Broadband Isotropic Probe System for Simultaneous Measurement of Complex E- And H-Fields

The present invention relates to radiofrequency (rf) electromagnetic (EM) field measurements and especially to such measurements made simultaneously and in the same location (same volume of space).

Measurements of rf complex, near field electromagnetic radiation sources, are frequently made to determine the intensity of the electric and magnetic fields at a given location in close proximity to RF sources, the reasons for such measurements being to ascertain electromagnetic radiation hazards to human beings.

At one time, field measurements were made with directive antennas but it was found that these antennas do not reliably measure complicated EM fields, such as those with reactive near-field components, multipath reflections, arbitrary polarization, multiple frequency components, complicated modulations and large field gradients. The accuracy of isotropic probes is independent of polarization of the measured radiation and direction of propagation. Those probes with square-law detection characteristics and broad bandwith overcome the uncertainties created by the complex EM fields because the outputs of three orthogonal antennas (either loops or dipoles) can be summed to obtain the square of the magnitude of the total field.

Previous EM field measurement systems have utilized two probes, one to measure the E-field and one to measure the H-field. In the near field of a radiation source, the EM field changes very sharply per unit volume, i.e., a very small change in spatial position of a three-dimensional probe can result in a large change in measured field intensity. Also, under typical usage situations, potentially hazardous fields generated by industrial or medical devices change rapidly with time (energy is generated for a few seconds per minute in a random pattern). If it is desired to measure, in the same spot, electric and magnetic field strengths which vary spatially and temporally, one must place two separate probes in exactly the same place at the same time, a physical impossibility. Even if the two probes are placed closely together, they cannot measure the respective fields at exactly the same point (volume) in space.

Alternatively, the probes may be placed at the same spot if used sequentially in time. However, the field under study may vary in intensity over the finite interval of time which elapses between performance of each measurement. Hence, measurement uncertainty exists in that the fields may vary rapidly temporally and spatially.

Accordingly, an object of the present invention is to overcome deficiencies in the prior art, such as indicated above; another object is to provide more accurate radiation measurements.

A further object is to measure simultaneously the electric and magnetic field intensities of a complex EM field in the same volume of space.

Yet another object is to provide an EM field probe which is isotropic for both the magnetic and electric fields.

Yet a further object is to provide an EM field probe which has a flat response over the frequency band of interest.

Yet another object is to provide in the same volume of space a set of three orthogonal E-field antennas and a set of three orthogonal H-field antennas.

Still another object is to provide in the same volume of space a set of three orthogonal E-field antennas and a set of three orthogonal H-field antennas which do not significantly perturb each other's field.

A further object of the invention is to provide a circuit means for simultaneously measuring the total electric (E) and total magnetic (H) field intensities. The circuit includes individual square law detectors for each of the three orthogonal E-field antennas and each of the three orthogonal H-field antennas. The circuit also includes additional converters, amplifiers, data processing and storing means, display means and the like, necessary to accomplish this purpose.

These and other objects and advantages of the present invention are achieved by constructing a set of three orthogonal electric-field-measuring antennas and a set of three orthogonal magnetic-field-measuring antennas and effectively locating them so that they encompass the same volume of space. The operating characteristics of the antennas are controlled by filters so that the antennas provide a substantially flat response over a desired frequency range, and also by constructing the antennas small enough, with relation to the wavelength of the highest frequency to be received, so that the EM fields to be measured are not significantly disturbed and the receiving characteristics of all antennas are optimal.

FIG. 1a is a schematic diagram showing a set of three mutually orthogonal dipoles and a set of three mutually orthogonal loop antennas covering the same spatial volume.

FIG. 1b is a schematic diagram showing how the set of dipoles of FIG. 1a may be rotated in their orientation relative to the set of loops to further reduce coupling between antennas.

FIG. 2a is a schematic diagram showing a different arrangement of the three dipoles and three loops in a cubic configuration.

FIG. 2b is a schematic diagram showing a still different arrangement of the dipoles and loops in a cubic configuration to further reduce coupling between antennas.

FIG. 3 is a schematic diagram showing a dipole and a loop antenna and the circuitry used to adjust the receiving characteristics of the antennas so that the response is flat over the frequency range of interest.

FIGS. 4a and 4b are schematic illustrations of orientations of a dipole and a loop with respect to the electromagnetic E- and H-field vectors where the loop pickup ratio of the two orientations is to be measured.

FIGS. 5a and 5b are schematic illustrations of orientations of a dipole and a loop with respect to the electromagnetic E- and H-field vectors where the dipole pickup ratio of the two orientations is to be measured.

FIG. 6 is a overall schematic system diagram of the invention.

A set of three mutually perpendicular dipole antennas (E-field antennas) D.sub.1, D.sub.2 and D.sub.3 and a set of three mutually perpendicular loop antennas (H-field antennas) L.sub.1, L.sub.2 and L.sub.3 are shown in FIG. 1a. These antennas are effectively located in and cover, or encompass, the electromagnetic fields in the same volume of space, which space can be described in relation to an assumed set of Cartesian coordinate axes x, y and z. Thus,

loop L.sub.1 lies in the y-z plane

loop L.sub.2 lies in the x-y plane

loop L.sub.3 lies in the x-z plane

dipole D.sub.1 lies on the y axis

dipole D.sub.2 lies on the x axis

dipole D.sub.3 lies on the z axis.

The antennas may be rotated from exact alignment with their respective coordinate planes and axes, as shown in FIG. 1b, where the dipole antennas are so rotated. The direction cosines of the antennas should be equal. In the figure, .alpha. is the angle dipole D.sub.3 forms with the x axis, .beta. is the angle dipole D.sub.3 forms with the y axis, and .gamma. is the angle dipole D.sub.3 forms with the z axis.

Another scheme for placement of the antennas so that they cover the same volume is shown in FIG. 2a. Here the antennas are located on three walls of a cubic structure 10, D.sub.1 and L.sub.1 being placed on one wall, D.sub.2 and L.sub.2 on another, and D.sub.3 and L.sub.3 on a third. This arrangement can be varied as shown in FIG. 2b, for example. Here the dipoles and loops are placed on different walls to reduce coupling between any two antennas. Thus,

D.sub.1 is on an x-z plane wall at y=0

D.sub.2 is on an y-z plane wall at x=0

D.sub.3 is on an x-y plane wall at z=0

L.sub.1 is on an x-z plane wall at y=1

L.sub.2 is on an y-z plane wall at x=1

L.sub.3 is on an x-y plane wall at z=1

Other antenna arrangements are possible.

An important aspect of the present invention is to assure that the antennas are isotropic (so as to receive arbitrary field polarizations equally), substantially flat in frequency response over the frequency band of interest, and minimally, or insignificantly, perturb the complex near or far EM fields. This can be accomplished by using band-pass filters to shape the diode-detected signals relative to frequency and by utilizing antennas having a length equal to or less than one-tenth the wavelength of the highest frequency to be received. Also, the use of high-resistance leads to the antennas tends to prevent disturbance of the EM fields by the leads. Possible configurations for a dipole antenna D.sub.1 and a loop antenna L.sub.1 are shown in FIG. 3. A Schottky diode d.sub.1 is placed across the gap in dipole D.sub.1 and high-resistance leads 12 and 14 and are used to bring out the detected signal. The inherent source capacitance of the dipole and load resistance of the diode, respectively, form a high pass RC filter. Such filter arrangements are shown in U.S. Pat. Nos. 3,750,017 and 4,008,477. Also see "EM Probe with Fiber Optic Telemetry System", by Bassen, Herman and Hoss, published in Vol. 20, April 1977, Microwave Journal.

It is also possible to utilize resistors R.sub.1, R.sub.2, R.sub.3 and capacitors C.sub.1, C.sub.2 to form the proper frequency shaping filter where inherent capacitive and resistive values of the antenna and diode are not of the proper value to provide desired filter characteristics.

The loop antenna also has a filter and diode arrangement with C.sub.3, C.sub.4, C.sub.5 and C.sub.6 and R.sub.4 forming the filter, and Schottky diode d.sub.2 across the loop antenna end of the high-resistance leads 16 and 18 performing the detection function.

In a test model in which the diameter of each loop antenna was 50 mm and the length of each dipole was 45 mm, both the loop and dipole antennas had a flat frequency response from 10 to 100 MHz within .+-.1 dB. The following component values would be typical for a 50 mm diameter loop antenna with a substantially flat response from 10-100 MHz:

C.sub.3 --20 nanofarads

C.sub.4 --20 nanofarads

C.sub.5 --20 nanofarads

C.sub.6 --20 nanofarads

R.sub.4 --5 ohms.

To minimize any possibility of the production of standing waves by the dipoles and loops, which would perturb the EM fields within the spatial volume of the probe and would interfere with the performance of the antennas, the length of the dipoles and the diameter of the loops are kept equal to or less than one-tenth of the wavelength of the highest frequency to be received by the probe.

FIGS. 4 and 5 indicate various orientations of a dipole and a loop antenna, each lying in the same plane, with respect to the electromagnetic fields. In all cases, the E-field vector lies in the plane of the loop and its direction is upward in the figures. The H-field vector is either perpendicular to the plane of the loop with its direction into the paper away from the observer, or parallel to the plane of the loop. The E-field pickup ratio (V.sub.2 /V.sub.1) of the loop (H-probe) for the antenna orientations of FIGS. 4a and 4b can be better than -13 dB with respect to H-field response. Dipole antenna pattern degradation (due to high-resistance-lead pickup and loop influence on the dipole) is determined by measuring a ratio V.sub.2 /V.sub.1 (for the E-probe) of better than -25 dB can be obtained for the orientations shown in FIGS. 5a and 5b.

FIG. 6 shows an overall schematic circuit of the invention including the circuitry necessary to measure the three individual vector components of the total magnetic field intensity and the three individual vector components of the total electric field intensity, and to display these results separately.

The three sets of antennas are indicated by the legends H.sub.1 through H.sub.3 and E.sub.1 through E.sub.3, together with more or less conventional symbols for magnetic and electric field antennas. The difference in the orientation of the antennas is to indicate the mutual orthogonal arrangement of each set of antennas. Each antenna is associated with a square law detector circuit 30 and a signal conditioning means 32. The output of the signal conditioner 32 is passed on to an analog to digital converter 34, and from there onto a temporary data processing and storage means 36. Thereafter, the accumulated data is sent onto the data display 38. A permanent data recorder 44 can be associated with the system to record results permanently, as needed.

Alternatively, each set, that is the magnetic and electric field antennas, also have the results passed on from their signal conditioners 32 to respective summing amplifiers, an amplifier 40 for the electric field, and a similar amplifier 42 for the magnetic field. The lines from each respective antenna's associated signal conditioner are also controlled by a manual switch, one such switch, unnumbered, being provided in each of the input lines to each respective summing amplifier 40 or 42.

The summed outputs of the two summing amplifiers 40 and 42 are also fed into the main data display 38. A permanent data recorder 44 can be associated with the system to record results permanently, as needed.

The invention system is uniquely capable of simultaneous measurement of all the vector components of both the electric (E) and magnetic (H) fields existing at a single location, at a single instant in time, particularly where steep field gradients exist. This task can be performed with non-directional, electrically-small antennas and the signal processing circuitry described. The prior art for multiple antenna arrays containing both loop (magnetic field ) and dipole (electric field) antennas utilized only electrically large, directional antennas, was without such circuitry to provide the vector sum of both the E and H fields. The present invention includes a total of six non-directional, electrically small antennas, all placed within a small volume of space which has dimensions much less than one wavelength of the electro-magnetic fields being measured.

The square law detectors for each of the three dipole (E) antennas and for each of the three loop antennas provide the squared value of the instantaneous amplitude of each received field vector component. The individual signal conditioners process the outputs of each detector's output voltage, and compensate for imperfections in the detector's squaring performance. All six of the conditioned signals are then sent to the digital signal processing system or alternatively to the analog electronic processing system. In the case of the analog electronic system, the two summing amplifiers 40 and 42 are employed to separately provide the sum of the three electric field antenna's detected output voltages, and the sum of the three magnetic field antenna's detected output voltages. The input channels to each of the summing amplifiers contain switches to select any or all of the three signals, for measurement of portions of, or the total magnitudes of the E-and the H-fields. By summing the squared magnitudes of the all of the vector components, the total E-field and H-field can be measured.

In the case of the digital electronic signal processing system, the multiple channel analog-to-digital converter 34 is used to provide a digital representation of each of the six detected vector(squared) field components. The digital processing sub-system 36 containing temporary data storage capabilities is then used to sum any combination of the three electric field (squared) signals and any combination of the three magnetic field (squared) signals. In addition, all six squared field components are stored individually for later analysis and use.

The final sub-systems include display 38 and recorder 44 which are used for either the analog or the digital signal processing versions of the system. They provide data display and recording functions so that the separate, instantaneous values of the electric and the magnetic field are provided for immediate viewing, and for later analysis using recorder 44.

While the invention has been described in detail above, it is to be understood that this detailed description is by way of example only, and the protection granted is to be limited only within the spirit of the invention and the scope of the following claims.