There is a significant error in the equation referring to the rule of thumb for ‘line-of-sight’ analogue transmissions given in the paper by William Cairns, in the May 2005 issue (Cairns, 2005). This incorrect formula has previously been quoted in other sources, and possibly the author obtained it from one of these. In addition, the term ‘line-of-sight’ implies geometrical optics, whereas use of the term ‘radio horizon’ would be more appropriate since this takes account of the extension of radio wave transmission distance over the earth's surface caused by the normal water vapour content of the lower atmosphere over sea.