FIELD OF THE INVENTION

The present invention relates generally to gradiometer probes, and more specifically to a shielded-loop-resonator based gradiometer probe that operates at distances comparable with the size of the probe.

BACKGROUND

A gradiometer measures the gradient (numerical rate of change) of a physical quantity, such as a magnetic field or gravity. When a plane radio frequency (RF) wave reaches a gradiometer, a zero reading is produced. However, when the front of the RF wave that reaches the gradiometer is curved, the value of the electrical field near the gradiometer is not constant and has a gradient. This causes the gradiometer to generate a signal that is proportional to the gradient.

A basic gradiometer includes two coils and a metrology circuit that measures the difference of the signals from the coils as a result of an electrical field near the coil. When a plane wave reaches these two coils, the coils produce identical signals and the difference will be zero and thus the gradiometer measures zero gradient. When the wave front is not planar, the signals are different and the resulting signal measurement is proportional to the gradient of the electrical field.

This gradiometer can operate only in the far field, which is the area where the distance between the source and the gradiometer is very large with respect to wavelength. If the source is near the gradiometer, the latter still measures some output but, since different parts of the gradiometer are exposed to very different areas of the field, the output is nearly impossible to relate to a gradient of the electric field. When the source is much closer to one part of the gradiometer than the other part, the signal from that part dominates the output and the gradient is not measured at all. Therefore, coil-based gradiometers do not work in the near field.

Moreover, the above-mentioned gradiometer does not explain the relationship between the wavelength of the radiation and the size of the gradiometer probe. While in many RF applications, the wavelength of the radiation is comparable or smaller than the size of the source and receiver antennas, in other applications the opposite is true, that is, the radiation wavelength is significantly greater than the size of the receiver antenna.

For the purposes of this disclosure, the far field is defined as the electromagnetic field at distances much greater than the distance from a receiver to the source. The near field is defined as the area in close proximity to the antenna, and is comparable in size with the size of the antenna. The source, or object to be detected, is placed in the near field region.

A major concern in systems that use near-field probes and related technologies, especially for detection and screening of materials such as explosives or nuclear material, is suppression of radio frequency interference (RFI). Suppression of RFI is particularly relevant for the systems, which rely on detection of a relatively weak or small signal. Detection of these weak or small signals, using near-field probe (antenna) systems such as loop antennas, can be difficult in the presence of strong far field noise sources/signals, such as AM radio transmitters, and nearby noise sources/signals, such as automobile ignitions, computers, mobile phones, and other electronics.

FIG. 1shows a prior art antenna. As shown, antenna1100can be substantially flat, occupying a plane1150. The antenna1100includes first loop1110, a second loop1120, and a third loop1130which are substantially coplanar in plane1150. A coupler element1142supplies a current from the first loop1110to the second loop1120. Coupler element1140juts out a distance from the plane1150to couple the third loop1130to the second loop1120. The current flows in a first polarity through the third loop1130, and in a second polarity opposite to the first polarity in first and second loops1110,1120. The loops1110,1120, and1130may be manufactured on a single side of an insulating material, such as a printed circuit panel or a panel, for ease of fabrication. The outer loop1130has an outer loop enclosed area equal in size to the sum of each of the inner loop enclosed areas. With this configuration, the far-field radiation is cancelled to a high degree, while the near-field energy is not as substantially impacted.

However, the bandwidth of this noise cancelling gradiometer probe (antenna) is limited by the self-resonance of the gradiometer probe. The sensitivity of the probe grows with the number of turns in the probe; however so does its inductance. A large inductance coupled with parasitic capacitance causes self-resonances to appear, rendering the probe less effective. In addition, introduction of objects (especially metals, cars, and the like) near the probe detunes the probe, again diminishing the effectiveness of the probe.

SUMMARY

Some embodiments are directed to a noise cancelling gradiometer probe utilizing a coaxial cable to form shielded loop resonator wires and a tuning circuit for tuning the probe. The shielded-loop-resonator based gradiometer probe operates at distances comparable with the size of the probe, operating as a gradiometer with respect to far away sources and as an antenna for near objects.

Some embodiments are directed to a gradiometer probe. In some embodiments, the probe includes: an insulating material having a first side and a second side; a first coaxial cable forming a first loop, wherein a portion of the first loop is locating on the first side of the insulating material and a second portion of the first loop is locating on the second side of the insulating material; a second coaxial cable forming a second loop, wherein a portion of the second loop is located on the first side of the insulating material and a second portion of the second loop is located on the second side of the insulating material; and a third coaxial cable forming a third loop, wherein a portion of the third loop is located on the first side of the insulating material and a second portion of the third loop is located on the second side of the insulating material. A first antenna is formed by the first portions of the first, second and third loops on the first side of the insulating material, and a second antenna is formed by the second portions of the first, second and third loops on the second side of the insulating material. The first antenna may act as a receive antenna and the second antenna may act as a transmit antenna.

Some embodiments are directed to a method of operating a gradiometer probe to detect a material. In some embodiments, the probe includes: connecting the gradiometer probe to a control box including a power supply; providing power to the gradiometer probe to transmit a transmit signal; tuning the gradiometer probe for detecting the material; and detecting a receive signal from excitation of the material to be detected by the transmit signal; analyzing the receive signal to detect the material. The gradiometer probe may include: an insulating material having a first side and a second side; a first coaxial cable forming a first loop, wherein a portion of the first loop is located on the first side of the insulating material and a second portion of the first loop is located on the second side of the insulating material; a second coaxial cable forming a second loop, wherein a portion of the second loop is located on the first side of the insulating material and a second portion of the second loop is locating on the second side of the insulating material; and a third coaxial cable forming a third loop, wherein a portion of the third loop is locating on the first side of the insulating material and a second portion of the third loop is locating on the second side of the insulating material. A first antenna is formed by the first portions of the first, second and third loops on the first side of the insulating material, and a second antenna is formed by the second portions of the first, second and third loops on the second side of the insulating material.

In some embodiments, the gradiometer probe further includes a tuning circuit, which may be a balun transformer. In some embodiments, the gradiometer probe further includes a fourth coaxial cable that forms a fifth loop larger than the fourth loop. In some embodiments, the gradiometer probe further includes an adjustment device to change an area of the third loop by expanding or contract the third loop outward or inward to tune the gradiometer probe.

