Interference Mitigation Using an Array Feed
Chad K. Hansen, Brigham Young University
Karl F. Warnick, Brigham Young University
Brian D. Jeffs, Brigham Young University
J. Richard Fisher, National Radio Astronomy Observatory
Richard Bradley, National Radio Astronomy Observatory

Spatial filtering techniques for RFI mitigation require multiple, spatially separated looks at the interfering signal. For a large reflector, this can be accomplished with an array feed. We show using simulations that an array of electrically small elements in conjunction with a sp\begin{displaymath} {S}_{\mbox{{\it\scriptsize{eff}}}} = \frac{G}{T_{sys} + T_{int}} \end{displaymath}atial filtering algorithm can provide high antenna sensitivity even in the presence of RFI.

We first consider 7 and 19 element dipole arrays and a 25 meter Very Large Array (VLA) type paraboloidal reflector with no interference, to determine the achievable aperture and spillover efficiencies. For a receiver temperature of 15K and 300K background, the sensitivity in Jy-1 obtained for the 7 element array was approximately % 100*(.0093-.007)/.007 (optimized) 35% higher for the array feed than that of a circular waveguide feed with diameter optimized for maximum sensitivity at 1612 MHz. The sensitivity of the 19 element array feed was % 100*(.012-.008)/.008 (max-SNR - analytical noise correlation model) 50\% greater than the 7 element array.

In the presence of an interferer, we define an effective sensitivity,

Seff = G / (Tsys + Tint)

where G is antenna gain in K/Jy, Tsys is the combined receiver and antenna spillover temperature, and Tint is the interferer power at the output of a beamformer which combines inputs from the array feed elements using the max-SNR algorithm. For the 7 element array and a single interferer, the effective sensitivity is close to the sensitivity with no interferer except for a few interferer arrival angles at which the effective sensitivity drops by several dB. At these angles, the array feed response to the interferer is nearest to the response to a signal at boresight in an inner product sense. The optimal solution found by the spatial filtering algorithm admits more spillover noise than at other interferer arrival angles, so that Tsys increases. With a larger array (19 elements), variations in effective sensitivity with interferer arrival angle still occur, but are less severe. These results indicate that array feeds offer a promising solution for RFI mitigation, and may provide superior performance to standard feeds in the absense of RFI as well.