Resonant loop current disturbing probe

A resonant loop current disturbing probe. This probe comprises a conductive loop closed on a photoresistor, a light-emitting diode optically coupled to the photoresistor by an optical fiber and a diode control circuit. This probe is characterized in that it also comprises a variable capacitor connected in parallel to the loop. The loop-capacitor assembly forms a resonant circuit, the capacitor being adjusted in such a way that said circuit resonates at the frequency of the current to be disturbed. Application is to the measurement of the current in antennas.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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.