# Patent

## Title
Frequency detection system comprising circuitry for adjusting the resonance frequency of a high temperature superconductor self-resonant coil

## Abstract

The use of a circuit to adjust the resonance frequency of a high temperature superconductor self-resonant transmit, receive, or transmit and receive coil results in improved performance. The circuit is useful in a frequency detection system, especially in a nuclear quadrupole resonance detection system.

## Background
This application claims the benefit of U.S. Provisional Application No. 60/524,461, filed Nov. 24, 2003, which is incorporated in its entirety as a part hereof for all purposes.


FIELD OF THE INVENTION
This invention relates to the use of a circuit to adjust the resonance frequency of a high temperature superconductor self-resonant coil, and to a frequency detection system comprised of such a coil and circuit.
BACKGROUND OF THE INVENTION
The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives and other contraband has been recognized for some time—see e.g. T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980); A. N. Garroway et al, Proc. SPIE 2092, 318 (1993); and A. N. Garroway et al, IEEE Trans. on Geoscience and Remote Sensing, 39, pp. 1108-1118 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance. NQR is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies.
One technique for measuring NQR in a sample is to place the sample within a solenoid coil that surrounds the sample. The coil provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals. This is the typical apparatus configuration that might be used for scanning mail, baggage or luggage. There is also need for a NQR detector that permits detection of NQR signals from a source outside the detector, e.g. a wand detector, that could be passed over persons or containers as is done with existing metal detectors. Problems associated with such a detector using conventional systems are the decrease in detectability with distance from the detector coil, and the associated equipment needed to operate the system.
A detection system can have one or more coils that both transmit and receive, or it can have separate coils that only transmit and only receive. A transmit, or transmit and receive, coil of an NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample, and results in their producing their characteristic resonance signals that the coil detects. The NQR signals have low intensity and short duration.
The transmit, receive, or transmit and receive, coil preferably has a high quality factor (Q). The transmit, receive, or transmit and receive, coil has typically been a copper coil and therefore has a Q of about 102. It is advantageous to use a transmit, receive, or transmit and receive, coil made of a high temperature superconductor (HTS) rather than copper since the HTS self-resonant coil has a Q of the order of 103−106. The large Q of the HTS self-resonant coil produces large magnetic field strengths during the RF transmit pulse and does so at lower RF power levels. This dramatically reduces the amount of transmitted power required to produce NQR signals for detection, and thereby reduces the size of the RF power supply sufficiently so that it can be run on portable batteries.
The large Q of the HTS self-resonant coil also plays an important role during the receive time. In view of the low intensity NQR signal, it is important to have a signal-to-noise ratio (S/N) as large as possible. As the signal-to-noise (S/N) ratio is proportional to the square root of Q, the use of the HTS self-resonant coil results in an increase in S/N by a factor of 10-100 over that of the copper system. These advantages during both the transmit and the receive times enable a detector configuration that is small and portable. It is important that the transmit, receive, or transmit and receive, coil is tunable so that the resonance frequency of the respective coil can be adjusted after fabrication to the NQR frequency.
An object of the present invention is to provide for the adjustment of the resonance frequency of a high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil to enhance its use in a frequency detection system.
SUMMARY OF THE INVENTION
This invention provides a circuit to adjust the resonance frequency of a high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil, wherein the circuit is comprised of a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil; a reactance in series with the single loop or coil; and means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil.
Preferably, the reactance is one that can be varied in order to provide flexibility in adjusting the resonance frequency of the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil. The means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil may include at least one mechanical switch and/or electrical switch such as a diode. Preferably, the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil is a planar or surface coil. Preferably, the single loop or coil inductively coupling the circuit to the high temperature superconductor self-resonant transmit, receive, or transmit and receive coil, is a high temperature superconductor single loop or coil, and more preferably, is a high temperature superconductor single loop.
This invention also provides a frequency detection system comprising a high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil, and a circuit to adjust the resonance frequency of the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil; wherein the circuit is comprised of a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil, a reactance in series with the single loop or coil, and means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil. This frequency detection system, with the circuit to adjust the resonance frequency of the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil, is especially useful for detecting nuclear quadrupole resonance, and particularly, for detecting the nuclear quadrupole resonance of explosives, drugs and other contraband.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic drawing of the circuit of the invention for adjusting the resonance frequency.
FIG. 2 shows a schematic drawing of one embodiment of the circuit of the invention for adjusting the resonance frequency.
FIG. 3 shows the S21 of a HTS self-resonant transmit and receive coil under various conditions.
FIG. 4 shows the HTS coil design of the HTS transmit and receive coil used in Examples 1-30.
FIGS. 5A and 5B show the holder that was used in Examples 1-30 to hold the HTS coil that is comprised of two coupled HTS self-resonant planar coils and the single loop that inductively couples the circuit of the invention to the HTS coil.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The use of the circuit of this invention enables the adjustment of the resonance frequency of a high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil. This adjustment of the resonance frequency of the self-resonant coil can be very useful since it is difficult to fabricate a coil having the exact resonance frequency desired. In addition, it can be important to be able to adjust the resonance frequency of the self-resonant coil to match a specific frequency, e.g. the frequency, fNQR, of the NQR of interest.
When detecting a frequency, it is important that the resonance frequency of the transmit, or transmit and receive, coil during the transmit mode, i.e. the excitation mode, be identical to the resonance frequency of the receive, or transmit and receive, coil during the receive mode, i.e. the detection mode. This is especially important for low intensity and short duration signals such as NQR signals.
The circuit of the invention that provides for the adjustment of the resonance frequency is comprised of (i) a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil; (ii) a reactance in series with the single loop or coil; and (iii) means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil. The single loop or coil can be made of a regular conductor, such as copper, or a high temperature superconductor. The reactance can be an inductance, capacitance or combination of both. Preferably, the reactance can be varied so that the resonance frequency can be adjusted to more than one frequency. The means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, whether the reactance includes capacitors or inductors or both, may include at least one mechanical switch and/or electrical switch such as a diode.
One way of accomplishing a variable reactance is to have the reactance comprised of a number of capacitors in parallel, each of which can be individually connected to or disconnected from the single loop or coil. Alternatively, a variable reactance can be comprised of a number of inductors in series, each of which can be individually connected to or disconnected from the single loop or coil by a switch or diode that can short-circuit the inductor and thereby essentially remove it from the circuit.
A schematic diagram of the circuit is shown in FIG. 1. The circuit is shown with a single loop 11 that inductively couples the circuit to the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil 12. Additional loops, i.e. a coil, can be used to provide the desired coupling. Connected to the single loop 11 is a reactance 13 and a switch 14 that connects and disconnects the reactance 13 to the single loop 11. The switch 14 can be a mechanical switch, or it can be an electrical switch such as a diode that conducts above a certain applied voltage.
Preferably, the reactance is variable and various portions of the reactance can be connected to and disconnected from the single loop. A schematic diagram of one such embodiment of the circuit of the invention is shown in FIG. 2. The circuit is shown with a single loop 21 that inductively couples the circuit to the high temperature superconductor self-resonant transmit, receive, or transmit and receive, coil 22. In this embodiment, the reactance is variable in the sense that it is comprised of four capacitors 23, 24, 25 and 26 in parallel, and four switches 27, 28, 29, and 30. This permits various combinations of the capacitors to be connected to single loop 21; which can thereby provide various capacitances and, as a result, provide different adjustments to the resonance frequency of the self-resonant coil. More or fewer capacitors can be used in parallel depending upon the desired variation in resonance frequency.
In another embodiment, the reactance is comprised of inductors placed in series with a switch in parallel with each inductor. When the switch is open the inductor is in series with the single loop or coil. When the switch is closed, the switch short-circuits the inductor and essentially removes it from the circuit.
The planar or surface coil preferred for use as the high temperature superconductor self-resonant transmit, receive or transmit and receive, coil has a HTS coil configuration on only one side of the substrate, or has essentially identical HTS coil configurations on both sides of the substrate.
It is often advantageous to be able to fine tune the resonance frequency. One means for accomplishing such tuning is to use two coupled high temperature superconductor self-resonant coils. The resonance frequency of the fundamental symmetric mode of the two coupled high temperature superconductor self-resonant coils can be varied by mechanically displacing one coil with respect to the other, and these coupled coils serve as the HTS transmit, receive or transmit and receive, coil.
Preferably, the single loop in the Q-damping circuit is a single loop of copper or HTS on the same substrate as the HTS transmit, receive, or transmit and receive, coil.
One difficulty in using a HTS self-resonant transmit, or transmit and receive, coil is that the resonance or center frequency of the coil is power dependent, and shifts to lower frequencies as the power coupled into the coil increases, e.g. as occurs during the transmit period. The S21, the magnitude of the transmitting coefficient from the input to the output as a function of frequency, is shown in FIG. 3 for a high temperature superconductor self-resonant transmit, or transmit and receive, coil designed to have a low-transmit-power resonance frequency, fNQR, equal to that of the NQR of interest. Curve 1 shows the low-transmit-power-mode, high-Q S21 with the resonance frequency fNQR. When high power is applied to the coil in the transmit mode, the resonance frequency of the system shifts to a lower frequency, and the Q degrades, as shown by Curve 2. The resonance frequency is no longer fNQR, and the fraction of NQR power coupled to the HTS receive, or transmit and receive, coil during the receive mode under these conditions would be small.
The circuit of the instant invention maintains the resonance frequency of the HTS transmit, or transmit and receive, coil at fNQR during the transmit mode. This result is accomplished by an adjustment to the resonance frequency during the high-transmit-power mode, which can be understood by referring to the schematic diagram of simple circuit shown in FIG. 1. The self-resonant coil 12 and the reactance 13, e.g. a capacitance, are chosen so that the low-transmit-power resonance frequency is fNQR when the switch 14 closed, and the reactance 13 is in series with the single loop 11. When high power is applied to the high temperature superconductor self-resonant transmit and receive coil 12 during the transmit mode, the switch is opened to essentially remove the reactance from the circuit, and the resonance frequency remains at fNQR. This is indicated by Curve 3 in FIG. 3 where the resonance frequency is fNQR, but the Q is still degraded due to the power dependence of the HTS coil. This allows power to be transmitted at the correct frequency during the transmit mode. In the case of a transmit and receive coil, the switch 14 in FIG. 1 is closed for the receive mode so that the resonance frequency is fNQR.
The circuit is designed in the following manner to adjust the power dependent resonance frequency. The HTS self-resonant transmit, or transmit and receive, coil and the circuit are designed to provide a resonance frequency equal to the desired fNQR. The power dependent resonance frequency shift can be attributed to a kinetic inductance. When the reactance is removed from the circuit by opening the switch, its removal balances the additional kinetic inductance during the high power mode. The power dependent shift in resonance frequency of the HTS self-resonant transmit, or transmit and receive, coil can be observed for the high power expected to be used, and the HTS coil design and the circuit reactance parameters are chosen accordingly.
High temperature superconductors are those that superconduct above 77K. The high temperature superconductors used to form the HTS self-resonant transmit and receive coil are preferably selected from the group consisting of YBa2Cu3O7, Tl2Ba2CaCu2O8, TlBa2Ca2Cu3O9, (TlPb) Sr2CaCu2O7 and (TlPb) Sr2Ca2Cu3O9. Most preferably, the high temperature superconductor is YBa2Cu3O7 or Tl2Ba2CaCu2O8.
The HTS self-resonant transmit, receive, or transmit and receive, coil can be formed by various known techniques. A preferred technique for forming Tl2Ba2CaCu2O8 coils is used in the examples.
Provision must be made for a power supply to supply power for transmitting the RF pulse as well as for related circuitry for processing the detected NQR signal. Provision must also be made for cooling the HTS coil to liquid nitrogen temperature.
The advantageous effects of this invention are demonstrated by a series of examples, as described below. The embodiments of the invention on which the examples are based are illustrative only, and do not limit the scope of the appended claims.
EXAMPLES 1-15
The purpose of these examples is to demonstrate the use of a circuit of the type shown in FIG. 2 to adjust the resonance frequency of a HTS self-resonant coil. The HTS self-resonant coil used is comprised of two coupled essentially identical Tl2Ba2CaCu2O8 planar coils. Each of the coupled coils is on a sapphire (Al2O3) substrate with the coil design configuration shown in FIG. 4 on both sides of the substrate.
A clean, polished single crystal sapphire substrate with a diameter of 2 inches (5.1 cm) and an approximate thickness of 0.018 inches (0.46 mm) was obtained from Union Carbide Corp. An epitaxial CeO2 buffer layer is grown on both sides of the substrate by off-axis sputter deposition with the substrate temperature held in the range of about 700-800° C. Off-axis magnetron sputtering of a Ba:Ca:Cu oxide target is used to deposit, at room temperature (about 20° C.), an amorphous precursor Ba:Ca:Cu oxide film on the CeO2 buffer layer on both sides of the substrate. This amorphous Ba:Ca:Cu oxide film is about 550 nm thick and had a stoichiometry of about 2:1:2. The precursor film is then thallinated by annealing it in air for about 45 minutes at 850° C. in the presence of a powder mixture of Tl2Ba2Ca2Cu3O10 and Tl2O3. When this powder mixture is heated, Tl2O evolves from the powder mixture, diffuses to the precursor film and reacts with it to form the Tl2Ba2CaCu2O8 phase.
The sample is then coated with photoresist on both sides and baked. A coil design mask with the design shown in FIG. 4 is prepared. The coil has an inner radius of about 10.5 mm and an outer radius of about 22.5 mm. The outermost HTS ring 41 of the coil is about 2 mm wide and the innermost HTS ring 42 is about 3.5 mm wide. The intermediate HTS rings 43 are about 250 μm wide with about 250 μm gaps between the rings. The coil design mask is then centered on the photoresist covering the Tl2Ba2CaCu2O8 film on the front side of the substrate and exposed to ultraviolet light. The coil design mask is then centered on the photoresist covering the Tl2Ba2CaCu2O8 film on the back side of the substrate and exposed to ultraviolet light. The resist is then developed on both sides of the substrate, and the portion of the Tl2Ba2CaCu2O8 film exposed when the resist is developed is etched away by argon beam etching. The remaining photoresist layer is then removed by an oxygen plasma.
The result is a coil structure comprised of the single crystal sapphire substrate with a CeO2 buffer layer and a high temperature superconductor Tl2Ba2CaCu2O8 pattern of the configuration shown in FIG. 4 centered on the CeO2 buffer layer on each side of the single crystal sapphire substrate. The process is repeated in essentially the same way to produce a second coil essentially identical to the first.
FIG. 5A is a schematic drawing of the top view of holder 50 that holds the two sapphire substrates and the copper single loop. FIG. 5B is a cross section of the holder 50 taken at points “5B”-“5B” of FIG. 5A. The holder 50 is a fiberglass epoxy laminate. A circular hole 51 of radius about 7⁄8 inch (2.2 cut) is cut near one end of the sample holder 50 A second hole with an outer radius of about 1 inch (2.5 cm) is cut leaving a ledge 52 to hold the one sapphire substrate 53 as shown. On the opposite side of the ledge 52, a cut-out 54 is made to accommodate moveable substrate holders 55 and 56 that serve to hold the other sapphire substrate 57. The two moveable substrate holders 55 and 56 also serve to translate sapphire substrate 57 laterally, i.e. horizontally, with respect to sapphire substrate 53, and thereby enable the resonance frequency of the two coupled HTS coils to be fine tuned. The distance 58 between the two sapphire substrates is 1 mm. A circular groove 59 of radius about 1.1 inch (2.8 cm) surrounding and concentric with the sapphire substrate 53 is cut into the surface of the sample holder 50 along with parallel grooves 60 and 61. A piece of 18 American Wire Gauge copper wire, not shown, is placed in groove 60 beginning at point 62 and extending along groove 60 to the circular groove 59, around circular groove 59 to parallel groove 61, and along groove 61 to point 63. The copper wire in the circular groove 59 Serves as a single loop to inductively couple the FITS transmit and receive coil to the Q-damping circuit. Connections to the four capacitors of the circuit shown in FIG. 2 are made at points 62 and 63. The HTS pattern 64 on one side of the sapphire substrate 53 is indicated in FIG. 5A.
The capacitors 23, 24, 25 and 26 of the circuit shown in FIG. 2 had capacitances of 20, 58, 100 and 220 pF, respectively.
The HTS self-resonant coil is immersed in liquid nitrogen held in a Nalgene® dewar. A pick-up coil comprised of a loop of copper wire is placed about 1 inch (2.5 cm) away from the HTS self-resonant coil with the plane of the pick-up coil parallel to the plane of the coil. The pick-up coil is formed by removing the outer jacket and dielectric spacer from a piece of 0.080 inch (2 mm) coax cable. The loop is formed by bending the inner conductor into a circle and soldering it to the outer jacket of the coax cable just outside the point where the jacket and dielectric are removed. The pick-up coil is connected to an Agilent 8753 Vector Network Analyzer (Agilent Technologies, Palo Alto, Calif.).
The frequency is swept, and the resonance frequency and Q of the HTS coil is measured for each combination of capacitors connected to the single loop. Substrate 57 has been positioned with respect to substrate 53 so that the HTS coil comprised of the two coupled Tl2Ba2CaCu2O8 planar coils has a resonance frequency of 3761.6 kHz and a Q of 2950. This measurement is made with all switches 27, 28, 29 and 30 shown in FIG. 2 open so that there are no capacitors in the circuit of the single loop 21, i.e. the circuit is open and has no effect.
The specification of which switches are closed, using the notation of FIG. 2, and the resulting capacitances in the circuit, are listed in Table I for each example along with the resonance frequencies and Q's of the HTS coil. For instance, for Example 1, switch 27 of FIG. 2 is closed, and switches 28, 29 and 30 are open, so that the capacitance in series with the single loop is 20 pF. For Example 15, all switches are closed, and the capacitance in series with the single loop is 398 pF.











TABLE I





Example
Switches
Capacitance
Resonance



Number
Closed
(pF)
Freq. (kHz)
Q



















Open Circuit
None
0
3761.6
2950


1
27
20
3761.5
2840


2
28
58
3760.5
2890


3
27, 28
78
3760.4
2800


4
29
100
3759.8
2710


5
27, 29
120
3759.5
2780


6
28, 29
158
3758.6
2710


7
27, 28, 29
178
3758.3
2690


8
30
220
3757.1
2850


9
27, 30
240
3756.9
2730


10
28, 30
278
3755.7
2540


11
27, 28, 30
298
3755.4
2650


12
29, 30
320
3754.5
2450


13
27, 29, 30
340
3754.2
2440


14
28, 29, 30
378
3752.9
2440


15
27, 28, 29, 30
398
3752.7
2400









These results show that the resonance frequency of the HTS coil can be adjusted in small increments by varying the reactance in the circuit inductively coupled to the HTS coil, and that this variation can be accomplished with no significant change in Q. With the set of capacitors used in these examples, the resonance frequency could be varied over a range of 8.9 kHz.
EXAMPLES 16-30
The purpose of these examples is to further demonstrate the use of a circuit of the type shown in FIG. 2 to adjust the resonance frequency of a HTS self-resonant coil. The HTS coil used in these examples is the same HTS self-resonant coil comprised of two coupled essentially identical Tl2Ba2CaCu2O8 planar coils used in Examples 1-15. The same holder, and the same copper single loop coupling the HTS coil to the circuit for adjusting the resonance frequency of the HTS coil as used in Examples 1-15, are used in Examples 16-30. The circuit is identical to that used in Examples 1-15 except that the capacitors 23, 24, 25 and 26 of the circuit shown in FIG. 2 had capacitances of 58, 100, 220 and 300 pF, respectively.
The resonance frequency and Q of the HTS coil is measured for each combination of capacitors connected to the single loop in the same manner as carried out for Examples 1-15. Substrate 57 has been positioned with respect to substrate 53 so that the HTS coil comprised of the two coupled Tl2Ba2CaCu2O8 planar coils has a resonance frequency of 3766.8 kHz and a Q of 2950. This measurement is made with all switches 27, 28, 29 and 30 shown in FIG. 2 open so that there are no capacitors in the circuit of the single loop 21, i.e. the circuit is open and has no effect. The specification of which switches are closed, using the notation of FIG. 2, and the resulting capacitances in the circuit, are listed in Table II for each example along with the resonance frequencies and Q's of the HTS coil.











TABLE II





Example
Switches
Capacitance
Resonance



Number
Closed
(pF)
Freq. (kHz)
Q



















Open Circuit
None
0
3766.8
2950


16
27
58
3765.8
2840


17
28
100
3765.4
2890


18
27, 28
158
3764.3
2800


19
29
220
3762.8
2710


20
27, 29
278
3761.4
2780


21
30
300
3760.8
2850


22
28, 29
320
3760.3
2710


23
27, 30
358
3759.3
2730


24
27, 28, 29
378
3758.8
2690


25
28, 30
400
3758.2
2540


26
27, 28, 30
458
3756.6
2650


27
29, 30
520
3754.7
2450


28
27, 29, 30
578
3752.8
2440


29
28, 29, 30
620
3751.4
2440


30
27, 28, 29, 30
678
3749.3
2400









These results show that the resonance frequency of the HTS coil can be adjusted in small increments by varying the reactance in the circuit inductively coupled to the HTS coil, and that this variation can be accomplished with no significant change in Q. With the set of capacitors used in these examples, the resonance frequency could be varied over a range of 17.5 kHz.
Where an apparatus of this invention is stated or described as comprising, including, containing, having, being composed of or being constituted by certain components, it is to be understood, unless the statement or description explicitly provides to the contrary, that one or more components other than those explicitly stated or described may be present in the apparatus. In an alternative embodiment, however, the apparatus of this invention may be stated or described as consisting essentially of certain components, in which embodiment components that would materially alter the principle of operation or the distinguishing characteristics of the apparatus would not be present therein. In a further alternative embodiment, the apparatus of this invention may be stated or described as consisting of certain components, in which embodiment components other than those as stated would not be present therein.
Where the indefinite article “a” or “an” is used with respect to a statement or description of the presence of a component in an apparatus of this invention, it is to be understood, unless the statement or description explicitly provides to the contrary, that the use of such indefinite article does not limit the presence of the component in the apparatus to one in number.

## Claims

1. A circuit to adjust the power dependent resonance frequency of a high temperature superconductor self-resonant transmit or transmit and receive coil, wherein the circuit is comprised of
a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant coil,
a reactance in series with the single loop or coil, and
means operative while the coil is transmitting for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the connection and disconnection of the reactance serving to adjust the power dependent resonance frequency of the coil.

2. The circuit of claim 1 wherein the circuit is comprised of a single loop, and the high temperature superconductor self-resonant transmit or transmit and receive coil is a planar coil.

3. The circuit of claim 1 wherein the circuit is comprised of a single loop, and the high temperature superconductor self-resonant transmit or transmit and receive coil is comprised of two coupled high temperature superconductor planar coils.

4. The circuit of any of claims 1-3 wherein the reactance can be varied.

5. A circuit to adjust the resonance frequency of a high temperature superconductor self-resonant coil, comprising:
a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant coil;
a variable reactance in series with the single loop or coil, wherein the reactance comprises two or more capacitors in parallel; and
means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil enables each of the capacitors to be individually connected to, and disconnected from, the single loop or coil.

6. The circuit of claim 5 wherein the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil comprises one mechanical or electrical switch for each of the capacitors.

7. A circuit to adjust the resonance frequency of a high temperature superconductor self-resonant coil, comprising:
a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant coil;
a variable reactance in series with the single loop or coil, wherein the reactance comprises two or more inductors in series; and
means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil enables each of the inductors to be individually connected to, and disconnected from, the single loop or coil.

8. The circuit of claim 7 wherein the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil comprises one mechanical or electrical switch for each of the inductors.

9. A frequency detection system, comprising:
a high temperature superconductor self-resonant transmit and receive coil; and
a circuit to adjust the power dependent resonance frequency of the high temperature superconductor self-resonant transmit and receive coil;
wherein the circuit is comprised of a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit and receive coil,
a variable reactance in series with the single loop or coil, and
means operative while the coil is transmitting for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the connection and disconnection of the reactance serving to adjust the power dependent resonance frequency of the coil.

10. The frequency detection system of claim 9 wherein the circuit is comprised of a single loop, and the high temperature superconductor self-resonant transmit and receive coil is a planar coil.

11. The frequency detection system of claim 9 wherein the circuit is comprised of a single loop, and the high temperature superconductor self-resonant transmit and receive coil is comprised of two coupled high temperature superconductor planar coils.

12. The frequency detection system of any of claims 9-11 wherein the reactance can be varied.

13. The frequency detection system of any of claims 9-11 which comprises a nuclear quadrupole resonance detection system.

14. The frequency detection system of claim 13 wherein the nuclear quadrupole resonance detection system detects explosives, drugs or other contraband.

15. A frequency detection system comprising:
a high temperature superconductor self-resonant transmit and receive coil;
a circuit to adjust the resonance frequency of the high temperature superconductor self-resonant transmit and receive coil, wherein the circuit comprises a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit and receive coil;
a variable reactance in series with the single loop or coil, wherein the reactance is comprised of two or more capacitors in parallel, and
means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the means enabling each of the capacitors to be individually connected to, and disconnected from, the single loop or coil.

16. The frequency detection system of claim 15 wherein the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil comprises one mechanical or electrical switch for each of the capacitors.

17. A frequency detection system comprising:
a high temperature superconductor self-resonant transmit and receive coil;
a circuit to adjust the resonance frequency of the high temperature superconductor self-resonant transmit and receive coil, wherein the circuit comprises a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit and receive coil;
a variable reactance in series with the single loop or coil, wherein the reactance is comprised of two or more inductors in series, and
means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the means enabling each of the inductors to be individually connected to, and disconnected from, the single loop or coil.

18. The frequency detection system of claim 17 wherein the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil comprises one mechanical or electrical switch for each of the inductors.

19. A frequency detection system, comprising:
a high temperature superconductor self-resonant transmit coil; and
a circuit to adjust the power dependent resonance frequency of the high temperature superconductor self-resonant transmit coil;
wherein the circuit is comprised of a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit coil,
a reactance in series with the single loop or coil, and
means operative while the coil is transmitting for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the connection and disconnection of the reactance serving to adjust the power dependent resonance frequency of the coil.

20. The frequency detection system of claim 19 wherein the single loop or coil is a single loop, and the high temperature superconductor self-resonant transmit coil is a planar coil.

21. The frequency detection system of claim 19 wherein the single loop or coil is a single loop, and the high temperature superconductor self-resonant transmit coil is comprised of two coupled high temperature superconductor planar coils.

22. The frequency detection system of any of claims 19-21 wherein the reactance can be varied.

23. The frequency detection system of any of claims 19-21 which comprises a nuclear quadrupole resonance detection system.

24. The frequency detection system of claim 23 wherein the nuclear quadrupole resonance detection system detects explosives, drugs or other contraband.

25. A frequency detection system comprising:
a high temperature superconductor self-resonant transmit coil;
a circuit to adjust the resonance frequency of the high temperature superconductor self-resonant transmit coil, wherein the circuit comprises a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit coil;
a variable reactance in series with the single loop or coil, wherein the reactance is comprised of two or more capacitors in parallel, and
means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the means enabling each of the capacitors to be individually connected to, and disconnected from, the single loop or coil.

26. The frequency detection system of claim 25 wherein the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil comprises one mechanical or electrical switch for each of the capacitors.

27. A frequency detection system comprising:
a high temperature superconductor self-resonant transmit coil;
a circuit to adjust the resonance frequency of the high temperature superconductor self-resonant transmit coil, wherein the circuit comprises a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit coil;
a variable reactance in series with the single loop or coil, wherein the reactance is comprised of two or more inductors in series, and
means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the means enabling each of the two or more inductors to be individually connected to, and disconnected from, the single loop or coil.

28. The frequency detection system of claim 27 wherein the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil comprises one mechanical or electrical switch for each of the inductors.

29. A frequency detection system comprising:
a high temperature superconductor self-resonant receive coil;
a circuit to adjust the resonance frequency of the high temperature superconductor self-resonant receive coil, wherein the circuit comprises a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit coil;
a variable reactance in series with the single loop or coil, wherein the reactance is comprised of two or more capacitors in parallel, and
means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the means enabling each of the capacitors to be individually connected to, and disconnected from, the single loop or coil.

30. The frequency detection system of claim 29 wherein the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil comprises one mechanical or electrical switch for each of the capacitor.

31. A frequency detection system comprising:
a high temperature superconductor self-resonant receive coil;
a circuit to adjust the resonance frequency of the high temperature superconductor self-resonant receive coil, wherein the circuit comprises a single loop or coil to inductively couple the circuit to the high temperature superconductor self-resonant transmit coil;
a variable reactance in series with the single loop or coil, wherein the reactance is comprised of two or more inductors in series, and
means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil, the means enabling each of the inductors to be individually connected to, and disconnected from, the single loop or coil.

32. The frequency detection system of claim 31 wherein the means for connecting the reactance to, and disconnecting the reactance from, the single loop or coil comprises one mechanical or electrical switch for each of the inductors.

33. The frequency detection system of any of claims 29-31 which comprises a nuclear quadrupole resonance detection system.

34. The frequency detection system of claim 33 wherein the nuclear quadrupole resonance detection system detects explosives, drugs or other contraband.