Abstract:
A compact and versatile power meter is created through the use of a discrete component network providing for phased splitting and combining of signals obtained at taps along a transmission conduit having a predefined phase separation. The use of the discrete component network eliminates the need for bulky waveguides or microstrip antenna designs, the latter providing phase shift through their physical dimensions.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional application 61/983,715 filed Apr. 24, 2014, and hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to radio/microwave power measurement devices and in particular to an extremely compact high-power measurement device. 
     Radiofrequency power meters are used to measure the power of a radiofrequency signal transmitted to an antenna or other load (forward power) as well as to measure reflected power back from the antenna or load (reflected power) such as may provide an indication of a voltage standing wave ratio (VSWR), power consumption or the like. These dual measurements allow determination of the actual amount of power delivered to the load and permit tuning and adjustment of the load or source for optimal power transfer. The term radiofrequency is used herein shall be considered to embrace high-frequency (HF), very high frequency (VHF), ultrahigh frequency (UHF) and microwave frequency signals. 
     Three common designs for radiofrequency power meters are those which employ waveguides (typically for microwave frequencies) and coaxial or microstrip transmission lines (typically for HF, VHF and UHF frequencies) placed in series between the power transmitter and the load. 
     In the former design, a primary waveguide is coupled to a secondary waveguide through two ports located to couple signals from the primary waveguide to the secondary waveguide at points with a 90-degree phase difference (one quarter wavelength) at the conducted signal frequency. The outputs of the secondary waveguide at opposite ends will individually isolate the forward and reflected power allowing these two different quantities be measured, for example, with a diode sensor. 
     In the latter design, signals from the primary transmission line are received by an a transmission line physically analogous to the secondary waveguide, again through openings separated by a 90-degree phase difference (one quarter wavelength) along the primary transmission line. Outputs from the opposite ends of the secondary transmission line isolate the forward and reflected power. 
     The process of isolating forward and reflected power in both of these designs requires analyzing structures (secondary waveguides or secondary transmission lines) having a length in excess of a quarter wavelength of the measured frequency. For UHF frequencies, for example, this can require constructing carefully tuned structures having a length many centimeters long. Power meters intended for different frequencies can require wide range of different analyzing structures. 
     SUMMARY OF THE INVENTION 
     The present invention provides a power meter that employs a discrete component phase shift and summing network for isolating forward and reflected power. This network permits the construction of an analyzer whose dimensions are largely independent of the frequency being analyzed and thus can be extremely compact. By eliminating the need to fabricate large tuned structures of a variety of different sizes, a compact power meter can be created at lower cost using standardized components for a range of frequencies. 
     In one embodiment, the invention provides an input port for communication with a radiofrequency power source for receiving forward radiofrequency power at a frequency for measurement and an output port separated along a transmission path from the input port for communication with a radiofrequency load for receiving reflected radiofrequency power at the frequency for measurement. A power conduit extending along the transmission path communicates power from the input to the output, and a first and second power tap are coupled to the power conduit and separated along the transition path by an odd integer multiple of a quarter wavelength distance at the frequency. A discrete component network has four ports and receives at a first and second port power, and outputs at a third and fourth port power being a sum of power received at the first and second ports, the power at the third port from the first port shifted by an odd multiple of 90-degrees relative to the power at the third port from the second port, and the power at the fourth port from the second port shifted by an odd multiple of 90-degrees relative to the power at the fourth port from the first port. A computer processor system receives signals from the third and fourth ports and communicates with a display to provide a display of radiofrequency power selected from the group consisting of: forward power, reverse power, voltage standing and wave ratio. 
     It is thus a feature of at least one embodiment of the invention to provide a power meter that eliminates the need for the construction of a waveguide or microstrip antenna with precise mechanical dimensions for the analysis of power. By employing a four-port network of discrete components, the cost and difficulty of manufacturing a range of power meters is greatly reduced. 
     The discrete component network may provide an interconnected transformer and one or more capacitors and resistors. 
     It is thus a feature of at least one embodiment of the invention to make use of standard commercially available power splitter components to isolate forward and reverse power. 
     An integrated housing of the discrete component network may have a volume with a longest dimension of less than one-quarter wavelength of the frequency; 
     It is thus a feature of at least one embodiment of the invention, to permit the analysis of radiofrequency signals using a device that may be smaller than a quarter of a wavelength of the signal greatly reducing the size of the power meter. 
     The computer processor may further provide an output selected from an instantaneous power value and a time-average power value over a longer time than measured by the instantaneous power value and/or selected from power measured in decibels and power measured in watts. 
     It is thus a feature of at least one embodiment of the invention to provide a set of varied measurements possible from fundamental measurements of forward power and reverse power. 
     The first and second power taps may be conductive pins having outer threads received by threaded sockets having inner threads in electrical communication with the first and second port of the quadrature combiner, and wherein the conductive pins may be rotated to extend perpendicularly toward and away from the power conduit perpendicular to the axis. 
     It is thus a feature of at least one embodiment of the invention to provide for a simple tuning method for controlling the balance of power to the first and second ports, the total power drawn from the power conduit. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the housing of the power meter of the present invention in a first embodiment for receiving coaxial cable; 
         FIG. 2  is a simplified diagram of the circuitry of the power meter of  FIG. 1  showing adjustable power taps communicating with an integrated discrete component network and with a microcontroller; 
         FIG. 3  is a fragmentary portion of the block diagram of  FIG. 2  showing a second embodiment for receiving a waveguide; 
         FIG. 4  is a phase diagram showing operation of the integrated discrete component network with 90-degree taps. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a power meter  10  for providing radiofrequency power measurements within a given frequency band may provide for a housing  12 , for example, of a conductive metal material exposing at an upper face a liquid crystal graphic display  14  for providing measurement readings and a display selector knob  16  for controlling the readings. 
     Releasable coaxial cable input  18  and output  20  may be aligned along an axis  22  on opposite vertical faces of the housing  12  so that a source of radiofrequency power may be connected to input  18  to pass through the meter  10  to be output at output  20  where it may be attached to a load such as an antenna or the like. 
     Referring now to  FIG. 2 , a power conduit  24 , for example, a solid metal conductor, may extend along the axis  22  from input  18  to output  20  as surrounded by an insulating dielectric  26  and an outer coaxial conductor  28 , for example, a metal tube. The outer coaxial conductor  28  may have two openings  30   a  and  30   b  extending through the outer coaxial conductor  28  perpendicular to the axis  22  and the power conduit  24  and separated along the axis  22  by a distance  31  being an odd integer multiple of one quarter wavelength of the radiofrequency to be measured. Typically, this integer multiple will be one. 
     Positioned outside of the outer coaxial conductor  28  and aligned with the openings  30   a  and  30   b  are externally threaded coupling studs  32  extending perpendicularly to the axis  22  and received by internally threaded conductive collars  34 . As so held, the studs  32  may be rotated to move them toward and away from the power conduit  24  thereby changing the relative coupling of these conductive collars  34  (through the studs  32 ) to the power conduit  24 . It will be understood that adjustment of the studs  32  may be done to balance the received power at each of the conductive collars  34  and to control the total coupling between the studs and the power conduit  24 . 
     The conductive collars  34  are connected to a first and second port of a four-port hybrid combiner circuit  36 . The four-port hybrid combiner circuit  36  may make use of an integrated power splitter commercially available, for example, from Mini-Circuits of Brooklyn, N.Y., under the trade name of QCN-27 (for a frequency range of 1700 to 2700 megahertz) and QCN-5 (for a frequency range of 330 to 580 megahertz), as two non-limiting examples. These integrated power splitter/combiners have a dimension of 0.12 inches by 0.06 inches by 0.35 inches and are formed of an integrated transformer in an integrated sealed package with outwardly communicating solder terminals. In some embodiments additional discrete components including resistors and capacitors may be used. Example technologies for constructing the four-port hybrid combiner circuit  36  are described in U.S. Pat. Nos. 6,963,256 or 6,542,047 hereby incorporated by reference in its entirety. 
     The remaining third and fourth port of the four-port hybrid combiner circuit  36  may connect to radiofrequency detectors  40   a  and  40   b  (for example, diode demodulators) which communicate with analog-to-digital converter inputs of a microcontroller  42 . Microcontroller  42  may receive power from external power jack  44  passing through the housing  12 . Microcontroller  42  may also communicate with the display  14  to output data on the display  14 , and a selector encoder  46  may be attached to the knob  16  to allow user selection of particular displayed quantities as will be discussed below. 
     Referring now to  FIG. 3 , in an alternative embodiment, the input  18  and output  20  may be waveguide couplers and the power conduit  24  may be a waveguide channel having an outer conductive wall  50  also with openings  30   a  and  30   b  separated by an odd multiple of 90-degrees of waveform phase for use with the studs  32  and collars  34  which attach to the same circuitry described above with respect to  FIG. 2 . 
     Referring now to  FIG. 4 , in a simple case where the openings  30   a  and  30   b  are separated by 90-degrees at the wavelength of the measured frequency, for forward power passing from input  18  to output  20 , power received at opening  30   b  will have a 90-degree phase lag with respect to the power received at opening  30   a . The hybrid combiner circuit  36  will receive the power from opening  30   a  at first port  50   a  and the power from opening  30   b  at second port  50   b . This power from opening  30   a  will be transferred by first parallel kg  52   a  to third port  50   c  after the introduction of a 90-degree phase lead. Similarly, the power from opening  30   b  will be transferred by second parallel leg  52   b  to fourth port  50   d  after the introduction of a 90-degree phase lead. These phase leads are provided by normal operation of the commercial device discussed above. 
     Two crossing legs  52   c  and  52   d  also connect first port  50   a  to fourth port  50   d , and second port  50   b  to third port  50   c , respectively, without the introduction of phase lead. Thus, port  50   c  receives the sum of the signal received at port  50   a  shifted to lead by 90-degrees plus the unshifted signal from port  50   b . Similarly port  50   d  receives the sum of the unshifted signal from port  50   a  and a signal from port  50   b  with an added 90-degree phase lead. 
     It will be appreciated that this circuit  36  allows the distinguishing between forward and reverse (reflective) power in the following way. For power passing in the forward direction from input  18  to output  20 , the phase of that power received at port  50   a  shown by arrow  54   a  leads the phase of power received at port  50   b  as shown by arrow  54   b  by 90-degrees. After passing through the circuit  36  and as depicted in the upper left quadrant of a phase depiction diagram  56 , port  50   c  will show substantially zero output resulting from the destructive cancellation between the signals from ports  50   a  and  50   b  which after phase shifting are now in 180-degree opposition. In contrast, as shown in the upper right-hand quadrant of the phase depiction diagram  56 , port  50   d  will show a nonzero magnitude of power as a result of the constructive addition between the signals from ports  50   a  and  50   b  which are now in alignment. Thus the power at port  50   d  isolates the forward power. 
     Conversely, for reverse power passing from output  20  to input  18  being reflected power from the load, the power at port  50   a , shown by arrow  54   c , will lag the power at port  50   b  shown by arrow  54   d . In this case, as shown in the lower left quadrant of the phase depiction diagram  56 , port  50   c  will show a nonzero magnitude isolating reflected power, whereas port  50   d  depicted by the lower right-hand quadrant of the phase depiction diagram  56  will have a zero magnitude. Accordingly, forward power and reflected power may be independently resolved using this circuit. 
     The microcontroller  42  through selection by knob  16  operates on a selector encoder  46  and may show through display  14  forward power, reverse power, or combinations of forward power and reverse power including, for example, voltage, standing wave ratio or the difference between forward power and reverse power (being the power absorbed by the load). The microcontroller  42  permits the power display to be done on an instantaneous basis or over predefined averaging periods longer than the measurement provided by the instantaneous basis, for example, 10 seconds. Power may be depicted in watts or decibels. It will be appreciated that the microprocessor can practically swap the location of the input  18  and output  20 , at least by function, by simply swapping the measurements from ports  50   c  and  50   d  to allow more convenient connection of the device according to the location of the power transmitter. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a microprocessor” and “a controller” or “a microcontroller” or a “processor” should be under stood include all computing technology suitable for executing stored programs held in non-transitory form in computer memory associated with such devices. 
     The term “discrete component network” means networks that are principally constructed of discrete components having actual lumped element properties rather than components with distributed properties. The terms input and output are intended to cover coaxial and microwave couplers and any other communication path allowing for essentially unobstructed energy transfer at the described radio frequencies. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.