Minimum-Scattering Antennas

Abstract

Antennas with identical patterns differ to the extent in which they n~odify an incident wave, i.e., in the amount they scatter. An antenna is completely described by an (infinite dimensional) scattering matrix. The concept of a minimum scattering antenna introduced by Dicke is generalized to include antennas with a finite number of accessible waveguide ports and with non-reciprocal componants.A canoniral minimum scattering antenna is defined as on, which becomes "invisible' when the accessible waveguide term-nal.: arc. open circuited. Such an antenna is shown to be unique once the independent radiation patterns have been specified. Neither an impedance nor an --dm.ttaLce matrix for such an antenna exists.The physical significance of the' :n-iinurn scrttt-rirg antenna concept is examined from several points of view. Approirite eaecralizations of Dicke' a results are derived for mu3ktiport ari no--rezip:oczai antennas. The "scattered power 0, is introduced as a convenient measure of &c4Attering. It is demonstrated, for a large class of antennas, t•a the scaterced power is quite generally greater than the absorbed power, equality being attained for minimum scattering antennas cG this class. This result further justifies the mv 4 'niiur-scat.ering terminology.Arrays of canonical antennas are discussed briefly.

On channel capacity of communication via antenna arrays with receiver noise matching

“…The receive antennas are canonical minimum scattering antennas as defined in [18], have negligible Ohmic resistance, and are affected by uncorrelated thermal equilibrium noise.…”

“…PROPOSITION 1 Given Assumptions 1 to 4, the capacity of the multi-antenna channel from T transmit antennas with spatial distribution ρ(θ, φ, r), directivity D u (ϑ, ϕ), and path loss g(r) to a linear array of R receive antennas with directivity D a (ϑ, ϕ) = c D u (ϑ, ϕ + π) lim T →∞ ∞ 0 ρ(θ, φ, r)g(r)r 2 dr, (18) for some constant c, obeys…”

“…Note that (18) is easily fulfilled if the transmit antennas are isotropically distributed and the directivities of the transmit and receive elements differ only by a 180 degrees shift in azimuthal direction.…”

“…In practice, (18) is rarely fullfilled and a result as explicit as (19) is hard to obtain. However, one can get rid of the determinants in (9) in the limit of vanishing inter-element distance.…”

Response of an Antenna to Arbitray Incident Fields

“…1 [1][2][3] where at the input port a and b represent the conventional incident and reflected scattering variables at the physical antenna port (which could be, e.g., a transmission line) while a,6 and bpare infinitely-dimensional column vectors representing, respectively, the where F is the input reflection coefficient, s is an infinite-dimensional column vector representing the so-called modal radiation pattern (T denotes the transpose). In reception, a =0 and the incoming modal amplitudes af8 generate the output b=sTa,, saf6…”