Structure characteristic impedance estimator using current probe

A method for estimating the characteristic impedance of a structure comprising the following steps: providing a current probe comprising a magnetic core having an aperture therein and a primary winding wrapped around the core; measuring, with a calibrated vector network analyzer (VNA), the impedance (Zop) of the current probe while in an open configuration wherein nothing but air occupies the aperture and the current probe is isolated from a ground; measuring, with the VNA, the impedance (Zsh) of the current probe while in a short configuration, wherein the current probe is electrically shorted; measuring, with the VNA, the impedance (Zin) of the current probe while the current probe is mounted to the structure such that the structure extends through the aperture; and calculating an estimated characteristic impedance (Z′mast) of the structure according to the following equation: Z′mast=(Zin−Zsh)(Zop−Zsh)/(Zop−Zin).

BACKGROUND OF THE INVENTION

When a specially-designed current probe is clamped to a conductive structure, such as a ship's mast, the combination of the current probe and mast can function as an antenna (hereinafter referred to as a mast clamp current probe (MCCP) antenna). Current probes used in MCCP antenna applications must be specifically designed for the platform on which the current probe will be used. In order to properly match the current probe to the structure, the current probe designer needs to know the characteristic impedance of the structure. The “characteristic impedance” is the impedance of the structure that the probe will see. Previously the characteristic impedance of the structure was determined by feeding a brass, scale model of the structure as if it were an antenna.

SUMMARY

Disclosed herein is a method for estimating the characteristic impedance of a structure comprising the following steps: providing a current probe comprising a magnetic core having an aperture therein and a primary winding wrapped around the core; measuring, with a calibrated vector network analyzer (VNA), the impedance (Zop) of the current probe while in an open configuration wherein nothing but air occupies the aperture and the current probe is isolated from a ground; measuring, with the VNA, the impedance (Zsh) of the current probe while in a short configuration, wherein the current probe is electrically shorted; measuring, with the VNA, the impedance (Zin) of the current probe while the current probe is mounted to the structure such that the structure extends through the aperture; and calculating an estimated characteristic impedance (Z′mast) of the structure according to the following equation: Z′mast=(Zin−Zsh) (Zop−Zsh)/(Zop−Zin).

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1is a flowchart of a method10for estimating the characteristic impedance of a conductive structure11by using a current probe12, without having to build a scale model of the structure. The elements and configurations mentioned in method10are described below and depicted inFIGS. 2a,3a, and4a. In a mast clamp current probe (MCCP) antenna, the impedance that the current probe12will “see” varies depending on where the current probe12is mounted on the structure11. Different locations on the structure11have different impedance properties. The impedance estimation method10may be used to identify the best locations on structure11to place the current probe12to provide desired antenna performance.

Step10aof the impedance estimation method10calls for providing the current probe12. As shown inFIG. 6aand described below, the current probe12comprises a magnetic core14having an aperture16therein and a primary winding18wrapped around the core14. Step10bprovides for measuring, with a calibrated vector network analyzer (VNA)20, the impedance (Zop) of the current probe12while in an open configuration. The VNA20may be any vector network analyzer or performance network analyzer (PNA). For example, a suitable VNA20is a model 8753ES, 2-port VNA made by Agilent Technologies. In the open configuration, depicted inFIG. 2a, nothing but air occupies the aperture16and the current probe12is isolated from a ground source22.FIG. 2bshows an example Smith chart of the measurement conducted in step10b. In step10cthe impedance (Zsh) of the current probe12is measured with the VNA20while in a short configuration. In the short configuration, depicted inFIG. 3a, the current probe12is electrically shorted.FIG. 3bshows an example Smith chart of the measurement conducted in step10c. In the embodiment shown inFIG. 3a, the current probe12is shorted by covering the inner walls of the aperture16with a conductive tape17. Any conductive tape can be used as the conductive tape17. A suitable, non-limiting example the conductive tape17is copper tape. The current probe12may also be shorted by clamping the current probe12around a shorted measurement fixture or by wrapping a metallic wire around a conductive housing24(shown inFIG. 6a) in a closed-loop configuration. Step10dprovides for measuring, with the VNA20, the impedance (Zin) of the current probe12while the current probe12is mounted to the structure11at a location A.FIG. 4adepicts an example of a mounted configuration that may be used in step10d. In the mounted configuration the current probe12is mounted to the structure11such that the structure11extends through the aperture16. In the embodiment shown inFIG. 4a, the current probe12is mounted to the structure11at location A, which is positioned a distance L from the base of the structure11. It is to be understood that the distance L can be any desired distance.FIG. 4bshows an example Smith chart of the measurement conducted in step10d. Step10eprovides for calculating an estimated characteristic impedance (Z′mast) of the structure11at location A according to the following equation: Z′mast=(Zin−Zsh) (Zop−Zsh)/(Zop−Zin).

FIG. 5shows a simplified Thevenin equivalent circuit model21for an MCCP antenna. The circuit model21illustrates how the equation utilized in step10eabove was derived. The completion of steps10a-10cof the impedance estimation method10with a given current probe12results in the characterization of that particular current probe12. A given current probe12need only be characterized once and then that current probe12can be used to estimate the characteristic impedance of different structures11and different locations on a single structure11without having to repeat steps10a-10c. By comparing calculated Z′mastvalues from multiple locations on the structure11, one can identify a location on the structure11that has substantially optimal impedance for radio frequency resonance.

FIG. 6ashows a horizontal cross-sectional view of a current probe12exposing the relationship of the magnetic core14and its primary winding18. The current probe12may also comprise a housing24and a feed connector26.FIGS. 6aand6bshow the features that allow the shown embodiment of the current probe12to be clamped around the structure11. A hinge28allows the depicted embodiment of the current probe12to be hinged open and positioned around the conductive structure11. In this embodiment, a releasable latch30allows the two core halves to be latched together.

Also shown in the embodiment of the current probe12depicted inFIG. 6a, the magnetic core14and primary winding18are contained within the housing24. The magnetic core14may be comprised of any suitable magnetic material with a high resistivity. The primary winding18may be wound around the magnetic core14for any number of desired turns. The number of turns of the primary winding18and the magnetic core14materials will provide different inductive and resistive characteristics, affecting the frequency response and thus the insertion loss of the current probe12. The primary winding18may consist of a single turn around the magnetic core14or several turns around the magnetic core14. The primary winding18may cover only one half of the magnetic core14, or may extend around both core halves. The primary winding18may be terminated with a connection to the housing24as a ground, or it can be terminated in a balanced to unbalanced transformer (typically referred to as a BALUN). A radio frequency (RF) signal may be coupled into the current probe12through the feed connector26. Examples of the feed connectors26include, 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 shield32portion of the connector26may be coupled to the housing24, while the inside conductor34of the connector26is coupled to the primary winding18. The primary winding18and magnetic core14may be insulated from the housing24by an electrical insulating layer36. The insulating layer36may comprise any suitable electrical insulating materials. The core halves of the magnetic core14are generally in contact with each other when the current probe12is closed. AlthoughFIGS. 6aand6bshow the current probe12as configured to clamp around the conductive structure11, it is to be understood that the manner of mounting the current probe12to the conductive structure11is not limited to clamping, but any effective manner of mounting the current probe12to the conductive structure11may be used. The current probe12may be any desired size and shape.

FIG. 7ais an illustration of a basic transformer38. An MCCP antenna can be compared to the basic transformer38. The transformer38comprises a primary winding18, a secondary winding40, and a magnetic core14. The transformer38works on Faraday's law of induction principle of sharing common magnetic flux in the magnetic core14to transfer the electric energy from one winding to the other.FIG. 7bshows the similarities between a transformer38and a simple depiction of an MCCP antenna41. Like the transformer38, the current probe12also has a magnetic core14and a primary winding18. The electromagnetism principle of the MCCP antenna is that a varying magnetic field generated in the magnetic core14induces a current I into the conductive structure11, which forms a secondary winding40. The primary winding18of the current probe12comprises a coil of wire wrapped around the magnetic core14. The conductive structure11may be any type of conductive structure that a portion of which will fit inside the aperture16of the current probe12. The structure11may be closed loop or open-ended. For example, in the embodiment depicted inFIG. 7b, the structure11has a grounded end and an open end.

Faraday's law of induction helps explain the functioning of an MCCP antenna. Faraday's law of induction formalized the interrelationship between electromotive force (EMF) or “voltage” and magnetic flux in the following equation:

E=EMF in volts

ΦB=magnetic flux in webers

The MCCP antenna is an electromagnetic energy transducer much like the transformer38. In both cases, electrical energy fed into the primary winding18is magnetically coupled into the magnetic core14, which in turns couples the energy into the secondary winding40. In the case of the transformer38, the secondary winding40delivers this energy to a load. But in the case of the MCCP antenna, the conductive structure11(i.e. the secondary winding) radiates this energy into space.

FIG. 8is a circuit diagram of a transformer model. The transformer has NPturns in the primary winding, NSin the secondary. The reference characters shown inFIG. 8may be defined as follows:

RC=Core loss equivalent resistance

IP=Primary winding current

IS=Secondary winding current

EP=Electromotive force of the primary winding

ES=Electromotive force of the secondary winding

IC=Current into RC

IM=Magnetizing current into XM

I0=No load current

In the electrical model depicted inFIG. 8, the secondary winding resistance RSand the secondary leakage reactance XShave been reflected to the primary circuit by the turns ratio squared. For the MCCP antenna, the turns ratio is approximately unity (because both primary and secondary windings have about one turn of coil, i.e. NP˜NS). When the current probe12encloses an open-ended structure11, the magnetic energy couples into the structure11; thus forming a half-turn coil pathway for the RF energy.

FIG. 9ashows another example embodiment of the structure11. In this case, the structure11is a pickup truck and the current probe12is clamped around the tail pipe42. The characteristic impedance at different locations on the pickup truck can be determined with method10.FIG. 9bis a Smith Chart showing the resulting cascade impedance of the current probe12clamped around the tail pipe42.FIG. 9cis a Smith Chart showing the resulting characteristic impedance of the tail pipe42.FIGS. 10aand10bshow different example embodiments of the structure11. InFIG. 10a, the current probe12is shown clamped to the superstructure44of a ship46. InFIG. 10b, the current probe12is depicted as clamped to a vehicle chassis48.

While the current probe12is in the open configuration, S-parameters of the current probe12may be measured by the VNA20, and the open configuration impedance (Zop) of the current probe12may be calculated. While the current probe12is in the short configuration, S-parameters of the current probe12may be measured by the VNA20, and the short configuration impedance (Zsh) of the current probe12may be calculated. While the current probe12is in the mounted configuration, S-parameters of the current probe12may be measured by the VNA20, and the input impedance (Zop) of the current probe12may be calculated.

From the above description of the impedance estimation method10, it is manifest that various techniques may be used for implementing the concepts of method10without departing from its scope. The described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that method10is not limited to the particular embodiments described herein, but is capable of many embodiments without departing from the scope of the claims.