Conventional antennas primarily receive the electric field component of the electromagnetic radiation; the electric field component induces a voltage in the antenna which is amplified through resonance. A conventional antenna is referred to as "electrically small" if its size is less then one-quarter of the wavelength for which the antenna is tuned. Attempts to construct an efficient, electrically small, antenna have met with several obstacles.
Generally, conventional antennas (i.e., dipoles) have bandwidths of less than 20% of their resonant frequency for useful operation. Larger bandwidths can be obtained with "frequency independent" antennas (i.e., equiangular spirals), however, even they tend to have maximum bandwidths of about 10:1 (i.e., 2-18 GHz). In such spirals, the bandwidth is set by the size of the antenna being .lambda./3 at the lower end of the band and by the electrical size of the antenna feed on the high end of the band. In either case, the size of conventional antennas places at least a lower limit on the frequency of electromagnetic radiation that can efficiently be received. This limit is merged by the sensitivity wavelength correlation. Also, the size of an efficient, low frequency antenna can be prohibitively large for most platforms, so efficiency is often sacrificed to make them smaller. For example, a conventional .phi..sub.0 dBi antenna for detecting 1 MHz signals would be 400 feet in diameter for optimal efficiency.
Another obstacle to constructing an electrically small antenna is that a reduction in the size of the antenna generally results in a corresponding reduction in its bandwidth because of the sensitivity: wavelength correlation. Electrically small antennas must be resonant to absorb power effectively and efficiently from the incident energy. Since electrically small antennas also have a small impedance as seen at the antenna feed, additional methods for achieving resonance will be narrow band.
Still another obstacle associated with conventional antenna systems is the limited linear dynamic range of any preamplifiers connected to the passive antenna. Typically semiconductor preamplifiers have about a 100 dB linear dynamic range in the power output of the amplified signals over a 1 Hz bandwidth. In many applications this dynamic range, along with the associated sideband level increase (due to nonlinearities), is unacceptable. Quite often linear dynamic range requirements of over 130 dB are required in a 1 Hz bandwidth.
Further, since the efficiency of conventional antennas is reduced with their size, noise and other inherent losses become more important when post-processing the antenna's received signal. Increased inefficiency for small antennas is an unavoidable consequence of the low radiation resistance compared to resistive losses of the antenna. Still further inefficiency for small antennas can result from an impedance mismatch between the antenna impedance and the feed line impedance which is typically 50 ohms.
Superdirectivity (i.e., supergain) principles introduce additional problems. Superdirectivity refers to the ability of an electrically small antenna to have the same antenna pattern as an electrically larger antenna. Superdirectivity is typically obtained by producing a phased array of closely spaced conventional antennas. For traditional phased arrays the spacing of the elements is typically less than one half wavelength at the highest operating frequency. The size of the antenna element will determine the phased array bandwidth. For superdirective arrays with even smaller inter-element spacing, the size of each antenna element becomes more important, because further reductions in the antenna efficiency arise from strong mutual coupling between the plurality of closely spaced antenna elements. Consequently, conventional superdirectivity (phased) arrays are inefficient and impractical.
Welker attempted to provide an electrically small, high bandwidth antenna using superconducting quantum interference devices (SQUIDs) as the preamplifier. Welker et al., "A Superconductive H-Field Antenna System," Laboratory for Physical Sciences, College Park, Md. FIG. 2 of the Welker article provides a schematic illustration of the manner in which the SQUID preamplifier is coupled to the antenna in an attempt to improve the bandwidth and sensitivity of an electrically small antenna, but this arrangement suffers from some of the same disadvantages of conventional antenna systems. For example, the pickup loop is inherently narrow band because of its size and method of construction (i.e., the use of resistors and capacitors). Furthermore, the Welker system uses a single inefficient RF biased SQUID which dictates using a much larger pickup loop and produces a reduced linear dynamic range.
Accordingly, it is desirable to provide a small antenna capable of wideband operation, especially an antenna that is efficient and has a large linear dynamic range. It is further desirable to provide an antenna that, instead of detecting the electric component of electromagnetic radiation, produces an output signal in response to the incident magnetic field component.