With economical factors favouring the use of guyed structures for terrestrial microwave radio system towers of heights above 40 to 50 metres, situations are arising where information on antenna-pattern degradation due to guy wire scattering is required for route-design purposes. A qualitative understanding can be gained of the worst-case situations, where the wire runs near to, and directly in front of, the antenna, by regarding the illuminated section of guy as a long linearly phased cylindrical source. This produces a well-defined cone of maximum scattering, having the guy as its axis. Experimental measurements, and an approximate analytical study based on the above concept, show that within the cone region, antenna sidelobes may be degraded by as much as 20 dB. In the azimuthal plane the cone-region cuts can be quite narrow, so that interference with other cochannel terrestrial systems or repeaters will not always be significantly increased. However, co-ordination with satellite systems may often prove difficult, as a large fraction of the cone energy is radiated out into space, either directly, or after reflection from the ground.