Physical limitations on interference reduction by antenna pattern shaping

The problem of minimizing the signals received from interfering or undesirable signal sources by appropriately modifying the antenna radiation pattern is addressed. The solution is presented in terms of the modal expansions of the monochromatic electromagnetic fields outside the radiating structure, and appropriate optimization of these radiation patterns. Physical realizability of the results is assured by requiring that the allowable fields are derived from nonsuper gain antenna excitations; hence the results represent an upper bound on the performance. We consider two complementary, one-dimensional antenna structures, i.e., a circumferentially symmetric line source of length
a
, and an axially independent cylindrical antenna of radius
\rho_{0}
. We consider first the case of
N
discrete interfering signals and compute the loss in antenna directivity when the radiation pattern is modified so as to place a null at each angular position of the interfering signal sources. The results indicate that if all interfering signals are located outside the main beam, the loss in directivity is negligible. When one or more undesired signal sources are in the main beam, the antenna directivity will be reduced appreciably. It is shown that when uniformly distributed noise is superimposed over the discretely located interfering signals, the same antenna pattern maximizes the signal/noise ratio for those practical cases when the interfering signal power is significantly greater than the uniformly distributed noise source power. It is further shown that introducing noise uniformly distributed over all space is equivalent to adding thermal noise at the antenna terminals, as would occur with the receiver. Finally, the effects of bandwidth on the system performance are considered.