Perhaps the earliest application of the active antenna concept (following that of Hertz) was aimed at solving the small — antenna problem. As we recall, an antenna can be modeled (roughly) by a series RLC network with the R representing the radiation resistance. The input impedance of such a combination is given by
1-і»2/<t>Q + ja>RC jo)C
and so we see that, when the operation frequency w is well below the resonant frequency
and the reciprocal of the RC time constant
t — RC
then the antenna appears as a capacitor and radiates quite inefficiently. The problem of reception is similar. Apparently, already in 1928 Westinghouse had a mobile antenna receiver that used a pentode as an inductive loading element in order to boost the amount of low-frequency radiation that could be converted to circuit current. In 1974, two works discussed transistor-based solutions to the short-aerial problem (36,37). In Ref. 37, the load circuit appeared as in Fig. 34. The idea was to generate an inductive load whose impedance varied with frequency, unlike a regular inductor, but so as to increase the antenna bandwidth. The circuit’s operation is not intuitively obvious. I think that it is possible that most AM, short-wave, and FM receivers employ some short-antenna solution whether or not the actual circuit designers were aware that they were employing active antenna techniques.
Another set of applications where active devices are essentially used as loading elements is in the greater-than-100-
Figure 34. A circuit taken from Ref. 37 in which a transistor circuit is used to load a short antenna. Analysis shows that, in the frequency regime of interest, the loading circuit appears, when looking toward the antenna from the amplifier terminals, to cancel the strongly capacitive load of the short antenna.
GHz regime. Reviews of progress in this regime are given in Refs. 1 and 38. To date, most work at frequencies greater than 100 GHz has involved radio-astronomical receivers. A problem at such frequencies is a lack of components, including circuit elements so basic as waveguides. Microstrip guides already start having extra-mode problems at Ku band. Copla — nar waveguides can go higher, although to date, rectangular metallic waveguides are the preferred guiding structures past about 60 GHz. In W band (normally narrowband, about 94 GHz—see Table 1), there are components, as around 94 GHz there is an atmospheric window of low propagation loss. However, waveguide tolerances, which must be a small percentage of the wavelength, are already severe in W band, where the wavelength is roughly 3 mm. Higher frequencies have to be handled in free space or, as one says, quasi-optically. Receivers must therefore by nature be downconverting in this >100 GHz regime. Indeed, these types of solutions are the ones being demonstrated by the group at Michigan (38), where receivers will contain multipliers and downconverting mixers right in the antenna elements in order that CPW can be used to carry the downconverted signals to the processing electronics. Millimeter-wave-terahertz radio astronomy seems to be a prime niche for quasioptical active antenna solutions.
The first applications of active antennas where solid-state components were used as gain elements were primarily for power boosting (39-44). Power combining (see reviews in Refs. 45 and 46) can be hard to achieve. There is a theorem that grew out of the early days of radiometry and radiative transfer (in the 1800s), known variously as the brightness theorem, the Lagrange invariant, or (later) the second law of thermodynamics. (See, for example, Ref. 8, Chap. 5.) The theorem essentially states that one cannot increase the brightness of a source by passive means. This theorem practically means that, if one tries to combine two nominally identical sources by taking their outputs, launching them into waveguides, and then bringing the two waveguides together in a Y junction into a single waveguide, the power in the output guide, if the output guide is no larger than either of the input guides, can be no greater than that of either of the nominally identical sources. This seems to preclude any form of power combining. There is a bit of a trick here, though. At the time the brightness theorem was first formulated, there were no coherent radiation sources. If one takes the output of a coherent radiation source, splits it in two, and adds it back together in phase, then the brightness, which was halved, can be restored. If two sources are locked, they are essentially one source. (As P. A. M. Dirac said, a photon only interferes with itself. Indeed, the quantum mechanical meaning of locking is that the locked sources are sharing a wave function.) Therefore, locked sources can be coherently added if they are properly phased. We will take this up again in a following paragraph.
An alternative to power combining that obviates the need for locking and precise phase control is amplification of the signal from a single source at each element. By 1960, solid — state technology had come far enough that antennas integrated with diodes and transistors could be demonstrated. The technology was to remain a laboratory curiosity until the 1980s, when further improvements in microwave devices were to render it more practical. Recent research, however, has been more concentrated on the coherent power combining of self-oscillator elements. This is not to say that the element — mounted amplifier may not still be of practical use. The main research issue at present, though, is the limited power available from a single active element at millimeter-wave frequencies.
Another application area is that of proximity detection (47). The idea is that an oscillator in an antenna element can be very sensitive to its nearby (several wavelengths) environment. As was discussed previously, variation in distances to ground planes changes impedances. The proximity of any metal object will, to some extent, cause the oscillator to be aware of another ground plane in parallel with the one in the circuit. This will change the impedance that the oscillator sees and thereby steer the oscillator frequency. The active antenna of Ref. 47 operated as a self-oscillating mixer. That is, the active element used the antenna as a load, whereas the antenna also used a diode mixer between itself and a low — frequency external circuit. The antenna acted as both a transmitting and a receiving antenna. If there were something moving near the antenna, the signal reflected off the object and rereceived might well be at a different frequency than the shifting oscillator frequency. These two frequencies would then beat in the mixer, be downconverted, and show up as a low-frequency beat note in the external circuit. If such a composite device were to be used in a controlled environment, one could calibrate the output to determine what is occurring. Navarro and Chang (1, p. 130) mention such applications as automatic door openers and burglar alarms. The original paper (47) seemed to have a different application in mind, as the term Doppler sensor was in the title. If one were to carefully control the immediate environment of the self-oscillating mixer, then reflections off more distant objects that were received by the antenna would beat with the stable frequency of the oscillator. The resulting beat note of the signals would then be the Doppler shift of the outgoing signal upon reflection off the surface of the moving object, and from it one could determine the normal component of the object’s velocity. It is my understanding that some low-cost radars operate on such a principle. As with other applications, though, the active antenna principle, if only due to size constraints, becomes even more appealing at millimeter-wave frequencies, and at such frequencies power constraints favor use of arrays.
An older antenna field that seems to be going through an active renaissance is that of retroreflection. A retroreflector is a device that, when illuminated from any arbitrary direction, will return a signal directly back to the source. Clearly, retro — reflectors are useful for return calibration as well as for various tracking purposes. An archetypical passive retroreflector is a corner cube. Another form of passive reflector is a Van Atta array (48). Such an array uses wires to interconnect the array elements so that the phase progression of the incident signal is conjugated and thereby returned in the direction of the source. As was pointed out by Friis already in the 1930s, though, phase conjugation is carried out in any mixer in which the local oscillator frequency exceeds the signal frequency (49). (A phase conjugate signal is one that takes on negative values at each phase point on the incoming wave.) This principle was already being exploited in 1963 for implementing retroreflection (50). This work did not catch on, perhaps for technical reasons. A review in 1994 (51) and designs for such arrays were demonstrated and presented at the 1995 International Microwave Symposium (52,53). Although both demonstrations used transistors and patch-type elements,
Source bias Gate bias
Figure 35. Schematic depiction of (a) the active surface of a grid oscillator and (b) a breakout of an internal region of the grid showing the active device placement relative to the bias lines.
both also employed circulators for isolation and therefore were not actually active array demonstrations. It would seem that retroreflection should motivate an active self-oscillating mixer solution, which will perhaps appear in the future.
As was mentioned earlier in this article, a quite important application area for active antennas is free-space power combining. As was pointed out then, a number of groups are working on developing compact elements such as those of Fig. 14 (7) and Fig. 30 (21). As was also previously mentioned, in order to do coherent power combining, the elements must be locked. In designs where the elements are spatially packed tightly enough, proximity can lead to strong enough nearest — neighbor coupling so that the array will lock to a common frequency and phase. Closeness of elements is also desirable in that arrays with less than A/2 spacing will have no side — lobes sapping power from the central array beam. In designs that do not self-lock, one can inject a locking signal either on bias lines or spatially from a horn to try to lock to all elements simultaneously. Of course, the ultimate application would be for a high-bandwidth, steerable, low-cost transceiver.
Another method of carrying out power combining is to use the so-called grid oscillator (54,55). The actual structure of a grid appears in Fig. 35. The operating principle of the grid is quite a bit different from that of the arrays of weakly coupled individual elements. Note that there is no ground plane at all on the back, and there is no ground plane either, per se, on the front side. Direct optical measurements of the potentials on the various lines of the grid (56), however, show that the source bias lines act somewhat like ac grounds. In this sense, either a drain bias line together with the two closest source biases, or a gate bias line together with the two horizontally adjacent bias lines, appears somewhat like CPW. The CPW lines, however, are periodically loaded ones with periodic active elements alternated with structures that appear like slot antennas. The radiating edges of the slots are, for the drain bias lines, the vertical ac connection lines between drain and drain or, for the gate bias CPW, the horizontal ac gate-to-gate connection lines. Indeed, the grid is known to lock strongly between the rows and more weakly between columns. As adjacent row elements are sharing a patch radiator, this behavior should be expected.
In a sense, this strong locking behavior of the grid is both an advantage and a disadvantage. It is advantageous that the grid is compact (element spacing can be < A/6) and further that it is easy to get the rows to lock to each other. However, the compactness is also a disadvantage in that it is quite hard to get any more functionality on the grid. Much effort has been made in this area to generate functionality by stacking various grid-based active surfaces such as amplifying surfaces, varactor surfaces for frequency shifting and modulation, doubling surfaces, etc. A problem with stacking is, of course, diffraction as well as alignment. Alignment tolerance adds to complexity. Diffraction tends to ease alignment tolerance, but in an inelegant manner. A 100-transistor array with A/6 spacing will have an extent of roughly 1.5A per side. As the diffraction angle is something like the wavelength divided by the array diameter, the diffraction angle for such an array is a good fraction of a radian. One can say that grids are quasi-optical, but in optics one generally doesn’t use apertures much smaller than a millimeter (center optical wavelength of micrometers), for which the diffraction angle would be roughly a thousandth of a radian. As far as pure combining efficiency goes, grids are probably the optimal solution. However, more functionality may well be hard to obtain with this solution.
As we have mentioned, there are a number of techniques for steering being investigated. There seems to be less work on modulation, and I do not know of any simultaneous steering of modulated beams to date. Although the field of active antennas began with the field of radio frequency, it still seems to be in its infancy. However, as I hope this article has brought across, there is a significant amount of work ongoing, and the field of active antennas will grow in the future.
Figure 1. Resistor configuration for the ‘‘T’’ attenuator. Zj and Zo are the resistive impedances presented to the attenuator by external circuits.