form of antenna pattern
Specific features of constructing highly informative radio systems which use short ultrawideband (UWB) signals and antennas with large aperture are considered. As examples of such systems, we study radars ensuring resolution of closely neighboring targets (for instance, MIRV warheads) and radars detecting a target type by the set of its resolved elements. Another example of highly informative radio systems are radiocommunication systems which provide both transmission of large amounts of information due to the wide frequency band and stealth communication due to concentration of energy in the narrow (aiming) lobe of the antenna.
As a rule, in the such type of radio systems, a signal duration in space is shorter than physical dimensions of the antenna aperture. Due to this fact, a signal substantially varies its shape during the process of radiation and reception. In its turn, variation of the signal shape yields the change of the structure of antennas radiation patterns unlike the narrowband antennas which are excited by a harmonic oscillator.
To understand the nature of such variations, the process of antenna excitation by a short pulse signal is considered. A linear radiator is taken as an antenna model, which is excited from its one end and has a matched load. Using the expressions for the far-zone electric field of the Huygens element with an arbitrary current time distribution, the field of the linear radiator was determined as a superposition of fields of its elementary segments. Possibility of using currents with arbitrary time distribution allows us to obtain relations between an exciting current shape and a form of the radiated field at different ratios between signal duration in space and antenna aperture dimensions for different angles of observation. Obtained expression gives the possibility to find the radiation pattern as a function of the electric field component. It is shown that this pattern changes its spatial location as the current pulse “travels” along the radiator. Since such radiation pattern is not convenient for computations of radio systems parameters, an energy radiation pattern is introduced, which characterizes the distribution of radiated energy in angular coordinates (the power radiated during the time of current pulse travel along the radiator).
Application of the analogous approach for analysis of processes in the receiving antenna, gives us the dependence of voltage on the load from angular coordinates, i. e., its radiation pattern. It is shown that in this case it is also necessary to use the energy radiation pattern whose shape depends on the shape of the incident field, i. e., on the relative position of radiating and receiving antennas.
If the signal spatial duration is essentially (several times) shorter than the antenna aperture physical dimensions, antenna radiation and receiving patterns are different. The latter fact does not allows using the same antenna both in radiation and in receiving regimes. Here, according to the theorem of reciprocity, shapes of both antennas patterns in the same regime (radiation or receiving) are identical. After changing antennas regimes they exchange their patterns.
Signal delay in antennas and mutual position of antennas influence not only the pattern shape but also shapes of signals during their passage from the radiating antenna input to the receiving antenna output. Variations of complex Gaussian pulses with or without high-frequency filling, as well as complex signals with intra-impulse linear frequency modulation, are shown in radiation and receiving regimes as a function of the signal delay during its passage along the antenna. Due to the complex signal structure variations, its processing by the matched filter placed on the receiving antenna output is ineffective.
The mentioned above processes are simulated in MathCad 13. Physical experiments confirm variations of the UWB signal shape at its radiation by an antenna with large aperture.