Ionospheric Scatter

This article is biased toward 144 MHz, although ionospheric scatter is more pravalent (and much easier to work) on 50 MHz, where reasonably well equipped stations very often utilize the mode. Further experimentation is needed, particularly at 144 MHz.

Ionospheric Scatter is a forward scatter mode occurring in the lower ionosphere, at D or E layer height. It results from ionospheric irregularites and turbulence. Signals are generally weak, and for reasonable success something on the order of EME capable stations will be required most of the time (at 50 MHz, a few hundred watts and a 3 or 4 element beam will do). For this reason CW is the preferred mode, although signals may occasionally be strong enough for SSB. A very slow fading (on the order of several minutes) may be observed, and signals often exhibit a characteristic flutter.

Because signals are weak and high gain (narrow beamwidth) antennas are generally required,  pre-arranged schedules offer the best chance for success. Generally, antennas should be aimed along the great circle path, but occasional path skewing of 10 degrees or so has been reported. Elevating the antennas a few degrees may often be helpful on the shorter paths. As with any weak signal work, some sort of timed transmit/receive sequencing is suggested. Schedules of 10 to 15 minute duration should suffice; if signals are not heard during that time, chances are good that a contact is not possible until conditions change significantly (perhaps a matter of several hours or even days).

Optimum time seems to be between 10am and 2pm, local time, when the sun is highest and D layer ionization is greatest. There is a marked daily variation in the intensity of ionospheric scatter, and some days will be completely unusable. Experience in Europe several years ago suggested the mode may be more prevalent during the summer months, on perhaps as many as 5 out of every 7 days in northern Europe. However, based on limited experimentation in the early 90s, it was suspected that ionospheric scatter may actually work better during the winter months in the northeastern U.S. This is based on very limited available data, and may not be accurate. No direct correlation with other propagation modes has been reported by European experimenters. In fact, there is considerable evidence to suggest that the occurance of other modes, such as aurora and sporadic E (even if only in the lower VHF range) may have an adverse affect on ionospheric scatter at upper VHF frequencies. There is reason to expect ionoscatter to be best at times of high solar activity.

Good results have been obtained on paths ranging from 750 to 1250 miles, with the optimum distance being around 900 miles under typical conditions. The scatter mode may distinguished from tropo on the shorter paths by checking to see if elevating the antennas helps.

Despite limited knowledge of the factors affecting ionospheric scatter at 144 MHz, N1BUG (FN55mf) and KB8RQ (EN80ad) found they could work almost every day during mid winter over an 836 mile path, signlals often strong enough for good copy on SSB. The K0IFL (EM48mk) to N1BUG path, 1220 miles, was workable on the better days, but with extremely weak signals. Several midwestern stations (700 to 1000 miles) were added to the N1BUG January VHF contest log in 1991 and 1992 that would not have been possible by any other means (except EME).


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