Apparatus for an antenna with a plurality of antenna elements (8; 23), the antenna providing a radiation pattern including one or more beams and the apparatus comprising: a digital beam-former (14; 16) for digitally weighting component signals corresponding to the one or more beams with respective beam-forming weights to provide each component signal of a beam with a phase relationship and amplitude relationship to other component signals of the same beam; and a processing arrangement comprising: a converter for converting a composite signal comprising a plurality of antenna element signals stacked with respect to each other in frequency from analogue to digital form and a demultiplexer for demultiplexing the digital signal into component signals present in said antenna element signals for providing to the digital beam-former; or a multiplexer for multiplexing beam-formed component signals, for forming a number of antenna element signals, from the beam-former into a composite signal comprising said number of antenna element signals stacked with respect to each other in frequency and a converter for converting the composite signal from digital to analogue form; the apparatus further comprising compensation means (30, 28; 33, 34)for compensating for at least one out of phase and amplitude distortions between component signals associated with the same frequency channel and related to the same beam arising in the processing arrangement, wherein for a given component signal, the compensation means are configured to determine a plurality of compensation complex weights each corresponding to a relative distortion introduced by a component within the processing arrangement, and multiply the plurality of compensation complex weights to determine a final compensation weight with which to weight the component signal. a digital beam-former (14; 16) for digitally weighting component signals corresponding to the one or more beams with respective beam-forming weights to provide each component signal of a beam with a phase relationship and amplitude relationship to other component signals of the same beam; and a processing arrangement comprising: a converter for converting a composite signal comprising a plurality of antenna element signals stacked with respect to each other in frequency from analogue to digital form and a demultiplexer for demultiplexing the digital signal into component signals present in said antenna element signals for providing to the digital beam-former; or a multiplexer for multiplexing beam-formed component signals, for forming a number of antenna element signals, from the beam-former into a composite signal comprising said number of antenna element signals stacked with respect to each other in frequency and a converter for converting the composite signal from digital to analogue form; a converter for converting a composite signal comprising a plurality of antenna element signals stacked with respect to each other in frequency from analogue to digital form and a demultiplexer for demultiplexing the digital signal into component signals present in said antenna element signals for providing to the digital beam-former; or a multiplexer for multiplexing beam-formed component signals, for forming a number of antenna element signals, from the beam-former into a composite signal comprising said number of antenna element signals stacked with respect to each other in frequency and a converter for converting the composite signal from digital to analogue form; the apparatus further comprising compensation means (30, 28; 33, 34)for compensating for at least one out of phase and amplitude distortions between component signals associated with the same frequency channel and related to the same beam arising in the processing arrangement, wherein for a given component signal, the compensation means are configured to determine a plurality of compensation complex weights each corresponding to a relative distortion introduced by a component within the processing arrangement, and multiply the plurality of compensation complex weights to determine a final compensation weight with which to weight the component signal.