Method and apparatus to quantify mast clamp current probe effective loss on pole-mast antenna

A method for quantifying the effective loss of a mast-clamp-current-probe (MCCP) antenna comprising the following steps: providing a vector network analyzer (VNA) having first and second ports (Ports 1 and 2 respectively); coupling a transmit monopole antenna to Port 1 via a first transmission line; coupling a receive antenna to Port 2 via a second transmission line; determining the S-parameters of the two coupled antennas; calculating the power at the receive antenna (Pr); converting the transmit antenna to the MCCP antenna by shorting the transmit antenna to ground and clamping a current probe around the transmit antenna; determining the S′-parameters of the MCCP antenna and the receive antenna; calculating the power at the receive antenna (Pr′) when coupled to the MCCP antenna; and quantifying the effective loss of the MCCP antenna as the difference Pr−Pr′.

BACKGROUND OF THE INVENTION

Mast Clamp Current Probe (MCCP) technology offers a means to eliminate some of the traditional HF antennas in locations where available real estate for antenna placement is limited, such as building rooftops and ships. In the case of ships, MCCP antennas can utilize parts of the shipboard structure as the antenna element for receive and transmit functions. There exists a need for a method for quantifying the attenuation, or effective loss, of a MCCP antenna.

SUMMARY

Disclosed herein is a method for quantifying the effective loss of a mast-clamp-current-probe (MCCP) antenna comprising the following steps: providing a vector network analyzer (VNA) having first and second ports (Ports 1 and 2 respectively); coupling a transmit monopole antenna to Port 1 via a first transmission line; coupling a receive antenna to Port 2 via a second transmission line; determining the S-parameters of the two coupled antennas; calculating the power at the receive antenna (Pr); converting the transmit antenna to the MCCP antenna by shorting the transmit antenna to ground and clamping a current probe around the transmit antenna; determining the S′-parameters of the MCCP antenna and the receive antenna; calculating the power at the receive antenna (Pr′) when coupled to the MCCP antenna; and quantifying the effective loss of the MCCP antenna as the difference Pr−Pr.

DETAILED DESCRIPTION OF EMBODIMENTS

FIGS. 1aand1bare block diagrams showing the components needed to conduct a method10(as depicted inFIG. 2) for quantifying the effective loss of a Mast Clamp Current Probe (MCCP) antenna.FIG. 2is a flowchart which shows the individual steps of method10. The first step of method10, step10a, calls for providing a vector network analyzer (VNA)12having a first port14and a second port16. Step10bprovides for coupling a transmit monopole antenna18to the first port14via a first transmission line20. Step10cprovides for coupling a receive antenna22to the second port16via a second transmission line24. Step10dprovides for determining the S-parameters of the two coupled antennas18and22. The next step, step10e, provides for calculating the power at the receive antenna (Pr). Step10fprovides for shorting the transmit antenna18to ground and clamping a current probe26around the transmit antenna18—thus converting the transmit antenna18into an MCCP antenna28. Then, the S′-parameters of the two coupled antennas28and22is determined in step10g. Step10hprovides for calculating the power (Pr′) at the receive antenna22when coupled to the MCCP antenna28. Step10iprovides for quantifying the effective loss of the MCCP antenna28as the difference Pr−Pr′.

The VNA12may be any vector network analyzer or performance network analyzer (PNA) that has at least two ports and is capable of measuring both the amplitude and phase properties of a device under test. For example, a suitable VNA12is a model 8753ES, 2-port VNA made by Agilent Technologies. Monopole and dipole antenna designs may be used for the receive antenna22.

Any desired frequency may be used with method10, depending on the VNA/PNA frequencies range and the MCCP antenna28desired to be tested. Any size of current probe26can be used as long as the inside diameter aperture of the current probe26is large enough to accommodate the pole mast29of the transmit monopole antenna18. The size of current probe26depends on the outer diameter of the pole mast29under test. The impedance of the MCCP antenna28is a function of the current probe26attachment point. The current probe26may be clamped near the base of the transmit monopole antenna18pole mast29to create a monopole MCCP antenna28. For example, in one embodiment, the current probe26may be clamped approximately 15.25 cm (6 in) from the base of the transmit monopole antenna18pole mast29. Current probes26having more that one port may also be used with method10—in such cases, the unused port(s) may be terminated with a 50 ohm load.

A suitable example of the first and second transmission lines20and24is shielded co-axial cable. The length of the first and second transmission lines20and24may be any desired length. For example, the first and second transmission lines20and24may both be approximately 18.3 meters (60 ft) in length. The length of first and second transmission lines20and24depend on whether the analyzer being used in the test can calibrate a particular cable length. Cables can be of different lengths so long as they are calibrated with the analyzer prior to the S-parameter test.

The MCCP antenna28is essentially a specially designed wideband RF current transformer that inductively or magnetically couples energy from a transmitter output signal voltage to a structure (e.g. the superstructure of a ship) for antenna communication.FIG. 3ais an illustration of a basic transformer30. The transformer30comprises a primary winding32, a secondary winding34, and a magnetic core36. The transformer30works on Faraday's law of induction principle of sharing common magnetic flux in the core36to transfer the electric energy from one winding to the other.FIG. 3bis a simple illustration of an MCCP antenna28. The current probe26also has a magnetic core36and a primary winding32. The electromagnetism principle of the MCCP antenna28is that a varying magnetic field generated in the magnetic core36induces a current I into an open-ended conductive structure38, which acts as the antenna element. The conductive structure38may be an existing antenna pole, such as the pole mast29, the superstructure of a ship, or any other desired conductor that fits inside the center aperture of the current probe26. The primary winding32of the current probe26has a coil of wire wrapped around the magnetic core36. The close-ended or grounded-end superstructure38passing through the magnetic core36forms a coil of wire for the secondary winding. Maximum coupling energy transfer is obtained with a high permeability core minimizing flux leakage and core loss. Permeability is a measure of how well the current probe core material conducts or guides a magnetic field.

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 antenna28is an electromagnetic energy transducer much like the transformer30. In both cases, electrical energy fed into the primary winding32is magnetically coupled into the core36, which in turns couples the energy into the secondary winding34. In the case of the transformer30, the secondary winding34delivers this energy to a load. But in the MCCP antenna28, the conductive structure38(i.e. the secondary winding) radiates this energy into space. Operationally, the quantities of interest of the MCCP pole-mast antenna28are bandwidth, attenuation, pole-mast impedance, and radiation efficiency. Bandwidth specifies the operational frequency range of the MCCP antenna28. This range is directly related to the reflection (input VSWR) of the MCCP antenna28. Attenuation specifies the signal loss as it propagates through the MCCP antenna28.

FIG. 4is a circuit diagram of a transformer model. The first port14may be thought of as the primary winding32of the current probe26. In which case, the transformed normalized input feed-point impedance is then the internal impedance of the current probe26together with the reflected transmit monopole antenna18mast pole impedance from the secondary winding34. The physical quantity being transformed in this case is suggested by the transformer model shown inFIG. 4. The transformer has NPturns in the primary winding, NSin the secondary. The reference characters shown inFIG. 4may 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 above electrical model, the secondary winding resistance RSand the secondary leakage reactance XShave been reflected to the primary circuit by the turns ratio squared. For the MCCP antenna28, 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 probe26encloses an open-ended structure, the magnetic energy couples into the structure; thus forming a half-turn coil pathway for the RF energy.

Direct measurement of the effective loss of the current probe26on the transmit monopole antenna18can be deduced from the measurable scattering parameters or S-parameters using two antenna coupling techniques. S-parameters describe the signal voltage flow in a terminated network. To measure the S-parameters for the two antenna systems, the VNA12is used. The S-parameters produced by the VNA12have the following meaning:

S11=Reflected signal ratio at the first port

S12=Transmitted signal ratio from the second port to the first port

S21=Transmitted signal ratio from the first port to the second port

S22=Reflected signal ratio at the second port

S11=the ratio of a reflected signal at the first port to an incident signal at the first port,

S12=the ratio of a measured signal at the first port to an incident signal at the second port,

S21=the ratio of a measured signal at the second port to the incident signal at first port,

S22=the ratio of a reflected signal at the second port to the incident signal at the second port;

The total system power gain or attenuation can be related to the S-parameters. To uncover the effective loss due to the current probe26, a two antennas coupling reference system, such as is depicted inFIGS. 5aand5b, may be used.

FIG. 5ais a block diagram representing a reference system40which comprises the transmit monopole antenna18coupled to the receive antenna22via the VNA12. The mismatch loss Mtof the transmit monopole antenna18is the reflected signal ratio at the first port14, (i.e. S11). The mismatch loss Mrof the receive antenna22is the reflected signal ratio at the second port16(i.e. S22). In the reference system40, the power at the receiver (Pr) is given by:
Pr=Pt−Mt+(Gt−S+Gr)−Mr
where

Gt=Transmit antenna gain

Gr=Receive antenna gain

FIG. 5bis a block diagram representing a test system42which differs from the reference system40in that the current probe26is attached to the transmit monopole antenna18pole mast29, thus creating an MCCP antenna28. When the transmit MCCP antenna28is inserted into the transmission loop, the power at the receiver (Pr′) is given by:
Pr′=Pt−(Mc+Ct)−Mt′(Gt′−S+Gr)−Mr
where

Mc=Mismatch loss (reflection loss) of the current probe core=s11′

Mt′=MCCP antenna mismatch loss

Gt′=MCCP antenna gain

When using an identical transmit antenna, it is expected that: Gr==Gt′. However, in reality, the grounded base pole-mast and insulated base pole-mast antenna have slight differences in self-impedances and the insertion of the current probe26at the grounded base of the open-ended structure38further affects the pole-mast self impedance. If this compatibility is not satisfied, a matching network may be used. For similar current probe26and VNA12impedances, the following assumption is used: Mt≅Mt′.

The attenuation of the MCCP antenna28is estimated by evaluating the change in the signal at the receive antenna22between the reference system40and the test system42. Any changes in receive power is attributed to insertion of the current probe26; so for similar transmit powers, the MCCP effective loss should be given by:
Pr−Pr′=Mc+Ct

The following scenario is offered as a non-limiting example of the use of method10. In the following antenna comparisons, three requirements will be used: the structure pole-mast antenna feed-impedance, the reflection loss, and the transmission loss of the MCCP antenna28. To facilitate the loss estimation, the following example scenario is defined:(1) In this embodiment, the reference system40, as depicted inFIG. 6a, is comprised of an approximately 5.5 m (18 ft) traditional monopole whip antenna43as the receive antenna22, and an approximately 5.5 m (18 ft) simulated stub-mast mesh-pole44as the transmit monopole antenna18. Both the whip antenna43and the simulated stub-mast mesh-pole antenna44are insulated from the ground by insulators46.(2) The test system42, as depicted in the embodiment shown inFIG. 6b, differs from the reference system40ofFIG. 6ain that a current probe26is clamped around the base of the simulated stub-mast mesh-pole44, which is shorted to the ground plane at the base.(3) The VNA12is coupled to the transmit monopole antenna18and the receive antenna22by first and second transmission lines20and24respectively. In this embodiment, the first and second transmission lines are approximately 18.3 meters (60 ft) in length. The receiver power was measured from the input of the 0.91 m (3 ft) feed line attached to the feedpoint of the whip monopole receiving antenna. The feedpoint was about 0.61 m (2 ft) from the insulator base.
In the above-described systems, the VNA12measures the S-parameters for the two systems. The effective loss of the MCCP antenna28is the reduction in power as the current probe26transfers the power from a transmitter to the mast structure (or from the mast structure to the receiver). As detailed above, the reduction is the algebraic difference between the receive power compared to that of the reference system40. To maintain consistency in interpretation, the power gain (minus attenuation) is estimated for the systems described in the embodiment above. In that embodiment, a negative gain denotes a loss. Across most of the frequency band, insertion of the current probe26incurs a loss between 0-15 dB when compared to the reference system40; except around 3 MHz, there's actually a gain over the reference system40.

From the above description of the Method for Quantifying the Effective Loss of a Mast Clamp Current Probe (MCCP) Antenna, 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.