The paper analyses the ranges obtained using digital transmissions and frequencies in the range 30–300kc/s. Parameters are assumed which could be met by a mobile installation using solid-state amplifiers and a wire antenna supported by a captive helicopter. The calculations are for both binary and 32-ary f.s.k. modulation, and they take into account groundwave and skywave propagation, fading and noise. The results are expressed in terms of the percentage probability of achieving a given range during the two least favourable periods, namely, summer afternoons and late-spring evenings. With a radiated power of 50kW and a single teleprinter channel operating at 60 words per minute, ranges of about 400 km can be achieved at all times with high for frequencies of 50kc/s or lower, ranges the order of 700km are possible. At these lower frequencies, the range can even be extended to 2000km, if reliabilities of only 90% and error rates of 1 in 102are tolerable.