WEBVTT NOTE EME, Meteor Scatter, and Wave Properties 1 00:00:00.000 --> 00:00:08.893 Wave properties, precisely. Three facts the Technician track stated loosely and Extra tests exactly. 2 00:00:08.893 --> 00:00:15.201 Question. How are the component fields of an electromagnetic wave oriented? 3 00:00:16.101 --> 00:00:19.700 Answer. They are at right angles. 4 00:00:20.300 --> 00:00:26.235 Question. In what direction does an electromagnetic wave travel? 5 00:00:27.135 --> 00:00:33.576 Answer. It travels at a right angle to the electric and magnetic fields. 6 00:00:34.176 --> 00:00:43.536 All three, electric field, magnetic field, direction of travel, are mutually perpendicular. 7 00:00:43.536 --> 00:00:50.162 Question. What determines the speed of electromagnetic waves through a medium? 8 00:00:51.062 --> 00:00:54.684 Answer. The index of refraction. 9 00:00:55.284 --> 00:01:05.585 Which is where velocity factor comes from: a coax's dielectric has an index of refraction, and the wave slows accordingly. 10 00:01:05.585 --> 00:01:11.425 Question. What are circularly polarized electromagnetic waves? 11 00:01:12.325 --> 00:01:17.867 Answer. Waves with rotating electric and magnetic fields. 12 00:01:18.467 --> 00:01:29.227 The rotation produced by two perpendicular linear antennas fed in quadrature, from the antennas module. Earth-Moon-Earth. 13 00:01:29.227 --> 00:01:40.387 Question. What is the approximate maximum separation measured along the surface of the Earth between two stations communicating by EME? 14 00:01:41.287 --> 00:01:48.087 Answer. 12,000 miles, if the moon is "visible" by both stations. 15 00:01:48.687 --> 00:02:00.489 The only requirement is that both stations can see the Moon at once, which, given the Earth's curvature, means up to about half the planet apart. 16 00:02:00.489 --> 00:02:06.763 Question. What characterizes libration fading of an EME signal? 17 00:02:07.663 --> 00:02:12.023 Answer. A fluttery, irregular fading. 18 00:02:12.623 --> 00:02:29.242 The Moon wobbles slightly in its rotation (libration), so the many reflecting features on its surface shift relative to each other and their reflections combine and cancel irregularly. 19 00:02:29.242 --> 00:02:37.823 It is multipath, on a 480,000-mile path, and it gives EME signals their characteristic flutter. 20 00:02:37.823 --> 00:02:47.703 Question. When scheduling EME contacts, which of these conditions will generally result in the least path loss? 21 00:02:48.603 --> 00:02:52.096 Answer. When the Moon is at perigee. 22 00:02:52.696 --> 00:03:00.872 Perigee is the Moon's closest approach, around 356,000 km against 406,000 at apogee. 23 00:03:00.872 --> 00:03:14.694 Path loss goes as the fourth power of distance for a two-way reflection, so the difference between perigee and apogee is a couple of decibels. 24 00:03:14.694 --> 00:03:24.136 On a path with almost no margin, a couple of decibels is worth scheduling around. Meteor scatter. 25 00:03:24.136 --> 00:03:34.302 Question. When a meteor strikes the Earth's atmosphere, a linear ionized region is formed at what region of the ionosphere? 26 00:03:35.202 --> 00:03:37.994 Answer. The E region. 27 00:03:38.594 --> 00:03:48.594 Around 100 km, the E region altitude, which is why meteor-scatter distances resemble sporadic-E distances. 28 00:03:48.594 --> 00:03:56.134 Question. Which of the following frequency ranges is most suited for meteor-scatter communications? 29 00:03:57.034 --> 00:04:03.554 Answer. 28 MHz - 148 MHz. 30 00:04:04.154 --> 00:04:20.034 Ten metres through 2 metres, with 6 metres the sweet spot from the Technician track. Below that the ionosphere carries the signal anyway; above it the trails do not reflect well. Auroral propagation. 31 00:04:20.034 --> 00:04:25.754 Question. What is most likely to result in auroral propagation? 32 00:04:26.654 --> 00:04:30.788 Answer. Severe geomagnetic storms. 33 00:04:31.388 --> 00:04:37.723 Question. Which of these emission modes is best for auroral propagation? 34 00:04:38.623 --> 00:04:41.263 Answer. CW. 35 00:04:41.863 --> 00:05:00.030 Auroral reflection Doppler-smears a signal badly, the "raspy" quality from the Technician track. SSB voice becomes unintelligible; CW survives because a smeared tone is still a tone with recognisable timing. 36 00:05:00.030 --> 00:05:12.668 This is the general rule for degraded paths: the less structure the signal carries per second, the more abuse it survives. Tropospheric ducting. 37 00:05:12.668 --> 00:05:22.268 Question. Atmospheric ducts capable of propagating microwave signals often form over what geographic feature? 38 00:05:23.168 --> 00:05:26.528 Answer. Large bodies of water. 39 00:05:27.128 --> 00:05:40.688 Water surfaces produce the stable temperature inversions ducting needs. This is why long microwave paths over sea are routine and the same paths over land are not. 40 00:05:40.688 --> 00:05:48.516 Question. What is a typical range for tropospheric duct propagation of microwave signals? 41 00:05:49.416 --> 00:05:54.056 Answer. 100 miles to 300 miles. 42 00:05:54.656 --> 00:05:57.296 When the MUF drops. 43 00:05:57.296 --> 00:06:06.682 Question. What should be done to continue a long-distance contact when the MUF for that path decreases due to darkness? 44 00:06:07.582 --> 00:06:11.952 Answer. Switch to a lower frequency HF band. 45 00:06:12.552 --> 00:06:32.355 Directly from the General MUF lesson: above the MUF the signal leaves. Move down until you are below it again, which is why an evening on HF is a walk down the bands as the ionosphere weakens. Check yourself. 46 00:06:32.355 --> 00:06:51.872 Why does a perigee EME schedule matter when the difference is only about 2 dB? An aurora is running and you want to work it. Which mode, and why not SSB? The band goes quiet mid-contact. Up a band or down? 47 00:06:57.872 --> 00:07:17.012 Because an EME path has almost no margin, two decibels is the difference between copy and no copy. CW. Auroral Doppler smearing destroys voice intelligibility, and a smeared tone is still readable. Down. 48 00:07:17.012 --> 00:07:24.460 The MUF has fallen below your frequency; a lower band brings you back under it.