The present invention relates to a system fortesting an object by means of electromagnetic radiation. Such a system emits electromagnetic radiation that penetrates into the object under test, such as the ground, a body, etc.. Use of such systems can thus be found in civil engineering, medical imaging, and so on.

The system comprises a signal generator for generating a testing signal within a desired ultra-wide frequency band, which frequency band has limits f _low and f _high , and an ultra-wide band antenna system, that in turn comprises a current loop, and a power unit arranged to receive the testing signal and to amplify and feed said testing signal into the current loop. The current loop comprises an electrically small and conducting radiator element to emit the amplified testing signal, an electrically conducting return plane, a layer of absorber material between the radiator element and the return plane to absorb electromagnetic radiation in said frequency band, and at least a first and second electrical lead for connecting said radiator element and said return plane. The power unit comprises a transistor circuit with at least one transistor.

Herein, the first electrical lead is connected between the radiator element, in the following sometimes called radiator plate, and the return plane, in the following sometimes called ground plane. The power unit forms part of the connection between the radiator element and the return plane, and may be connected between either and the first electrical lead. It is also possible to have a two-part first electrical lead, wherein the power unit is connected to the radiator element via a first part of the first electrical lead and is connected to the return plane via a second part of the first electrical lead.

The desired ultra-wide frequency band is the desired band of frequencies emitted electromagnetic radiation, that is to be used for the actual testing of the object. This desired frequency band has the lower and upper limits f _low , f _high , respectively. This band depends on the type of testing and the type of object to be tested. The antenna system should be able to receive the input signal and emit same over the complete desired ultra-wide frequency band. The frequency band which the antenna system should be able to handle in this way should be at least as wide as the desired frequency band, and preferably as wide as possible, to be able to deal with as many types of testing and objects as possible. The other way around, it is also possible to define the frequency band which the antenna system is able to deal with as the band from f _low to f _high , and define that the testing signal should then have limits between those f _low and f _high . For the rest of this description, use will be made of the former definition.

Furthermore, the power unit is arranged to receive the testing signal and to feed the testing signal into the current loop. Thereto, the power unit is of course electrically connected into the current loop.

Moreover, the radiator element being "electrically small", also "electrically short", has the standard meaning of the radiator element having a relevant dimension, such as the length of the radiator plate, that is much shorter than the "characteristic length" of the signal (e.g. the (lowest) wavelength of the emitted electromagnetic radiation, the coherence length, and so on). The behaviour of the antenna will then largely be that of a dipole, which has a number of advantages in applications of the system, such as a spherical wavefront of the emitted radiation, a short impulse response and a short nearfield zone.

Such systems are known in the art. The most basic ones are Hertzian dipoles, but modified dipole antenna systems are also known, that allow higher currents and thus higher signal strengths. More specifically, US4,506,267 discloses a "Large Current Radiator", with a current loop antenna said to have a frequency independent operation. However, the document is silent how this would be achieved, or how the system as a whole is operated.

However, low-frequency signals turn out to be suppressed in such systems, which is undesirable for many applications, since the penetrating power for higher frequencies if often quite low. Thus, low frequency signals would be emitted with low power, and high frequency signals have low penetrating power, so that in all the useful frequency range becomes rather small.

It is an object of the present invention to provide a system of the kind described above, that does not, or at least to a lesser extent, have the above disadvantage.

Thereto, the present invention provides a system according to claim 1 , in particular a system for testing an object by means of electromagnetic radiation, comprising a signal generator for generating a testing signal within a desired ultra-wide frequency band, which frequency band has limits f _low and f _high , and an ultra-wide band antenna system, comprising a current loop, and a power unit arranged to receive the testing signal and to amplify and feed said testing signal into the current loop, wherein the current loop comprises an electrically small and conducting radiator element to emit the amplified testing signal, an electrically conducting return plane, a layer of absorber material between the radiator element and the return plane to absorb electromagnetic radiation in said frequency band, and at least a first and second electrical lead for connecting said radiator element and said return plane, wherein the power unit comprises a transistor circuit with at least one transistor, wherein the transistor circuit is arranged to have a frequency-dependent gain curve that has a gain drop above a cut off frequency f1 of substantially -20 dB/decade, wherein f1 is smaller than f _high .

With such measures, the inventor has found that the useful frequency band becomes much wider, in particular the frequency band in which a signal is emitted with frequency independent behaviour/power. Without wishing to be bound by an explanation, it is believed to be caused by a compensation of the derivative action from the dipole antenna by a kind of integrating action from the behaviour of the transistor circuit. Herein, it is noted that the strength of the radiated electric field of the known system increases proportionally with frequency, which causes a "derivative action", that suppresses the lower frequencies. By operating the transistor circuit in the decaying part of the gain curve, this suppression is more or less compensated, and the nett response curve becomes a substantially flat line, i.e. frequency independent. So the idea behind it is to use the transistor circuit, in particular the transistor, in a frequency range in which it is normally not used precisely because the gain drops as a function of frequency. In the present invention, this gain drop is used to counteract a fundamental property of electrically small antennae, which antennae in turn are useful in many applications just because they are small.

It is expressly noted that, if f 1 is between f _low and f _high , the transistor circuit is able to compensate the derivative action in only a part of the desired frequency band, namely between f1 and f _high . This still opens up the possibility to use a signal generator that only needs to compensate this antenna-based gain drop for frequencies below f1. However, this still complicates the signal generator due to the demand of a frequency dependent output spectrum.

In the framework of the present invention, an "ultra-wide frequency band" means a frequency band between a lower frequency of at least 10 MHz and an upper frequency of up to 10 GHz, although it need not cover this whole range. The ultra-wide frequency band should also have a fractional bandwidth of at least 0.2 and, in an absolute sense, of at least 50 MHz. In specific cases, such as according to regulations, this may be limited to having an absolute bandwidth of at least 500 MHz. Examples of such ultra- wide frequency bands are 30-300 MHz for ground penetrating radar, 100 MHz - 2 GHz for pavement monitoring and 0.5 - 5 GHz for medical imaging. Of course, other limits are also possible, both for the mentioned applications and for other applications. Further advantageous measures and embodiments are described in the dependent claims, as well as in the introductory part that follows.

In embodiments, f1 is at most f _low . This means that the transistor circuit as a whole is now capable of compensating the derivative action in substantially all of the desired frequency band. This means that rather simple signal generators may be used, for example those that provide a frequency independent power, i.e. have a flat power spectrum in the desired frequency (band), such as simple sweeped or stepped sine wave generators.

In embodiments, the transistor circuit comprises a power transistor that has a gain cut-off frequency f _power , wherein f _power is between f _low and f _high , in particular between f1 and f _high . Power transistors are useful, often even necessary, to provide sufficient power to the signal for some applications, such as ground penetrating radar, or when crossing large distances. The present invention is particularly useful in such cases because such power transistors, in particular the simpler and cheaper ones, often have a relatively low cut-off-frequency, which is in fact desirable in the present invention. At least there are more kinds of power transistors to choose from when allowing such low cut-off frequencies.

In many cases, the gain of a power amplifier is not particularly high, so that the frequency dependency of the output current into the radiator element above f1 , i.e. in the used frequency band, relatively quickly drops below the noise level. This makes the useful bandwidth often relatively small. In order to compensate this, in particular, the transistor circuit comprises a preamplifier circuit with a negative feedback circuit, wherein the preamplifier circuit has a gain curve that has a gain cut-off frequency f _pre below f _power and has a gain drop of substantially -20 dB/decade. Preferably, f _pre is as low as possible, such as down to f _low . All this serves to lengthen the part of the gain curve that has a gain drop of 20 dB/decade. Put differently, the preamplifier and the network, that may comprise the preamplifier and optionally also the (power) transistor, besides other network components such as resistors and the like, are arranged such that the frequency dependent gain curve below f _power is no longer a constant value, but increases by 20dB/decade down to f _pre . Then the frequency dependent gain curve of the preamplifier circuit as a whole has a drop, with a steepness of -20 dB/decade, to substantially unity between f _pre up to f _power , and the total gain curve of the preamplifier circuit with the power amplifier has a gain curve that drops to unity between said frequency f _pre and f _high or the frequency determined by the noise level.

As mentioned above, the present invention allows the use of a simpler signal generator, that need not compensate the output signal for the derivative action of the antenna, at least not over all the desired frequency band. In embodiments, the signal generator comprises an arbitrary wave generator, a pulse generator or a binary pseudonoise generator (e.g. m-sequence generator) or a sine wave generator that steps or sweeps the frequency over the desired band.

In particular, f1 is between f _low and f _high , and the signal generator provides a spectral power that substantially increases by 20 dB/decade between f _low and f1 , and is substantially constant from f1 to f _high . In these embodiments, the signal generator can provide sufficient power to the signal to compensate for a constant, i.e. non-dropping, gain of the transistor circuit between f _low and f1 , i.e. in those frequencies where the gain is substantially frequency independent. Even though this limits the use of simple signal generators to a more narrow band, the advantages remain the same. However, it is more preferable to operate this system with a simpler signal generator, with a constant spectral power, and ensure that f1 is at most as large as f _low , such that the signal generator can operate it in the full desired frequency band from f _low to f _high .

The invention will now be explained further with reference to a number of exemplary embodiments as well as the drawings, in which:

Figure 1a, b shows a prior art antenna system,

Fig. 2 shows a diagrammatical cross-sectional view of a first system 100 according to the present invention,

Fig. 3 shows another embodiment of a system 100' according to the invention, in a diagrammatical cross-sectional view,

Fig. 4 diagrammatically shows an embodiment of a system 100" according to the invention,

Fig. 5 shows the behaviour of the system 100" of Fig. 4,

Fig. 6 shows the general frequency behaviour of the antenna system,

Fig. 7 shows an equivalent circuit for another embodiment 100", for high- power applications, and

Fig. 8a - 8c show diagrams of the transfer function (output current) of the LCR without (8a) and with (8c) a preamplifier of gain curve 8b.

Figure 1 a, b shows a prior art antenna system, as provided in US patent no. 4,506,267. Fig. 1a shows a cross-sectional view, and Fig. 1b a diagrammatic perspective view. The system comprises a signal generator 10 in a metal casing 11 (= 21), and connected by means of wires A and trapezoidal sections B (=16) and D (= 17) to a radiating plate C (=15). The casing extends in a metal plane 18 that is clad with an absorber 22 for the emitted electromagnetic radiation.

The source "G" (=10) is shown to provide some signal to the radiating part C. However, no details are given as to its nature or properties. However, it is known that electrically short antenna function as a (Hertzian) dipole. This means that there is a kind of "derivative" action to the electrical field strength, in that that field strength E varies with frequency. For sinusoidal currents, this becomes E(f, r) = (C ₁ /r) _. f _. exp[j _. C 2 _. f _. r]., wherein C1 and C2 are constants, and r is the distance to the antenna. Thus, low frequencies are suppressed, while high frequencies are relatively strong. This is undesirable, since it lowers the effective useful bandwidth.

Fig. 2 shows a diagrammatical cross-sectional view of a first system 100 according to the present invention. The system 100 comprises a power unit 101 with a transistor circuit 102 and a power source 103 and a power supply bypass capacitor 104, across an input source indicated at 105, between ground and the gate of the transistor.

The antenna part comprises a large current radiator (LCR), that is a current loop with a radiating plate 110, a first electrical lead 111 , a second electrical lead 112, and a return plate or ground plane 113. A layer of EM radiation absorbing material is indicated with 114.

In this embodiment, an input signal is supplied at the input source 105, such as by an arbitrary wave generator, a sine wave generator, that is stepped or swept, a pseudonoise generator or the like. The supplied signal is amplified by means of a transistor circuit 102, that is powered by a DC-power supply, indicated diagrammatically at 103. In order to short-circuit the DC-source for AC-signals, it is bridged by the power supply bypass capacitor 104, as is usual. Together, the parts 102, 103 and 104 form a (general) power unit 101.

The amplified signal is supplied to the current loop of the LCR, that is, via the first electrical lead 111 to the radiator plate 110, and then via the second electrical lead 112 and the return or ground plane 113 back to the power source 103. The effectively radiating part is the radiating plate 110, since the far-field contributions of the two electrical leads 111 and 112 cancel out, due to the opposite direction of the respective currents. The radiating plate 110 is an electrically short plate with a length h, in the applied range of frequencies, according to the invention an ultra-wide frequency band as explained in the introduction. In order to prevent negative effects from the radiated electromagnetic waves on the emitted field, in particular a cancelling out by radiation emitted by the ground plane 113, there is provided a layer 114 of material that absorbs a (very) large part of the emitted electromagnetic energy. The thickness and other properties thereof should be adapted to the particular frequencies used.

There are many variations possible to the basic set-up of the system 100 as sketched above. For example, The absorber layer 114 may be sandwiched to the ground plane 113, with the first electrical lead 111 now penetrating both plane 113 and layer 114. The transistor circuit 102 may then e.g. be connected "above" the first electrical lead 111 , and thus directly at the radiating part, which may lower the lead inductance. Furthermore, the transistor circuit 102 may be connected according to a common source circuit, or a common drain circuit, as is known in the field.

The present invention relates to operating the transistor circuit in the decaying part of its gain curve. Below, besides a more general explanation, some advantageous embodiments will be shown.

So in general, having a transistor circuit, one can choose to operate the system 100 in a frequency band in which the gain of the transistor circuit 102 drops. This drop may itself have any of a number of different values, depending on the set-up of the transistor circuit 102. However, since the electrically short antenna (radiating plate 110) has a "derivative action", which means a gain rise of 20 dB/decade, the gain drop of the transistor circuit 102 is chosen, according to the invention, as -20 dB/decade, above the cut-off frequency of the transistor circuit. By itself, a transistor has a gain drop of 20 dB/decade, and the transistor circuit as a whole should have that same gain drop. Of course, the cut-off frequency should be within the required frequency band. Herein, it is noted that any kind of gain or gain drop in the system 100 may be compensated by correspondingly adapting the signal, such as by means of a completely programmable arbitrary wave generator. However, this is a trivial, but technically complicated and expensive solution, that is not encompassed by the invention. Rather, according to the invention, the gain drop above the transistor circuit's cut-off frequency is actively used to counteract the frequency dependent gain by the antenna's "derivative action", resulting in a rather broad frequency independent transfer function of the system 100.

Fig. 3 shows another embodiment of a system 100' according to the invention, in a diagrammatical cross-sectional view. Herein, as in the rest of the drawings, similar parts are denoted with the same reference numerals, possibly with one or more primes.

The system 100' comprises again a radiating plate 110', with first and second electrical leads 111' and 112' and a return or ground plane 113', as well as a layer 114' of absorbing material, now oriented vertically in the drawing.

The power unit 10T now comprises basically the same transistor circuit 102', that is extended by a (negative) feedback loop, comprising feedback element FB and a resistor R around a preamplifier 106, to further control the frequency transfer function of the feeding transistor circuit 102'. This will be explained further with reference to Fig. 4. It is noted that the absorber layer 114' is now positioned "vertically", but this only means that a different part of the current loop effectively radiates into the far field. Of course, the dimension h should then be adapted accordingly.

Fig. 4 diagrammatically shows the equivalent circuit of an embodiment of a system 100" according to the invention. Herein, the transistor circuit 102" is shown to comprise an operational amplifier 107, and a feedback network of two resistors R ₁ and R ₂ , the current sensor R ₃ and an optional capacitor 108. The current loop of the LCR is now shown generally as an inductivity 109.

This embodiment is particularly useful in case only a low power electromagnetic field is needed, such as for medical imaging or short range sensing. In these cases, low feeding currents are sufficient. Thus, power transistors are not necessary, and a simple operational amplifier 107 suffices. The resistor R ₃ converts the output current / into a voltage, that is fed back via R ₂ to the input of the amplifer 107. With the help of selecting R ₁ and R ₂ , the gain of the circuit, and the length (cut-off frequency) of the decaying part may be selected.

The behaviour of the system 100" of Fig. 4 will be explained with reference to Fig. 5. This shows the output current i ₀ as a function of frequency, with a constant, frequency-independent input voltage. The scales are logarithmic.

The amplifier circuit provides the highest current at lower frequencies. In principle, the gain, and thus the output current, is constant below a cut-off frequency. For a small ratio R ₂ /R ₁ , the circuit has low gain. The maximum output current i ₀ is then e.g. i ₁ . The related gain curve is composed of branches 42 (below a cut-off frequency f ₁ ), and 40 (above f ₁ ) in the diagram, dropping with 20 dB/decade (in branch 40). Above a frequency f _um , the output current is only composed of noise level 44, and is no longer useful.

Herein, the cut-off frequency f ₁ is determined by semiconductor technology. The useful frequency band UB1 for frequency independent antenna operation runs from f ₁ to f _um . In other words, the bandwidth from f ₁ to f _um is useful to counteract the differentiation action of the electrically short antenna, and obtain a constant electric field strength in the far field.

Fora higher ratio R ₂ /R ₁ , the circuit gain is higher, the cut-off frequency goes down to f' ₁ , and the maximum output current becomes i ₂ . The gain curve is now composed of the constant gain branch 45 and the branch 41+40 with decaying gain, again up to f _um . The useful frequency independent antenna operation band UB2 extends from f' ₁ to f _um .

Thus, by selecting the ratio of the resistors R ₁ and R ₂ , one can play with the useful bandwidth, to adapt it to the desired frequency band. Of course, the properties of the operational amplifier 107 should also be accounted for in such a selection. In case its high-frequency properties are better than required, one can shift the decaying part of the gain curve (branches 40/41) to lower frequencies f” ₁ ... f” _um (dashed branch 43) by modifying the feedback network, such as by adding a parallel capacitor 108 in the feedback loop in Fig. 4. One then obtains useful frequency band UB2'. In all this, it is noted that the respective band UB1 , UB2 or UB2' is in each case, according to the invention, the maximum useful "desired frequency band" that makes full use of the inventive thought of using the transistor circuit's gain drop to counteract the derivative action of the electrically short antenna. In each case, the bandwidth for the desired frequency band may be increased, i.e. widened, to lower frequencies by accepting that for the frequencies under f ₁ , f' ₁ , f” ₁ , respectively, the signal generator should compensate said derivative action by accordingly lowering the power in the output spectrum for those frequencies.

In the above, it was assumed that the amplifier-LCR-loop does not limit any further. However, in practice, the actual properties of the output loop of the amplifier are determined by the output admittance γ ₂₂ , the inductor and radiation losses, which are summarized by R _α and the total LCR inductance L. This leads to a cut-off frequency f _u = 1/2πτ _L of the output loop, with T _L = LIR _α . This frequency f _u should again be at most equal to f _um , and preferably sufficiently far removed from f _um in order to have a signal that is sufficiently free from interference by the noise signals.

Fig. 6 shows the general frequency behaviour of the antenna system. The diagram thus shows the strength of the electric field E normalised to the observation distance r as a function of the frequency f. Both the horizontal and the vertical scale are logarithmic.

At low frequencies, i.e. branch "A", below f _1x (i.e. f ₁ l f' ₁ l f” ₁ ), the differentiation ("derivative action") of the radiating plate of the LCR dominates, and the strength of the electromagnetic radiation increases (with 20 dB/decade). Above a higher frequency f _u , i.e. branch "C", the twofold integration (explained below) dominates, and the strength of the electromagnetic radiation drops with 20 dB/decade. Between f _1x and f _u , the two actions, viz. differentiation of the radiator and integration of the transistor circuit, cancel out, and the behaviour is a constant, frequency-independent strength of the electromagnetic radiation. This operational band B between f _1x and f _u thus gives a truly frequency-independent system, which greatly increases the possibilities for providing an output signal, i.e. emitted electromagnetic radiation, over a broad frequency range.

The frequencies f _1x and f _u < f _um respectively, may be determined from the properties of the circuits, according to f _1x = lowest of [1/(2π _. τ _L ), 1/(2π _. τ _T )], wherein

T _L = LR _α , in which L represents the total inductance of the large current loop conductor and R _α covers the dissipation and radiative losses of the loop radiator as well as the output admittance of the transistor circuit, and T _T is the characteristic time constant of the transadmittance y ₂₁ behaviour of the transistor (or transistor circuit including feedback components), according to y ₂₁ = y ₀ /(1+j2πf T _T ), wherein yo is the dc-transadmittance of the transistor or the cascade of amplifier and transistor.

Fig. 7 shows an equivalent circuit for another embodiment 100", for high- power applications. The power unit has a power transistor 102" that is cascaded with a preamplifier circuit 115 with an operational amplifier 106” and a feedback circuit FB1 , FB2, that covers only the preamplifier (operational amplifier). For the analysis of its behaviour, both parts have to be analysed, as follows.

Fig. 8a shows the spectrum of the output current 50 of the power transistor 102". The maximum useful part, i.e. in the decaying part of the gain curve, covers the frequency band UB1 from f _power to f _um , with f _power the cut-off frequency for the transistor, and f _um the noise limit, as above.

Fig. 8b shows the gain curve 51 of the preamplifier, with a decaying part between f ₁ and f _power , with f ₁ as described above.

Fig. 8c shows the result of combining the preamplifier 106" and the power transistor 102". Here, the decaying part extends from f ₁ to f _um . This is now an enlarged maximum useful bandwidth UB2 for frequency-independent operation of the system.

Note that the restriction/assumption mentioned for Figure 5, concerning the upper cut-off frequency, holds here as well. Also note that, since the present invention is primarily interested in the decaying part of the gain/transfer function, power transistors and/or (pre)amplifiers with a relatively low cut-off frequency may be selected, which increases the number of possibilities to select appropriate transistors and lowers their cost. However, in many cases this will come at the cost of gain, thus leading to a shorter, more narrow useful frequency band. Yet, the invention opens up a number of possibilities for selecting components for a useful desired ultra-wide frequency band antenna system, that uses the insight to combine an electrically short radiator with a power unit with a gain that decays (with 20 dB/decade) in that desired frequency band.