WEBVTT NOTE MUF, LUF, and Choosing a Path 1 00:00:00.000 --> 00:00:12.623 Two frequencies that bracket every path. For any two points on Earth at any moment, there is a window of usable frequencies with a ceiling and a floor. 2 00:00:12.623 --> 00:00:17.543 Question. What does MUF stand for? 3 00:00:18.443 --> 00:00:24.963 Answer. The Maximum Usable Frequency for communications between two points. 4 00:00:25.563 --> 00:00:30.073 Question. What does LUF stand for? 5 00:00:30.973 --> 00:00:38.693 Answer. The Lowest Usable Frequency for communications between two specific points. 6 00:00:39.293 --> 00:00:54.429 Above the MUF, the ionosphere no longer bends the wave back, it passes through into space. Below the LUF, absorption is so heavy the signal does not survive the trip. 7 00:00:54.429 --> 00:01:05.149 Question. How does the ionosphere affect radio waves with frequencies below the MUF and above the LUF? 8 00:01:06.049 --> 00:01:10.089 Answer. They are refracted back to Earth. 9 00:01:10.689 --> 00:01:18.246 Question. What usually happens to radio waves with frequencies below the LUF? 10 00:01:19.146 --> 00:01:24.450 Answer. They are attenuated before reaching the destination. 11 00:01:25.050 --> 00:01:43.111 Note the asymmetry in the failure modes: above the MUF your signal leaves; below the LUF it is absorbed. Two different physical mechanisms, both resulting in silence. Where in the window to operate. 12 00:01:43.111 --> 00:01:50.631 Question. Which frequency will have the least attenuation for long-distance skip propagation? 13 00:01:51.531 --> 00:01:55.651 Answer. Just below the MUF. 14 00:01:56.251 --> 00:02:13.703 The highest frequency that still comes back. Higher frequencies penetrate deeper into the ionosphere before refracting, spending less time in the absorbing lower regions, so the closer to the MUF, the less loss. 15 00:02:13.703 --> 00:02:26.771 The practical instruction is simple: work the highest band that is open. If you can hear the path on 15 metres, use 15 rather than 20. When the window closes. 16 00:02:26.771 --> 00:02:35.011 Question. What happens to HF propagation when the LUF exceeds the MUF? 17 00:02:35.911 --> 00:02:44.351 Answer. Propagation via ordinary skywave communications is not possible over that path. 18 00:02:44.951 --> 00:03:00.709 The floor rises above the ceiling and there is no usable frequency at all. This happens during severe ionospheric disturbances, and it is why a path can be completely dead rather than merely difficult. 19 00:03:00.709 --> 00:03:14.116 MUF depends on, the pool answers "all these choices are correct", the type of ionospheric layer involved, the amount of solar radiation, and the distance between stations. 20 00:03:14.116 --> 00:03:25.719 Distance matters because a longer hop strikes the ionosphere at a shallower angle, and a shallower angle refracts a higher frequency. Hop distances. 21 00:03:25.719 --> 00:03:31.991 Two numbers to memorise, and they follow directly from the height of the region: 22 00:03:31.991 --> 00:03:42.673 Question. What is the approximate maximum distance along the Earth's surface normally covered in one hop using the F2 region? 23 00:03:43.573 --> 00:03:47.710 Answer. 2,500 miles. 24 00:03:48.310 --> 00:03:58.710 Question. What is the approximate maximum distance along the Earth's surface normally covered in one hop using the E region? 25 00:03:59.610 --> 00:04:03.770 Answer. 1,200 miles. 26 00:04:04.370 --> 00:04:12.259 F2 is roughly twice as high, so its hop is roughly twice as long. Geometry, not physics. 27 00:04:12.259 --> 00:04:26.243 Useful consequence: a 5,000-mile contact is two F2 hops, and each hop costs absorption at both ionospheric passes and at the ground reflection between them. 28 00:04:26.243 --> 00:04:44.530 Long paths are lossy in a way the numbers do not immediately suggest. Long path and short path. A radio wave can reach any point by the direct great-circle route or by going the other way round the world. 29 00:04:44.530 --> 00:04:55.290 Question. What is a characteristic of skywave signals arriving at your location by both short-path and long-path propagation? 30 00:04:56.190 --> 00:05:01.230 Answer. A slightly delayed echo might be heard. 31 00:05:01.830 --> 00:05:16.366 The long path is longer, so it arrives later, tens of milliseconds, enough to hear as a distinct echo on your own or the other station's signal. It is unmistakable once you have heard it. 32 00:05:16.366 --> 00:05:31.834 Long path is worth trying when short path is dead, particularly around sunrise and sunset, because the long way round may lie entirely in darkness or entirely in daylight when the short way does not. 33 00:05:31.834 --> 00:05:34.710 Checking a band without transmitting. 34 00:05:34.710 --> 00:05:43.070 Question. Which of the following is a way to determine current propagation on a desired band from your station? 35 00:05:43.970 --> 00:05:53.123 Answer. Use a network of automated receiving stations on the internet to see where your transmissions are being received. 36 00:05:53.723 --> 00:06:03.669 Reverse beacon networks and WSPR: automated receivers worldwide continuously report what they hear and how strongly. 37 00:06:03.669 --> 00:06:13.959 Send a short CW call or a WSPR transmission and within a minute you can see, on a map, exactly where your signal landed. 38 00:06:13.959 --> 00:06:26.563 This is a far better answer to "is the band open" than tuning around, and it is the modern replacement for guesswork. Seasonal and daily character. 39 00:06:26.563 --> 00:06:33.739 Question. Which of the following is typical of the lower HF frequencies during the summer? 40 00:06:34.639 --> 00:06:40.359 Answer. High levels of atmospheric noise or static. 41 00:06:40.959 --> 00:06:58.147 Thunderstorms radiate broadband noise that propagates on the low bands. Summer nights on 80 and 160 metres are noisy for exactly the reason that summer afternoons have storms, often thousands of miles away. 42 00:06:58.147 --> 00:07:15.167 Which is why the low bands are winter bands and the high bands are summer-and- solar-maximum bands. Check yourself. You can hear a station in Japan on both 20 and 15 metres. Which should you use, and why? 43 00:07:15.167 --> 00:07:24.679 You hear an echo on your own signal. What is happening? A contact 4,000 miles away via F2. How many hops, minimum? 44 00:07:30.679 --> 00:07:51.853 15 metres, the highest open band is closest to the MUF and suffers the least attenuation. Your signal is arriving by both short path and long path, and the long path's extra travel time is audible as a delayed echo. Two. 45 00:07:51.853 --> 00:07:55.799 A single F2 hop covers about 2,500 miles.