direct signals from the transmitters to the combiner and/or from the combiner to either an antenna for broadcast or a resistive dissipative load device for test. Use of such mechanical switches generally requires shutting off power to the transmitters and may call for performing partial disassembly of high-power apparatus to reroute signals. A desirable capability would be to alleviate the need for one or more mechanical switches as well as to allow testing and maintenance functions to proceed without shutting off known-good transmitters and without the necessity of taking a programming source off the air altogether.  
           [0005]    A type of hybrid known in the art as a “magic tee” or 180 degree hybrid differs from a standard or rectangular 90 degree hybrid in producing a substantially full-power output from an in-plane output port for two coherent inputs, and a substantially evenly split output between the in-plane and orthogonal output ports for two inputs out of phase by 90 degrees. Where the inputs have opposite phase, substantially all of the energy exits by the orthogonal port.  
           [0006]    Accordingly, there is a need in the art for a switching system for broadcast transmission that overcomes, at least to some extent, the problems associated with the use of mechanical switches along with combiners to switch individual transmitters in and out of the broadcast signal stream.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore a feature and advantage of the present invention to eliminate high-power mechanical switching devices from a broadcast signal path. It is another feature and advantage of the present invention to eliminate power dissipating devices from a broadcast signal path. It is another feature and advantage of the present invention to support filtering and combining of signals. It is another feature and advantage of the present invention to allow a single transmitter to be redirected from an antenna to a nonradiating load and back without shutting down power to that transmitter. It is another feature and advantage of the present invention to allow a single transmitter to be redirected from an antenna to a nonradiating load and back without shutting down power to other transmitters comprising the system. The above and other features and advantages are achieved through the application of a novel combination of switchless combiners and filter-combiners as herein disclosed.  
           [0008]    In one aspect, the invention provides an output directing apparatus for RF transmission, comprising a four-port switchless combiner configured to accept input from two RF signal sources and to output two signals corresponding to the inputs, altered in phase relationship and relative magnitude by an adjustable amount; and a four-port filter-combiner configured to accept input from one or two RF signal sources of the same broadcast channel and to output one or two signals, as determined by the phase relationship between the input signals.  
           [0009]    In another aspect, the invention provides an apparatus for directing high-power RF transmission signals, comprising means for accepting synchronous signals from a plurality of transmitters; means for optionally combining the constituent signals; means for directing the constituent signals to a plurality of output destinations in a plurality of configurations; and means for retaining signal path integrity during transitions between signal direction configurations, whereby impinging signals can continue to be accepted at representative power levels during transitions between configurations.  
           [0010]    In yet another aspect, the invention provides a method of directing high-power RF transmitter signals, comprising the following steps: accepting signals from a plurality of synchronous broadcast transmitters at any level of matching of their respective signal strength; directing signals from a plurality of transmitters to a plurality of output destinations in a plurality of configurations; altering the phase relationship between the signals to a selectable degree; combining the signals from the transmitters to a selectable degree; and varying the directing and combining of the signals continuously without requiring interruption of transmitter signal flow.  
           [0011]    There have thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and that will form the subject matter of the claims appended hereto.  
           [0012]    In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.  
           [0013]    As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is an overall schematic diagram summarizing a representative signal path, according to a preferred embodiment of the invention.  
         [0015]    [0015]FIG. 2 is a plan view of a switchless combiner with two conventional hybrids that comprises a portion of a switching system.  
         [0016]    [0016]FIG. 3 is a plan view of a filter-combiner that comprises a portion of a switching system.  
         [0017]    [0017]FIG. 4 is a block diagram of a switchless combiner with two conventional hybrids and a filter-combiner, together forming an operational switching system.  
         [0018]    [0018]FIG. 5 is a plan view of a switchless combiner with one conventional hybrid and one magic tee combiner, comprising a portion of a switching system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    A preferred embodiment of the present invention includes a switchless combiner for each two input signal paths, and sufficient switchless combiners and filter-combiners to combine all of the available signals into a single output. A preferred embodiment of the present invention also employs one input port per signal source. Each signal source may be a high-power RF signal, typically a single UHF-band television channel signal, although a variety of other sources and frequency bands can be used with a suitably configured embodiment of the invention. A preferred embodiment of the present invention further employs a switchless combiner to avoid the need to deenergize any transmitter devices when redirecting one or more transmitter outputs. Preferred embodiments will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout.  
         [0020]    As shown in FIG. 1, a complete signal path for a preferred embodiment of a transmitter switching device  10  carries RF transmitter energy from a transmitter amplifier  12  via a coupling device  14  to a switchless combiner  16 , from the output of which the signal travels via a filter-combiner  18  to two outputs, an antenna  20  (by way of a transmission line  22  of any length) and a station load  24  (by a length of transmission line  26 ).  
         [0021]    Various terms of art for load resistors may be used herein. These include for example the generic term dummy load for any nonradiating RF absorber; the terms load and station load for a device with sufficient capacity to provide continuous (indefinite) dissipation of all RF transmitter outputs together; and the terms reject load and ballast load for a device typically intended to dissipate the off-frequency energy filtered out of a single transmitter, and thus commonly smaller in size and capacity than a station load.  
         [0022]    [0022]FIGS. 2 and 3 approximate the physical appearance of typical devices for implementation of the preferred embodiment. In the plan view of the switchless combiner, FIG. 2, input signals A and B are fed by way of first adapter fitting  28  and second adapter fitting  30  into a first hybrid  32 . The first hybrid first input port  34  and first hybrid second input port  36  accept RF from two transmitters  12  (shown in FIG. 1 and FIG. 4); the first hybrid first output port  38  and first hybrid second output port  40  alter the signals in the following way: first hybrid first output port  38  has a divided share of first hybrid output A RF energy at nominal phase and a divided share of first hybrid output B RF energy delayed 90 degrees; first hybrid second output port  40  has a divided share of first hybrid output B RF energy at nominal phase and a divided share of first hybrid output A RF energy delayed 90 degrees. These signals can be combined or switched by the preferred embodiment of the inventive apparatus. The energy proportionality in the divided shares is preferably roughly equal.  
         [0023]    The first hybrid first output port  38  feeds into the first phase shifter  42 . The first phase shifter  42  includes a first dielectric block  44  positioned either automatically by a first motorized positioning apparatus  46  or manually by a first override device  50 . The first hybrid second output port  40  feeds similarly into a second phase shifter  52  with apparatus elements comprising a second dielectric block  54 , a second positioner  56 , and a second override  58 . When fully retracted, the first and second dielectric blocks  44  and  54 , respectively, have no effect on propagation rate, which is the default propagation value for the first and second phase shifter assemblies  42  and  50  with the first and second blocks  44  and  54  fully retracted. Extending the first and second blocks  44  and  54  causes increased delay in signal propagation in their respective phase shifters  42  and  52 . Maximum feasible delays can exceed 270 degrees of a cycle of RF energy when compared to the retracted rate for a realizable phase shifter.  
         [0024]    The first and second dielectric blocks  44  and  54  are commonly made from solid polytetrafluoroethylene (PTFE) (available under for example the trade names Teflon® and Dyneon™), which is preferred for its low dissipation factor. Because of its low dissipation factor, the PTFE block can provide the necessary delay while minimally absorbing the RF energy and turning it into heat. Alternative materials can be used in substantially the same way as PTFE.  
         [0025]    Positioning of first and second dielectric blocks  44  and  54  can be sufficiently repeatable using mechanical limit switches controlling drive motors in the first and second positioners  46  and  56  that producing a particular phase shift at a given channel frequency does not require feedback control on block position.  
         [0026]    Block position accuracy can be verified in some block positions by detecting power level in the station load  24  (FIG. 1). When the station load  24  dissipated power is at a minimum, for example, it may be reasonably deduced that the blocks are positioned to maximize the power directed to the antenna  20 . Complete system designs can, for example, use this property as a calibration test.  
         [0027]    The phase shifters  42  and  52  shown in FIGS. 2 and 3 are illustrated using low-dissipation factor dielectric blocks  44  and  54 , respectively, to perform any required phase shifting. Alternative phase shifter designs can be used. One such, a so-called trombone design, achieves phase shift with a transmission line that features a series of right-angle bends to allow a section of itself to be slid in and out, thereby shortening and lengthening the signal path while leaving the ends fixed. Another alternative phase shifter design inserts and retracts a series of pins spaced along a waveguide section, where increased insertion can correspond to increased phase shift. Other methods are likewise readily available to those knowledgeable in the art.  
         [0028]    The transmitter amplifiers  12  (FIG. 1) that serve as the source of input signals to the switching apparatus can be matched in physical properties and output levels, and can typically be fed by a synchronous driver circuit  60  (FIG. 1) to provide excitation matched in amplitude and phase. Where appropriate, one of the individual amplifier drive signals, i.e., the inputs to the amplifiers, may be altered in phase, such as with a delay line  62  (FIG. 1), to establish a fixed delay of one signal with respect to the other. An equivalent technique can interpose an additional waveguide section in one of the two amplifier  12  output waveguides  14  feeding the combiner circuit  16 , and thereby provide an equivalent fixed delay to that created by the delay line  62  feeding the switchless combiner  16 . The delay device here termed fixed can be adjustable or otherwise subject to alteration, such as for fine tuning.  
         [0029]    Referring again to FIG. 2, in the preferred embodiment, outputs of the first and second phase shifters  42  and  52  feed a second hybrid  62 . If the B input signal lags the A by 90 degrees at the input to the switchless combiner  16  and the second phase shifter  52  is so adjusted as to delay the hybrid output by another 90 degrees, then there is effectively no signal present on a second hybrid first output port  68 , and both signals will be present substantially at full strength and in phase on a second hybrid second output port  70 . Adjusting the second phase shifter  52  to 0 degrees or 180 degrees instead of 90 degrees shifts the B signal, in the 0 degree case, or the A signal, in the 180 degree case, to the second hybrid first output port  68 . Shifting the second phase shifter  52  to 270 degrees lag at the transmit frequency places both amplifier signals on the second hybrid first output port  68 .  
         [0030]    Referring to FIG. 3, for the waveguide filter-combiner  18 , the preferred embodiment can use input hybrid  72  and output hybrid  92  with first waveguide filter section  82  and second  84  waveguide filter section between, and can have the filters of the first and second waveguide filter sections  82  and  84  comprised of cascaded cavity resonators tuned to a specific channel frequency and dimensioned for the required filter performance at that frequency. If substantially all of the RF energy from the switchless combiner  16  is directed to the second input  76  of the input filter hybrid  72 , then substantially all of the energy will be directed by the filter-combiner  18  to the first output  98  of the output filter hybrid  92 , from which port it can be fed by way of a first output adapter  102  and the high-power RF conductor  22  to the antenna  20 . If substantially all of the RF energy is instead directed to both the first input port  74  and the second input port  76  of the input hybrid  72 , then the energy directed to the second port  76  can still go by way of first output adapter  102  to the antenna  20 , but the energy directed to the first port  74  can instead be directed to the second output port  100  of the filter-combiner  18 , and thence by way of second output adapter  104  to the station load  24 . Similarly, substantially all of the energy can be input to the first port  74 , in which case substantially all of the energy can be directed to the station load  24 . This set of options allows substantially all of the transmitter  12  energy to be broadcast on the antenna  20 , and further allows either one of the transmitters  12  to drive into the station load  24  for testing, as well as allowing the entire station signal to be directed into the station load  24 .  
         [0031]    As indicated in the discussion of FIG. 2, the second phase shifter  52  can provide continuous adjustment of the phase angle of the signal passing through it. As the phase angle changes, the balance between the outputs can likewise change continuously, including placing the entire signal from either one or both of the transmitters onto the station load  24  or onto the antenna  20 . Because the low dissipation factor is intrinsic to the PTFE or equivalent material used for the dielectric blocks  44  and  54 , a phase shifter designed and sized according to the preferred embodiment can be one that can operate continuously at any power setting and at any phase angle setting without damage to itself and without producing significant changes in impedance that would create reflections. This can permit the same functional performance otherwise achieved by a system with mechanical switches to be achieved by adjusting controls by hand or under the control of motors and limit switches.  
         [0032]    In the event of a shutdown-type failure of either transmitter, for a system configured to drive the antenna with all of the power of two substantially equal transmitters, a system designed according to the preferred embodiment can divide the power from the remaining transmitter equally between the output ports  68  and  70  without adjustment. This mode can be readjusted; setting the dielectric blocks  44  and  54  to an intermediate position can redirect all of the remaining energy to the switchless combiner output port  70 , which will direct the energy to the antenna  20 . A power sensor  48  can be embedded in the station load  24  (FIG. 3) to confirm that the single-transmitter power distribution arrangement is optimal, that is, that the amount of power sent to the station load  24  is minimized. The setting, however, can be sufficiently repeatable to allow the reconfiguration to be controlled by motors and position sensing switches rather than power or temperature sensors on the load.  
         [0033]    [0033]FIG. 4 shows a schematic of the preferred embodiment. A signal source  106  drives a power divider  108 ; from this a time delay  110  adjusts the phase on one signal  112  so that the first amplifier  114  leads the second amplifier  116  by 90 degrees at the center frequency of the broadcast signal. The preferred embodiment provides for a system in which low-level signals can be used to drive the amplifiers  114  and  116 , which can output signals at the level of multiple kilowatts.  
         [0034]    As illustrated in FIG. 4, the RF amplifier signals, outputs of the first amplifier  114  and the second amplifier  116 , differ in phase by 90 degrees. Fed into the first hybrid  32  of the switchless combiner  16 , each of the signals exits by both output ports, as represented in this schematic by a first signal path  118  and a second signal path  120 . The output energy emitted diagonally across from each input lags the output energy emitted horizontally across from the input by 90 degrees. The two signals in each waveguide phase shifter are isolated by about 30 dB.  
         [0035]    Passing through the adjustable phase shifters  42  and  52 , the signals can retain their isolation but may be delayed to the extent required by the application and permitted by the details of phase shifter design. At the intermediate nodes  126  and  128 , the signals may be found to have been altered in relative phase, so that their recombination in the second switchless combiner hybrid  62  may produce effectively any desired phase relationship at the switchless combiner  16  output nodes  130  and  132 .  
         [0036]    If the switchless combiner  16  is so configured that the signal at the second switchless combiner hybrid  62  second output node  132  contains substantially all of the RF energy from the transmitters, the energy can be directed by the filter combiner  18  to the transmission lines  22  leading to the antenna  20 . In that case, passage of the RF energy through the filter-combiner  18  consists of division of the signal in the filter-combiner first hybrid  72  into an in-phase component found at a first hybrid second output node  136  and a lagging component at a first hybrid first output node  134 , followed by filtering of these two components in the first filter  82  and the second filter  84 , respectively, followed by recombination into an in-phase signal in the output hybrid  92  at the entry to the antenna transmission line  22 .  
         [0037]    As illustrated in FIG. 5, which is an alternative embodiment of the switchless combiner function  138  of the improved switching system  10 , the phase shifter  52  output can feed an alternative final switchless combiner element, a style of hybrid known in the art as a magic tee or 180-degree hybrid  140 . The magic tee  140  differs from a standard hybrid in producing a substantially full-power output from an in-plane output port  142  for two coherent inputs, and a substantially evenly split output between the in-plane  142  and orthogonal  144  output ports for two inputs out of phase by 90 degrees. Where the inputs have opposite phase, substantially all of the energy exits by the orthogonal port  144 . Use of the magic tee  140  as a component of a switchless combiner may require that a signal impinging on the first input port  34  be out of phase by 90 degrees from a signal impinging on the second input port  36 . If the lagging signal is on the first input port  34  and if the second phase shifter  52  is set to introduce a lag of zero degrees, then the signals impinging on the magic tee  140  itself may be effectively in phase, allowing substantially all of the energy applied to the switchless combiner to emerge at the in-plane port  142  of the magic tee  140 . Both outputs of the switchless combiner  138  must still be applied to the filter combiner  18  to form the complete embodiment of the improved switching system.  
         [0038]    It may be possible to use a single phase shifter, corresponding to the second phase shifter  52  in FIG. 2, and to make no provision for a first phase shifter, such as element  42  of FIG. 2, using instead a fixed-delay waveguide section  146 . A sufficient range of adjustment in the sole phase shifter  54  allows the full range of function to be maintained. The orthogonal output port  144  of the magic tee  140  may be fitted with appropriate waveguide to allow it to feed the first input port  74  of the filter combiner without using a ballast load.  
         [0039]    The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, that fall within the scope of the invention.