Patent Publication Number: US-8111205-B1

Title: Mast clamp current probe (MCCP) insertion loss determining methods and systems

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention (Navy Case No. 099086) is funded by the United States Department of the Navy. Licensing inquiries may be directed to the Office of Research and Technical Applications, Space and Naval Warfare Systems Center, San Diego, Code 72120, San Diego, Calif., 92152; voice 619-553-2778; email T2@spawar.navy.mil. 
    
    
     BACKGROUND 
     1. Field 
     This disclosure relates to communication systems. More particularly, this disclosure relates to determining signal insertion loss for antenna systems. 
     2. Background 
     A Mast Clamp Current Probe (MCCP) is a device operable to couple various communication systems to various ship structures, such as a ship&#39;s mast, in order to transform such structures into an operable antenna. Mast Clamp Current Probes have been successfully demonstrated to produce broadband receive antennas using available shipboard structures, such as stub masts. The receive MCCP is robust, low maintenance, and affordable. 
     However, the transmit MCCP is currently under development. A key design consideration for the transmit MCCP is the insertion loss, and a number of attempts have been made to quantize the loss using empirical and numerical techniques, with mixed success. 
     The empirical approach is to infer system efficiency by measuring the radiated field at a distance and comparing the test antenna to a standard. Such measurements are typically made in the open environment and must be performed very carefully to achieve a modicum of accuracy—and even then test results are often subject to various interpretations. 
     Numerical techniques involve the art of developing a model for the MCCP core and principal surroundings in sufficient detail to predict antenna performance. Numerical techniques have provided much needed insight into the design process, but ultimately the results rest on the accurate measurement of material characteristics. As the MCCP materials are typically anisotropic and frequency dependent, any measurement of these properties is an art form in itself. Thus, new approaches to determining MCCP insertion loss are desirable. 
     SUMMARY 
     The foregoing needs are met, to a great extent, by the present disclosure, wherein systems and methods are provided that in some embodiments provide for a broadband antenna composed of open wires disposed over a ship&#39;s mast. 
     In various embodiments, a method for determining insertion loss for a mast clamp current probe (MCCP) coupled to a monopole antenna includes determining a first power radiated by the monopole antenna across a first range of frequencies while driving the monopole antenna using a base-feed arrangement to produce a first power-frequency measurement, determining a second power radiated by the monopole antenna across the first range of frequencies while driving the monopole antenna using an MCCP-feed arrangement to produce a second power-frequency measurement and to determine impedance mismatch (MM), and determining insertion loss using the first power-frequency measurement, the second power-frequency measurement and the impedance mismatch. 
     In various other embodiments, a system for determining insertion loss for a mast clamp current probe (MCCP) coupled to a monopole antenna includes a first testing means for determining a first power radiated by the monopole antenna across a first range of frequencies while driving the monopole antenna using a base-feed arrangement to produce a first power-frequency measurement, a second testing means for determining a second power radiated by the monopole antenna across the first range of frequencies while driving the monopole antenna using an MCCP-feed arrangement to produce a second power-frequency measurement and to determine impedance mismatch (MM), and computer equipment operable to determine insertion loss using the first power-frequency measurement, the second power-frequency measurement and the impedance mismatch. 
     In various other embodiments, a system for determining insertion loss for a mast clamp current probe (MCCP) coupled to a monopole antenna includes a base-feed test set that includes a signal generator and a current measuring device, wherein the signal generator is operable to drive the monopole antenna across a first range of frequencies using a base-feed arrangement while the current measuring device is operable to measure a first current of the monopole antenna, wherein a first power-frequency measurement may be determined using the first measured current, an MCCP-feed test set that also includes a signal generator and a current measuring device, wherein the signal generator is operable to drive the monopole antenna across a first range of frequencies via the MCCP while the current measuring device is operable to measure a second current of the monopole antenna, wherein a second power-frequency measurement may be determined using the second measured current, and wherein the MCCP-feed test set is operable to determine impedance mismatch (MM), and computer equipment operable to determine insertion loss using the first measured current, the second measured current and the impedance mismatch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a seagoing craft with an exemplary mast. 
         FIGS. 2 and 3  depict two separate testing arrangements for evaluating MCCP insertion loss for the exemplary mast of  FIG. 1 . 
         FIGS. 4A-4C  depict details of an exemplary MCCP. 
         FIG. 5  depicts the characteristics of an exemplary MCCP mounted  5 ′ above the base on the  35 ′ whip monopole antenna. 
         FIG. 6  is a flowchart outlining exemplary operations for evaluating insertion loss of a mast antenna. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed methods and systems below may be described generally, as well as in terms of specific examples and/or specific embodiments. For instances where references are made to detailed examples and/or embodiments, it should be appreciated that any of the underlying principles described are not to be limited to a single embodiment, but may be expanded for use with any of the other methods and systems described herein as will be understood by one of ordinary skill in the art unless otherwise stated specifically. 
     A Mast Clamp Current Probe (MCCP) is a device operable to couple various communication systems to various ship structures, such as a ship&#39;s mast, in order to transform such structures into an operable antenna. However, transmit MCCPs must be carefully characterized to be a useful part of a ship&#39;s communication systems. 
       FIG. 1  is a diagram illustrating a ship  100  having a hull  110 , a superstructure  120  and mast  140 . In the present example, an MCCP (not shown in  FIG. 1 ) may be used to convert the mast  140  to act as a monopole antenna for communication system  130 , which may include one or both of transmit and receive capabilities. 
       FIGS. 2 and 3  depict two separate testing arrangements useful for evaluating MCCP insertion loss for the exemplary mast/monopole antenna  140  of  FIG. 1 . For both configurations of  FIG. 2  and  FIG. 3 , the geometric configuration of the mast/monopole antenna  140  and the MCCP  220  may be identical. Similarly, the electromagnetic coupling between MCCP and mast/monopole antenna  140  may be the same in both cases. 
     For the present example, the mast/monopole antenna  140  may range in height from about 20 to 90 feet long, and is secured to superstructure  120  at base  210 . An MCCP  220  having a feed point terminal  222  may be placed a short distance, e.g., five feet, above the base  210  to accommodate connection of a radio frequency (RF) ammeter  230  between the base  210  and the MCCP  220 . In one embodiment, the MCCP  220  may be placed at the lowest point permissible. 
     Continuing, the length of the mast/monopole antenna  140  (hereinafter just “monopole antenna”) may be selected for convenience such that, over a particular swept frequency range, the maximum current on the monopole antenna  140  will occur at the base  210 . Hence, in various embodiments, the monopole antenna  140  may be limited to not much longer than a half wavelength at the highest frequency of interest, and long enough at the lowest frequency of interest to provide an adequate signal-to-noise ratio (SNR). 
     The exemplary test equipment used in the present example includes a wideband current sampling Pearson probe and voltmeter to function as the RF ammeter  230 , various (optional) resistive pads (or attenuators)  240 , a signal generator  250  capable of producing a swept frequency, computer-based equipment  260 , a network analyzer  270  and an assortment of cables and connectors. 
     For the configuration of  FIG. 2 , the radiated power of the monopole antenna  140  is determined when the monopole is driven from the signal generator  250  in what may be thought of as the conventional “base-fed” method. Thus, the configuration of  FIG. 2  will be hereafter referred to as a “base-feed” or “base-fed” configuration. 
     For the configuration of  FIG. 3 , however, the power radiated by the monopole antenna  140  is determined when the monopole is driven from the signal generator through the MCCP. Thus, the configuration of  FIG. 3  will be hereafter referred to as a “MCCP-feed” or “MCCP-fed” configuration. 
     Referring back to  FIG. 2 , the base-feed testing configuration is shown with signal generator  250  coupled to the monopole antenna  140  at a point between the base  210  and the MCCP  220  via a first cable with the RF ammeter  230  and resistive pads  240  placed there between in series. This configuration may be used to measure and store current I 1  delivered to the monopole antenna  140  by the signal generator  250 . 
     The exemplary monopole antenna  140  dimensions for the present example have been appropriately selected for the frequency range of interest as per the above guidance. The pad  240  is optional and is only suggested as a means to isolate the signal generator  250  from the inherent load of the monopole antenna  140 . A constant power output by the signal generator  250  is desirable. The measurement of current I 1  can be done with the input MCCP-feed point, or primary, terminal  222  left open (unconnected to anything) and a secondary terminal (not shown in  FIG. 2 ) of the MCCP  220  short-circuited. By way of example, one way of short-circuiting the secondary terminal would be to use electrical conductive tape to span the space gap  434  (described below in association with  FIG. 4B ). In this configuration, no current will flow through the MCCP  220  during testing. 
     In operation, the signal generator  250  may be made to sweep across a first frequency range of interest, e.g., 2 MHz to 30 MHz. As power is fed to the monopole antenna  140 , the RF ammeter  230  can measure current I 1 , and feed its current measurement signals to the computer-based equipment  260 . In turn, the computer-based equipment  260  can receive the current measurement signals, as well as the signals produced by the signal generator  250  (for reference). Note that the power radiated by monopole antenna  140  is proportional to the square of current I 1 . 
     Continuing to  FIG. 3 , which depicts the MCCP-feed configuration, the test equipment may be re-arranged such that the signal generator  250  and pad  240  are removed from the RF ammeter  230  such that the RF ammeter  230  is connected close to the base of the monopole antenna  140  on one side and shorted to the superstructure  120  (or whatever other structure may qualify as ground) on the other side. The pad  240  may be reconfigured to be in series between the signal generator  250  and the feed terminal  222  so as to allow the signal generator  250  to provide a signal to the MCCP  220 . While not shown in  FIG. 3 , the second terminal of MCCP  220  is open-circuit/isolated with respect to the monopole antenna  140 . 
     In operation, the signal generator  250  again may sweep across the first frequency range. During the sweep, as power is fed to the MCCP  220 , the RF ammeter  230  can measure current I 2 , and feed its current measurement signals to the computer-based equipment  260 . In turn, the computer-based equipment  260  can receive the current measurement signals, as well as the signals produced by the signal generator  250 . Again, the power radiated by monopole antenna  140  is proportional to the square of current I 2 . Also, in this configuration the network analyzer  270  may be used to measure impedance. The impedance measurement may then be used to determine impedance mismatch (MM). 
     The difference between the current measurements for the base-feed and MCCP-feed configurations (=20 log I 1 −20 log I 2 ) represents the total system loss when the monopole antenna  140  is driven through the MCCP  220 . Using the total system loss and the impedance mismatch, the insertion loss may be calculated as =20 log I 1 −20 log I 2 −MM. 
       FIGS. 4A and 4B  show multiple views of one embodiment of the MCCP  220 .  FIG. 4A  shows a horizontal cross-section exposing the relationship of the ferrite core  416  and its primary winding  420  to a housing  418  and a feed connector  222 .  FIG. 4B  shows a vertical cross-section of one half of the MCCP  220 . In  FIG. 4B , the ferrite core  416  can be split lengthwise into two halves. 
     The embodiment of the MCCP  220  shown in  FIGS. 4A and 4B  may be clamped around a mast  140  (or other similar structure usable as an antenna), with  FIGS. 4A and 4C  showing features that allow such embodiments to be so clamped. A hinge  424  allows this embodiment of the MCCP  220  to be hinged open and positioned around the mast  140 . In this embodiment, a releasable latch  426  allows the two core halves to be latched together.  FIG. 4C  shows an embodiment of the MCCP  220  in an open position. Although  FIGS. 4A-4C  show the MCCP  220  as configured to be clamped around a pole-like/mast structure, it is to be understood that the manner of mounting the MCCP  220  is not limited to clamping, but any effective manner of positioning of the MCCP  220  may be used. 
     Returning to  FIG. 4A , the ferrite core  416  and primary winding  420  are contained within the housing  418 . The ferrite core  416  may be comprised of any suitable magnetic material with a high resistivity. The primary winding  420  may be wound around the ferrite core  416  for a plurality of turns. The number of turns of the primary winding  420  and the ferrite core  416  materials will provide different inductive and resistive characteristics, affecting the frequency response and thus the insertion loss of the device. The primary winding  420  may consist of a single turn around the ferrite core  416  or several turns around the ferrite core  416 . The primary winding  420  may cover only one half of the ferrite core  416 , or may extend around both core halves. The primary winding  420  may be terminated with a connection to the housing  418  as a ground, or it can be terminated in a balanced to unbalanced transformer (typically referred to as a BALUN) as described below. For transmitting, an RF signal can be coupled into the MCCP  220  through the feed connector  222 . Examples of usable feed connectors include, but are not limited to: BNC (bayonet Neill-Concelman), SMA (SubMiniature version A), TNC (threaded Neill-Concelman), and N-style coaxial connectors. If a coaxial connector is used, the shield  428  portion of the connector  222  can be coupled to the housing  418 , while the inside conductor  430  of the connector  222  is coupled to the primary winding  420 . The primary winding  420  is terminated with a connection to the housing  418 . The primary winding  420  and ferrite core  416  may be insulated from the housing  418  by an electrical insulating layer  432 . The insulating layer  432  may comprise any suitable electrical insulating materials. The core halves of the ferrite core  416  are generally in contact with each other when the MCCP  220  is closed, but, in some instances, an intentional air gap may separate the core halves. However, even when the core halves are in contact with each other, a minute air gap may still exist even though the core faces may be polished to a very smooth finish and pressed tightly against one another. This air gap may result in air gap losses. The so-called air gap loss does not occur in the air gap itself, but is caused by the magnetic flux fringing around the gap and reentering the core in a direction of high loss. As the air gap increases, the fringing flux continues to increase, and some of the fringing flux strikes the core perpendicular to the core, and sets up eddy currents. Core materials with high resistivity may reduce these currents. 
       FIG. 4B  shows a space gap  434  within the interior portion of the housing  418 . This space gap  434  may be used to prevent forming a shorted tertiary turn around the primary winding  420 . If no space gap  434  were present, the shorted turn of the shield  428  would prevent the MCCP  220  from coupling RF current to and from the mast  140 . Note that this shorted (or open) tertiary turn of shield  428  may act as the “second terminal” mentioned above with respect to  FIGS. 2 and 3 . 
     For transmitting, current flow in the primary winding  420  can induce a magnetic field with closed flux lines substantially parallel to the ferrite core  416 . This magnetic field can then induce current flow in the mast  140  clamped within the MCCP  220 , which results in RF energy radiation. A transmission line transformer may be used to couple the RF energy from a transmitter to the MCCP  220 . If the primary winding  420  is terminated to the housing  418 , an unbalanced to unbalanced (UNUN) transmission line transformer may be used to couple RF energy to the input end of the primary winding  420  of the MCCP  220 . A balanced to unbalanced transformer (BALUN) may alternatively be used to couple RF energy to the MCCP  220 . In this configuration, the primary winding  420  may not be terminated at the housing  418 . Instead, both the input end and the termination of the primary winding  420  may be connected to the balanced terminals of a BALUN. The unbalanced ends of the BALUN may be connected to a coaxial cable carrying the RF energy from a transmitter. A BALUN may also be used if the RF current injector has no external shield connected to ground. Both BALUNs and UNUNs are well known in the art and are commercially available. However, specially made UNUNs may possibly be required to properly match a transmitter output to the input of the MCCP  220 . 
       FIG. 5  is a bode plot  500  depicting the loss characteristics of an exemplary MCCP mounted five feet above the base on a  35  foot whip monopole antenna. As shown in  FIG. 5 , the bode plot  500  includes three lines including a mismatch (MM) loss curve  510 , an insertion loss curve  512  and a total loss curve  514 . All of the curves  510 - 514  were generated using the approach described above with respect to  FIGS. 2 and 3 . 
       FIG. 6  is a flowchart outlining exemplary operations for evaluating insertion loss of a monopole antenna. The process starts in step  602  where the monopole antenna is outfitted with an MCCP and configured with a test set according to the example of  FIG. 2 , i.e., a base-feed configuration. Next, in step  604 , a signal generator may be made to base-feed the monopole antenna over a first range of frequencies, e.g., 2 MHz to 30 MHz. Then, in step  606 , an RF ammeter at the base of the monopole antenna may be used to measure current I 1 , which again is proportional to the power radiated by the monopole antenna. The measured current I 1  may then be provided to a computer and stored. Control continues to step  612 . 
     In step  612 , the monopole antenna and MCCP can be reconfigured with a test set according to the example of  FIG. 3 , i.e., an MCCP-feed configuration. Next, in step  614 , a signal generator may be made to base-feed the monopole antenna over the first range of frequencies. Then, in step  616 , the RF ammeter at the base of the monopole antenna may be used to measure current I 2 , which again is proportional to the power radiated by the monopole antenna. Control continues to step  620 . 
     In step  620 , a network analyzer may be used to measure input impedance and determine impedance mismatch, and in step  622  total loss and insertion loss may be determined noting that the total loss and insertion loss may be determined using computing hardware. Control then continues to step  650  where the process stops noting that it should be appreciated that steps  602 - 622  may be repeated for a variety of configurations, e.g., where the MCCP is coupled at different points to the monopole antenna and/or where the monopole antenna is modified (by lengthening or shortening) or by extending a wire from the top of the monopole antenna. 
     What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments. It will, therefore, be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.