System to combine high frequency signals

A system to combine high frequency signals comprises a plurality of band-pass filters (F1a - F1b) which are coupled into a common uninterrupted signal line (L) by high impedance quarter wavelength loops (L01a - L04b) which are linked to earth. Pairs of filters are distributed non-uniformly along the line with a spacing at different multiples of a half wavelength. The pairs of filter modules (S1 - S4) may be distributed with an intermittent short and long spacing of a half wavelength and one wavelength respectively. The pairs of filter modules (S1 - S4) may also be arranged to give a minimum possible distance between an output (P1) and an input (P2) of two units (S1 and S2) by rotating one module (S2) by 180 degrees relative to the other module (S1).