WEBVTT NOTE VHF Digital Modes, EME, and APRS 1 00:00:00.000 --> 00:00:18.101 One mode per path. The WSJT-X family is not a set of alternatives, each mode was designed for one kind of propagation, and the design follows from the path's characteristics. 2 00:00:18.101 --> 00:00:37.034 Mode, MSK144; Designed for, meteor scatter; Why, very short transmissions, to fit inside a burst. Mode, Q65; Designed for, EME and other very weak paths; Why, long, robust, averaged. 3 00:00:37.034 --> 00:00:59.400 Mode, JT65; Designed for, weak-signal work generally; Why, decodes at very low signal-to-noise ratios. Mode, FT8; Designed for, HF and general weak signal; Why, 15-second cycles, high throughput for its sensitivity. 4 00:00:59.400 --> 00:01:05.984 Question. Which of the following digital modes is designed for meteor scatter communications? 5 00:01:06.884 --> 00:01:10.958 Answer. MSK144. 6 00:01:11.558 --> 00:01:26.878 A meteor trail lasts a fraction of a second, so MSK144 sends very short, rapidly repeated messages, the opposite design choice from EME, where the path is weak but continuous. 7 00:01:26.878 --> 00:01:34.318 Question. Which of the following digital modes is designed for EME communications? 8 00:01:35.218 --> 00:01:38.375 Answer. Q65. 9 00:01:38.975 --> 00:01:45.255 Question. How does Q65 differ from JT65? 10 00:01:46.155 --> 00:01:50.915 Answer. Multiple receive cycles are averaged. 11 00:01:51.515 --> 00:02:10.835 Averaging is what buys the extra sensitivity. Noise is random and averages toward zero over repeated cycles; the signal is consistent and does not. It costs time, which an EME path has and a meteor burst does not. 12 00:02:10.835 --> 00:02:16.715 Question. What type of modulation is used by JT65? 13 00:02:17.615 --> 00:02:22.335 Answer. Multitone AFSK. 14 00:02:22.935 --> 00:02:30.015 Question. Which of the following is a method for establishing EME contacts? 15 00:02:30.915 --> 00:02:37.672 Answer. Time-synchronous transmissions alternating between stations. 16 00:02:38.272 --> 00:02:50.406 Strict alternation on a clock, so both stations know exactly when to listen. Which is why every mode in this family needs an accurate computer clock. 17 00:02:50.406 --> 00:03:01.297 Question. What information replaces signal-to-noise ratio when using the FT8 or FT4 modes in a VHF contest? 18 00:03:02.197 --> 00:03:04.717 Answer. Grid square. 19 00:03:05.317 --> 00:03:19.037 The exchange is call signs and grid square rather than a signal report, which, for a VHF contact, is more useful information anyway. APRS. 20 00:03:19.037 --> 00:03:24.477 Question. What digital protocol is used by APRS? 21 00:03:25.377 --> 00:03:29.457 Answer. AX.25. 22 00:03:30.057 --> 00:03:36.942 The amateur adaptation of the X.25 packet standard, with call signs as addresses. 23 00:03:36.942 --> 00:03:44.567 Question. What type of packet frame is used to transmit APRS beacon data? 24 00:03:45.467 --> 00:03:48.947 Answer. Unnumbered Information. 25 00:03:49.547 --> 00:03:55.817 UI frames are connectionless, sent without establishing a link and without acknowledgement. 26 00:03:55.817 --> 00:04:05.531 That is exactly right for a beacon: you are broadcasting your position to whoever hears it, and there is nobody in particular to acknowledge. 27 00:04:05.531 --> 00:04:10.408 Question. How do APRS stations relay data? 28 00:04:11.308 --> 00:04:14.908 Answer. By packet digipeaters. 29 00:04:15.508 --> 00:04:21.788 Question. What does the packet path WIDE3-1 designate? 30 00:04:22.688 --> 00:04:28.728 Answer. Three digipeater hops are requested with one remaining. 31 00:04:29.328 --> 00:04:43.025 The path notation is WIDEn-m: n hops requested, m remaining. Each digipeater decrements the second number and stops relaying when it reaches zero. 32 00:04:43.025 --> 00:05:02.068 WIDE3-1 therefore means a three-hop path with one hop left, two digipeaters have already handled it. Understanding this matters because excessive hop counts are the main cause of APRS network congestion. 33 00:05:02.068 --> 00:05:11.168 WIDE2-2 is the usual courteous setting; WIDE7-7 floods a continent and is universally deprecated. 34 00:05:11.168 --> 00:05:19.728 Question. What technology is used for real-time tracking of balloons carrying amateur radio transmitters? 35 00:05:20.628 --> 00:05:23.931 Answer. APRS. 36 00:05:24.531 --> 00:05:44.479 High-altitude balloon tracking is the classic APRS application: a small transmitter, a GPS, and a network of digipeaters and gateways that puts the balloon's position on a public map in real time. Check yourself. 37 00:05:44.479 --> 00:06:03.297 You want to work EME on 2 metres. Which mode, and what does it do that JT65 does not? Your APRS packet shows a path of WIDE2-2. What does that request? Why does APRS use unnumbered information frames? 38 00:06:09.297 --> 00:06:24.023 Q65, which averages multiple receive cycles, trading time, which an EME path has, for sensitivity. Two digipeater hops, with both still remaining, nothing has relayed it yet. 39 00:06:24.023 --> 00:06:33.417 A beacon is connectionless broadcast: there is no link to establish and nobody in particular to acknowledge it.