Abstract:
An antenna system for electromagnetic compliance testing within a frequency range includes a driven element mounted on a boom and including opposed first and second length-adjustable conductors. A length-adjustable passive element is mounted on the boom and spaced apart from the driven element and including opposed third and fourth length-adjustable conductors. An RF input connector is coupled between a feed path to a feed end of each length-adjustable conductors in the driven element. The feed path is configured to minimize stray reactances and stub effects within the frequency range.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to antenna systems. More particularly, the present invention relates to antenna systems that are particularly useful for electromagnetic compatibility (EMC) testing. 
     2. The Prior Art 
     EMC testing systems are known in the art. EMC testing is used for radiation and susceptibility testing of electronic devices. Radiation testing is used to determine how much RF energy is radiated by electronic devices while they are operating. In the radiation mode of testing, the antenna is used as a receive antenna. Susceptibility testing is used to determine how much RF energy a device can be exposed to before it malfunctions. In the susceptibility mode of testing, the antenna is used as a transmitting antenna. 
     Susceptibility testing starts at 30 MHz and at that frequency a very large shielded room is required to accommodate a reasonably efficient antenna such as a half wave dipole, which is 16 feet wide at this frequency. These rooms are not practical for the vast majority of test facilities. In addition, because the testing is performed at frequencies up to at least several hundred megahertz, the presently used antenna systems must be broadbanded enough to operate over these ranges. 
     Presently-available broadband EMC antennas used for susceptibility testing include bi-conical antennas and log periodic arrays. Such antennas are inefficient and can also be expensive. These antennas are paired with RF amplifiers in EMC systems the total cost of which can exceed $500K to $700K. Because presently-available systems are so inefficient, the amplifiers have to be run at extremely high power levels (e.g., in excess of 5 KW). At such power levels, coupled with extreme impedance mismatches between the amplifier and antenna, the EMC antenna sometimes radiates more energy at the second or third harmonic than at the fundamental frequency of interest. This setup badly skews the test results and often renders them invalid according to the existing testing standards. 
     Other antennas such as e-field radiators, also known as billboard antennas, are sometimes used for susceptibility testing but cannot totally illuminate the device under test. These antennas are thus very inefficient because the test must be repeated. In addition, the test cannot always be performed according to the existing testing standards. 
     There currently exists a need in the EMC industry for EMC test systems that can generate very high E fields to facilitate susceptibility testing and that conform to new stricter standards which specify that little or no harmonic energy be present. As previously noted, presently-available antenna/amplifier combinations require impractical power levels and require moving the antenna many times at every frequency test point to cover the required illumination of the Equipment Under Test (EUT). This requires many additional hours to perform the test. Additionally, using the presently-available antenna/amplifier combinations results in radiating energy at harmonic frequencies at power levels often exceeding the desired test frequency. 
     Adjustable antennas using flexible elements currently known in the art have attributes that could make them viable candidates for solving these long known problems in EMC susceptibility testing, but their basic design severely limits their upper frequency limit. Testing standards, such as MIL-STD-461F, require 200V/meter fields, with the antenna one meter away, from 30 MHz to 200 MHz, which is very difficult to achieve with available equipment. Current adjustable-frequency antenna designs, such as those disclosed and claimed in U.S. Pat. No. RE42,087, employ stepper motors to drive copper tape from reels. The reels are in close proximity to each other, resulting stray capacitive and inductive coupling between the element halves that is substantial within the frequency range of EMC testing systems. This limits current designs to an upper frequency limit of about 55 MHz. 
     In addition, the use of brushes contacting the metal tape to couple the RF energy to and from the tape does not allow the RF to be transferred directly to the starting point of each element resulting in the inadvertent formation of an RF “stub” that can make the system inoperable at various frequencies in the 30 MHz to 200 MHz range. 
     A second deficiency of current adjustable antennas using elements that loop back on themselves to shorten the width of the antenna is that many shielded rooms are still not large enough to accommodate them even with the 40% shorter elements these antennas provide. 
     Shielded rooms cause a multitude of “room effects” that severely affect the input impedance of the antenna resulting in very high standing waves that put a tremendous strain on the amplifier, but more importantly cause the harmonic output of the amplifier to be greatly increased. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the present invention, an antenna system for electromagnetic compliance testing within a frequency range includes a driven element mounted on a boom and including opposed first and second length-adjustable conductors. A length-adjustable passive element is mounted on the boom and spaced apart from the driven element and including opposed third and fourth length-adjustable conductors. An RF input connector is coupled between a feed path to a feed end of each length-adjustable conductors in the driven element. The feed path is configured to minimize stray reactances and stub effects within the frequency range. 
     According to another aspect of the present invention, an antenna system for electromagnetic compliance testing includes a plurality of antenna elements, at least some of which are arranged in a loop or folded configuration to allow them to fit in a smaller testing chamber. 
     According to yet another aspect of the present invention, an antenna system for electromagnetic compliance testing includes a plurality of antenna elements, at least some of which are have ends oriented at an angle with respect to the plane of the elements. 
     According to a further aspect of the present invention an antenna system for electromagnetic compliance testing includes a tunable antenna according to the present invention coupled through an SWR measuring device to an RF amplifier that is mounted on or in close proximity to the antenna. An output signal from the SWR measuring device is coupled to an antenna tuning controller coupled to the antenna elements to adjust their lengths for minimum SWR. An e-field meter is positioned to receive radiation emitted from the antenna. An output signal from the e-field meter is coupled to a control unit through a RF signal conditioning circuit. The control unit receives input from a user and adjusts a signal generator coupled to feed RF energy into the RF amplifier and also adjusts the RF amplifier to generate an RF signal at the e-field meter of a selected frequency at a selected field strength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a diagram showing a length-adjustment mechanism for an illustrative adjustable EMC antenna according to the present invention. 
         FIG. 2  is a diagram showing an illustrative adjustable EMC antenna according to the present invention. 
         FIG. 3  is a diagram showing an end view of an illustrative adjustable EMC antenna according to the present invention. 
         FIG. 4  is a diagram showing an illustrative adjustable EMC antenna system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Persons of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons. 
     Referring first to  FIG. 1 , a diagram shows a length-adjustment mechanism  10  for an illustrative adjustable EMC antenna according to the present invention. Length-adjustment mechanism  10  is mounted on a frame or housing  12 . An opposed pair of length-adjustable elements  14   a  and  14   b  are preferably formed from a conductive material such as perforated beryllium-copper alloy tape fed from respective reels  16   a  and  16   b  by stepper motors  18   a  and  18   b . Sprocket drive wheels  20   a  and  20   b  are preferably used to engage holes formed in the length-adjustable elements  14   a  and  14   b  to prevent slippage of the conductive material so that their lengths can be precisely controlled by the stepper motors. 
     The length-adjustable elements  14   a  and  14   b  are directed into hollow tubes  22   a  and  22   b . Details of the operation of the mechanism described so far are known in the art and are described in U.S. Pat. No. RE42,087 as well as being observable as employed in various adjustable antenna products marketed and sold under the trademark “Steppir” by Fluid Motion, Inc., of Bellevue, Wash. Further description of the construction and operation of this mechanism will not be presented here to avoid unnecessarily overcomplicating the disclosure. 
     For operation under typical EMC testing conditions, the antenna element length-adjustment mechanisms of the prior art present several problems including reactive coupling and inadvertent RF stub formation as noted above. These problems are overcome by the present invention. RF energy is coupled into the length-adjustable elements  14   a  and  14   b  by brushes  24   a  and  24   b  that make contact with conductive flat surfaces at the top edges of reels  26   a  and  26   b . The brushes are mounted on a block  28  formed from a dielectric material that is inert with respect to RF within the frequency range of interest. 
     In a presently preferred embodiment the reels  26   a  and  26   b  are formed entirely from a conductive material such as silver-plated brass. The reels  26   a  and  26   b  are preferably mounted in the same horizontal plane and are spaced apart from one another so as to substantially reduce or eliminate any reactive coupling between them. Also, because the brushes  24   a  and  24   b  are located towards the inner edges of reels  26   a  and  26   b , the possibility of inadvertently creating parasitic RF stubs is substantially eliminated within the frequency range of interest. 
     The length-adjustable elements  14   a  and  14   b  of the present invention may be configured as driven elements or as passive elements. When length-adjustable elements  14   a  and  14   b  are employed as driven elements, RF energy is coupled into the brushes  24   a  and  24   b  by RF input relay  30 . If used alone, they function together as a dipole antenna when oriented as shown in  FIG. 1 . When length-adjustable elements  14   a  and  14   b  are employed as passive elements, shorting relay  32  is used to connect brushes  24   a  and  24   b  together. RF input relay  30  and shorting relay  32  may be mounted on a circuit board  34 . Persons of ordinary skill in the art will appreciate that circuit board  34  should be formed from a material that is compatible with high frequencies expected to be encountered during use of the antenna system of the present invention. 
     In a typical EMC testing configuration, a number of length adjustment mechanisms  10  of  FIG. 1  can be arrayed into a more complicated antenna design, such as a yagi antenna. Referring now to  FIG. 2 , a diagram shows an illustrative adjustable multi-element EMC antenna  40  according to the present invention. 
     Multi-element EMC antenna  40  includes three length-adjustable element pairs  42   a  and  42   b  fed into tubes  44   a  and  44   b ,  46   a  and  46   b  fed into tubes  48   a  and  48   b , and  50   a  and  50   b  fed into tubes  52   a  and  52   b , respectively, mounted on a boom  54 . In one embodiment of the invention, the configuration shown in  FIG. 2  may be used over a frequency range such as 30 MHz to 200 MHz, although persons of ordinary skill in the art will appreciate that, by appropriate scaling, such a configuration could be made to operate over other frequency ranges. 
     Persons of ordinary skill in the art will observe that tubes  44   a  and  44   b  and  52   a  and  52   b  are both formed in a loop configuration. Such a configuration is described in U.S. Pat. No. 7,463,211 as well as being observable as employed in various adjustable antenna products marketed and sold under the trademark “Steppir” by Fluid Motion, Inc., of Bellevue, Wash. Further general description of the construction and operation of such loop elements will not be presented here to avoid unnecessarily overcomplicating the disclosure. 
     Each of length-adjustable elements  42   a  and  42   b ,  46   a  and  46   b , and  50   a  and  50   b , respectively, are controlled by length-adjustment mechanisms  56 ,  58 , and  60  which may be formed as described with reference to  FIG. 1  herein. Persons of ordinary skill in the art will observe that because of the loop construction of the tubes  44   a  and  44   b  and  52   a  and  52   b  enclosing element pairs  42   a  and  42   b  and  50   a  and  50   b , the maximum length of these elements is about twice that of element  46   a  and  46   b.    
     Accordingly, over a first portion of the frequency spectrum within which the antenna shown in  FIG. 2  can be used, only element pairs  42   a  and  42   b  and  50   a  and  50   b  are employed. Over this frequency range, element pair  46   a  and  46   b  is not extended and does not form a part of the antenna. In a typical embodiment of an antenna shown in  FIG. 2 , element pair  42   a  and  42   b  is employed as a driven element and element pair  50   a  and  50   b  is employed as a passive element. In an embodiment where element pair  42   a  and  42   b  and element pair  50   a  and  50   b  are configured as a 2-element yagi antenna operable over a frequency range of about 30 MHz to about 55 MHz, tube pairs  44   a  and  44   b  and  52   a  and  52   b  will have a length of approximately 109″ measured tip to tip. 
     Over a second portion of the frequency spectrum within which the antenna shown in  FIG. 2  can be used, all three of element pairs  42   a  and  42   b ,  46   a  and  46   b , and  50   a  and  50   b  are employed. Over this frequency range, element pairs  42   a  and  42   b ,  46   a  and  46   b , and  50   a  and  50   b  are extended within the looped tubes  44   a - 44   b  and  52   a - 52   b  without significantly entering the looped portion of the tubes. Element pair  46   a  and  46   b  may extended to its longest length. In an embodiment where tube pairs  44   a  and  44   b ,  48   a  and  48   b , and  52   a  and  52   b  have a length of approximately 109″ measured tip to tip, element pair  42   a  and  42   b ,  46   a  and  46   b , and  50   a  and  50   b  may be configured as a 3-element yagi antenna operable over a frequency range of about 55 MHz to about 200 MHz. 
     In the 2-element configuration, element pair  42   a  and  42   b  are used as a driven element and element pair  50   a  and  50   b  are used as a passive element. In the 3-element configuration, element pair  46   a  and  46   b  are used as a driven element and element pairs  42   a  and  42   b  and  50   a  and  50   b  are used as passive elements. RF input relay  62  and shorting relay  64  are associated with element pair  42   a  and  42   b . RF input relay  62  is energized when element pair  42   a  and  42   b  are used as a driven element and shorting relay  64  is energized when element pair  42   a  and  42   b  are used as a passive element. Element pair  46   a  and  46   b  are either retracted and do not form a part of the 2-element antenna, or are used as the driven element in the 3-element configuration. Antenna relay  66  is energized when the antenna  40  is employed in the 2-element configuration to direct incoming RF energy to driven element pair  42   a  and  42   b  and antenna relay  68  is energized when the antenna  40  is employed in the 3-element configuration to direct incoming RF energy to driven element pair  46   a  and  46   b . Element pair  50   a  and  50   b  are always used as a passive element and the brushes associated with the reels in this element pair are always connected together as a short circuit. 
     Some existing EMC testing environments have limited volume. Modifying these environments can be considerably expensive. According to another aspect of the present invention, an EMC testing antenna system can be configured to be accommodated in existing EMC testing environments. Referring now to  FIG. 3 , a diagram shows an end view of an illustrative adjustable EMC antenna  70  according to the present invention. The frontmost element (not shown in this view) is driven by length-adjusting mechanism  72  into opposing tube segments  74   a  and  74   b . The end portions  76   a  and  76   b  of tube segments  74   a  and  74   b , respectively, are angled up (or down) out of the element plane. In a particular embodiment, about 10% of the overall length of the element at the outer end is angled upwards at an angle of about 40°. Using this configuration, the embodiment described above having an overall element length of approximately 109″ measured tip to tip, can be reduced by about 8″ to about 101″. 
     The above-described antenna can be integrated into a novel complete EMC testing system that has not been possible to configure using prior art methods and components. Referring now to  FIG. 4 , a block diagram shows an illustrative adjustable EMC testing system  80  according to the present invention. 
     EMC testing system  80  is built around length-adjustable antenna system  82  including an antenna formed from length-adjustable element  84  employing length-adjustment mechanism  86 , length-adjustable element  88  employing length-adjustment mechanism  90 , and length-adjustable element  92  employing length-adjustment mechanism  94 , mounted on boom  96 . 
     RF amplifier  98 , which may be mounted on boom  96  with the antenna, drives antenna system  82  through SRW measuring device  100 . The output signal from SWR measuring device  100  is provided as a feedback signal to antenna tuning system  102 . Antenna tuning system  102  may be of the type marketed and sold by Fluid Motion, Inc., of Bellevue, Wash. as the SDA  100  Controller including an SWR error signal servo loop routine to drive the length-adjusting mechanisms  86 ,  90 , and  94  to minimize the SWR in the coupling between RF amplifier  98  and antenna system  82 . Such routines are known in the art and may be provided to such controllers as a matter of routine engineering skill. 
     E-field sensor  104  is placed at a desired distance from the antenna system  82  to measure the e-field strength at that location. E-field sensors are known in the art. The output signal from e-field sensor  104  is first conditioned in RF signal conditioning circuit  106  as is known in the art and then sent to control unit  108 . Control unit  108  can use the output signal from e-field sensor  104  in a servo control loop to control the gain of RF amplifier  98  to generate an e-field of a desired magnitude at the location of sensor  104 . The desired level may be set by a user input to user interface  110 . User interface  110  is also coupled to a signal generator  112 , which may be integrated into the system  80  or simply used to drive the RF amplifier of the system  80 . Parameters in addition to desired RF output level of RF amplifier  98  and antenna system  82 , such as frequency, duration, modulation, etc. may be set at user interface  100 . 
     The antenna system and EMC test system of the present invention can generate very high E fields to facilitate susceptibility testing that conforms to new stricter standards which specify that little or no harmonic energy be present. The antenna system and EMC test system of the present invention eliminate the need to employ impractical power levels that are required by presently-utilized antenna/amplifier combinations and eliminate the requirement that the antenna be moved many times at every frequency test point to cover the required illumination of the equipment under test. This eliminates the many additional hours that have been needed to perform the test and using the antenna and amplifier combinations currently known in the art and eliminates the serious problem of emitted RF energy at harmonic frequencies often exceeding emissions at the desired test frequency. 
     Presently-used shielded EMC test rooms cause a multitude of “room effects” that severely affect the input impedance of the antenna resulting in very high standing waves that put a tremendous strain on the amplifier, but more importantly cause the amplifier to greatly increase it&#39;s harmonic output. The present invention including a tunable antenna with an appropriate feedback system as described herein can virtually eliminate this problem while greatly boosting system efficiency. 
     Unlike a log periodic which places the lowest frequency driven and passive elements farthest away from the EUT, the 2-element configuration of the present antenna system keeps the radiator closest to the EUT at the most difficult frequency range of about 30 MHz to about 80 MHz. 
     Radiated testing starts at 30 MHz and at that frequency a very large shielded room is required to accommodate a reasonably efficient antenna such as a half wave dipole, which is 16 feet wide at this frequency. These rooms are not practical for the vast majority of test facilities. The use of an adjustable antenna system of the present invention allows the already shortened element of existing tunable antennas to be altered in a variety of hitherto unachievable ways to shorten the antenna even further. As an example, the bending of the ends of the existing loops upward as disclosed herein shortens the antenna even further at very little expense in performance in either E-field strength or polarization shifting. Retaining pure horizontal/vertical radiation when placed in either mode is an important attribute and the element shapes disclosed here do not degrade these radiation properties. 
     In the reactive near field, the antenna system of the present invention has the unique ability to dynamically change element lengths to alter the E-field resulting in a substantial increase in magnitude in the direction of the EUT. 
     While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.