Resonant Loop Current Disturbing Probe

The present invention relates generally to a resonant loop current disturbing probe and more particularly to the measurement of the distribution of the current along an antenna, particularly in the so-called modulated return radiation method.

The modulated return method is shown in FIG. 1.

An antenna 1 is supplied by a generator 2 across a circulator 3 (or hybrid T or coupler). The operating frequency is fo. A small probe 4, formed from one or more conductive turns closed on a photoresistor is moved along the antenna. The probe photoresistor is illuminated by light radiation guided by an optical fibre 5 supplied by a light-emitting diode 6. The latter is controlled by a circuit 7 comprising an oscillator at a frequency fm. The light radiation amplitude-modulated in this way at frequency fm leads to a variation in the resistance of the photoresistor inserted in the probe. The probe is couple to the magnetic field produced by the current flowing in the antenna, so that the electric signal reflected by the antenna is amplitude-modulated at frequency fm. Thus, the spectrum of the reflected signal is formed by a carrier at frequency fo and by two side bands at frequencies fo-fm and fo+fm. Such a signal carries amplitude and phase informations relative to the current flowing in the antenna to the right of the probe. A circuit 100 makes it possible to read these informations.

For example, such a method is described in the article by K. Itzuka entitled "Photoconductive Probe For Measuring Electromagnetic Fields" published in Proceedings of the IEE, vol 110, No. 10, October 1963.

The attached FIG. 2 shows in greater detail the structure of probe 4, where it is possible to see a loop 10, a photoresistor 12 and the optical fibre 5 referred to hereinbefore. Although being satisfactory in certain respects, such a probe lacks sensitivity.

The object of the present invention is to obviate this disadvantage, which is brought about by making the probe resonant. For this purpose, a variable capacitor is connected to the loop and adjusted to a value such that the loop - capacitor assembly forms a resonant circuit for the frequency of the current flowing in the antenna, i.e. in the example of FIG. 1 for frequency fo. This gives a better sensitivity, as will be shown hereinafter. The dimensions of the probe can then be reduced.

The invention also relates to a further improvement, which is combined with the first and which consists of shielding the probe.

Finally, another improvement makes it possible to further increase the sensitivity of the probe and consists of exciting the light-emitting diode by a pulse signal, whose cyclic ratio is varied by giving it a value differing from 1/2 (which corresponds to the square signal), instead of by a square signal. the performance characteristics of the photo resistor are consequently improved.

The invention is described in greater detail hereinafter relative to non-limitative embodiments and the attached drawings, wherein show:

FIG. 1, already described, illustrates the modulated return radiation method.

FIG. 2, already described, shows a prior art aperiodic probe.

FIG. 3, the equivalent electric diagram of the probe according to the invention.

FIG. 4, the overvoltage variation of the probe according to the invention as a function of the frequency.

FIG. 5a and 5b, the constitution of the probe.

FIG. 6, a light-emitting diode control means.

The electric diagram of FIG. 3 shows a loop 10 closed or shorted by a photoresistor 12 and a variable capacitor 20. The loop-capacitor assembly forms a resonant circuit LC, which is adjusted so as to be resonant at the operating frequency fo.

FIG. 4 shows the voltage variation of said circuit as a function of the frequency. This voltage passes through a maximum for the resonant frequency fo and decreases on either side of this value. When no light strikes the photoresistor, the latter has a high resistance and the circuit is placed under and voltage, curve 22 being obtained. However, when the photoresistor is illuminated, its resistance drops and the voltage drops, so that curve 24 is obtained. Thus, at fixed frequency, the modulation of the illumination of the photoresistor leads to a considerable voltage variation and consequently to a modulation of the magnetic disturbance caused by the probe. When the probe is aperiodic, as in the prior art, its influence on the antenna varies much less when the illumination of the photoresistor changes. Thus, sensitivity is significantly improved by making the probe resonant.

An embodiment of the probe is shown from the front in FIG. 5a and in profile form in FIG. 5b. It is once again possible to see loop 10, photoresistor 12, optical fibre 5 and variable capacitor 20. There is also a shield 22 surrounding the capacitor and the lower half of the loop. The latter is also shielded on its other half by a sheath 24, which is perforated by a slot 26 enabling the magnetic field to enter the loop. Capacitor 20 is accessible from the outside due to a hole 28 made in the shield.

In order to operate in the range 150 to 250 MHz, the loop e.g. has a diameter of 15 mm, i.e. approximately X/100.

In order to excite the light-emitting diode, it is possible to advantageously use a circuit, like that of FIG. 6. A quartz oscillator 30 e.g. functions at 2 MHz. It is followed by a divider 32, e.g. by 4096, which makes it possible to obtain a pulse train with a repetition rate equal to approximately 488 Hz. A circuit 34 makes it possible to vary the cyclic ratio of this signal and optimize the response of the photoresistor. Thus, the response of the latter decreases when the modulation frequency increases, due to the migration time of the carriers in the photoconductor. By limiting the illumination time, saturation of the photoconductor is prevented, so that sensitivity is further improved by said control means.