WEBVTT NOTE Beyond the Horizon 1 00:00:00.000 --> 00:00:04.560 The normal case, and why it is the normal case. 2 00:00:04.560 --> 00:00:11.944 Question. Why are simplex UHF signals rarely heard beyond their radio horizon? 3 00:00:12.844 --> 00:00:19.236 Answer. UHF signals are usually not propagated by the ionosphere. 4 00:00:19.836 --> 00:00:39.238 The ionosphere reflects HF and stops reflecting somewhere in the VHF range. Above that, signals go straight out into space instead of bending back, so VHF and UHF operation is fundamentally line of sight. 5 00:00:39.238 --> 00:00:48.641 Question. What is a characteristic of HF communication compared with communications on VHF and higher frequencies? 6 00:00:49.541 --> 00:00:56.581 Answer. Long-distance ionospheric propagation is far more common on HF. 7 00:00:57.181 --> 00:01:15.084 That single difference is why Technician privileges feel local and General privileges feel global. It is also why every exception below is worth knowing, they are the occasions when VHF stops behaving like VHF. 8 00:01:15.084 --> 00:01:19.261 The radio horizon is further than the visual one. 9 00:01:19.261 --> 00:01:28.028 Question. Why is the radio horizon for VHF and UHF signals more distant than the visual horizon? 10 00:01:28.928 --> 00:01:33.984 Answer. The atmosphere refracts radio waves slightly. 11 00:01:34.584 --> 00:01:47.198 Air density falls with altitude, and the resulting gradient bends radio waves gently downward, enough to add roughly 15% to the geometric horizon. 12 00:01:47.198 --> 00:02:02.144 So "line of sight" is a little more generous than the phrase suggests, and antenna height buys range faster than power does. The exceptions, and what each needs. Sporadic E. 13 00:02:02.144 --> 00:02:17.024 Question. Which of the following types of propagation is most commonly associated with occasional strong signals on the 10-, 6-, and 2-meter bands from beyond the radio horizon? 14 00:02:17.924 --> 00:02:21.044 Answer. Sporadic E. 15 00:02:21.644 --> 00:02:33.755 Patches of intensely ionised E region form, usually in early summer, and reflect VHF for a few hundred to a couple of thousand miles. 16 00:02:33.755 --> 00:02:43.225 It appears without warning, lasts minutes to hours, and is the reason 6 metres is called the Magic Band. 17 00:02:43.225 --> 00:02:58.067 For a Technician this is the single most rewarding propagation mode to watch for: it works on 6 and 2 metres, where you hold full privileges. Tropospheric ducting. 18 00:02:58.067 --> 00:03:12.347 Question. What type of propagation is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis? 19 00:03:13.247 --> 00:03:16.967 Answer. Tropospheric ducting. 20 00:03:17.567 --> 00:03:21.821 Question. What causes tropospheric ducting? 21 00:03:22.721 --> 00:03:27.041 Answer. Temperature inversions in the atmosphere. 22 00:03:27.641 --> 00:03:38.798 A warm layer over a cool one forms a waveguide in the lower atmosphere, and VHF or UHF signals travel inside it for hundreds of miles. 23 00:03:38.798 --> 00:03:53.702 Unlike sporadic E, ducting often persists for hours or days, and it favours the higher bands, 70 centimetres ducts beautifully. Common on calm high-pressure nights and over water. 24 00:03:53.702 --> 00:04:02.361 If a distant repeater suddenly appears at full strength on a still evening, this is why. Meteor scatter. 25 00:04:02.361 --> 00:04:09.121 Question. What band is best suited for communicating via meteor scatter? 26 00:04:10.021 --> 00:04:13.021 Answer. 6 meters. 27 00:04:13.621 --> 00:04:30.779 Meteors leave brief trails of ionisation that reflect VHF for a fraction of a second to a few seconds. Six metres is the sweet spot: high enough to escape HF noise, low enough that the trails reflect well. 28 00:04:30.779 --> 00:04:42.581 The bursts are too short for conversation, which is why digital modes built for very short exchanges dominate this mode. Auroral backscatter. 29 00:04:42.581 --> 00:04:50.701 Question. What is one characteristic of VHF signals received via auroral backscatter? 30 00:04:51.601 --> 00:04:57.641 Answer. They are distorted, with a characteristic raspy sound. 31 00:04:58.241 --> 00:05:10.077 Signals bounced off the disturbed auroral zone are Doppler-smeared by the moving ionisation, so voice becomes barely intelligible and CW acquires a rough buzz. 32 00:05:10.077 --> 00:05:16.255 Distinctive enough that you will recognise it the first time you hear it. F region. 33 00:05:16.255 --> 00:05:27.506 Question. Which of the following bands may provide long-distance communications via the ionosphere's F region during the peak of the sunspot cycle? 34 00:05:28.406 --> 00:05:32.326 Answer. 6 and 10 meters. 35 00:05:32.926 --> 00:05:45.422 The F region is the workhorse of HF long-distance propagation, and at solar maximum it reaches high enough in frequency to include 10 metres and occasionally. 36 00:05:45.422 --> 00:05:52.382 This is the mode that lets a Technician with 10 metre privileges work other. continents. 37 00:05:52.382 --> 00:06:01.312 Question. What is generally the best time for long-distance 10-meter band propagation via the F region? 38 00:06:02.212 --> 00:06:10.052 Answer. From dawn to shortly after sunset during periods of high sunspot activity. 39 00:06:10.652 --> 00:06:28.750 Two conditions, both necessary: daylight, because the F region needs solar ionisation, and high sunspot activity, because that determines how high in frequency the region will reflect at all. 40 00:06:28.750 --> 00:06:46.090 In a solar minimum, 10 metres is quiet regardless of the hour. Summary table. Mode, Sporadic E; Bands, 10, 6, 2 m; Duration, minutes to hours; Needs, ionised E patches, mostly summer. 41 00:06:46.090 --> 00:07:02.957 Mode, Tropospheric ducting; Bands, VHF and UHF; Duration, hours to days; Needs, temperature inversion. Mode, Meteor scatter; Bands, 6 m; Duration, seconds; Needs, a meteor trail. 42 00:07:02.957 --> 00:07:21.339 Mode, Auroral backscatter; Bands, VHF; Duration, hours; Needs, geomagnetic disturbance. Mode, F region; Bands, 6 and 10 m; Duration, daylight hours; Needs, high sunspot activity. Check yourself. 43 00:07:21.339 --> 00:07:42.186 It is a still, clear evening under high pressure and a 70 cm repeater 200 miles away is full quieting. What mode? Which propagation mode would you plan a 6 metre contact around at solar maximum, in the middle of the day? 44 00:07:42.186 --> 00:07:47.492 Why is a UHF simplex contact beyond the horizon unusual? 45 00:07:53.492 --> 00:08:09.292 Tropospheric ducting, a temperature inversion, favouring UHF, lasting hours. F region propagation: dawn to shortly after sunset, during high sunspot activity. 46 00:08:09.292 --> 00:08:18.692 UHF signals are usually not propagated by the ionosphere, so they leave rather than bend back.