WEBVTT NOTE Reading Antenna Patterns, and Modelling Antennas 1 00:00:00.000 --> 00:00:16.219 Reading a polar pattern. Six pool questions ask you to read numbers off two published figures. The technique is the same each time, and it is worth doing once carefully rather than memorising six answers. 2 00:00:16.219 --> 00:00:32.279 This question refers to one of the exam's circuit diagrams, which you will need to look at in the written lesson. The rings are decibels below the peak, and the peak is normalised to the outermost ring. 3 00:00:32.279 --> 00:00:46.987 So every measurement is "how many rings down". 3 dB beamwidth, find where the main lobe crosses the ring 3 dB below the peak, on both sides, and read the angle between those two points. 4 00:00:46.987 --> 00:01:03.843 For figure E9-1 that is 50 degrees. Front-to-back ratio, the peak, minus the response 180 degrees away from it. For E9-1, 18 dB. Front-to-side ratio, the peak, minus the response 90 degrees away. For E9-1, 14 dB. 5 00:01:03.843 --> 00:01:17.438 Note that front-to-side is smaller than front-to-back here, which is typical: the pattern's sides are usually less well suppressed than its back. Azimuth versus elevation. 6 00:01:17.438 --> 00:01:26.422 This question refers to one of the exam's circuit diagrams, which you will need to look at in the written lesson. 7 00:01:26.422 --> 00:01:33.342 Question. What type of antenna pattern is shown in Figure E9-2? 8 00:01:34.242 --> 00:01:37.042 Answer. Elevation. 9 00:01:37.642 --> 00:01:53.076 The tell is the flat bottom half: an elevation plot shows only 0 to 180 degrees, because there is ground below. An azimuth plot is a full 360-degree circle. 10 00:01:53.076 --> 00:02:03.174 Question. What is the elevation angle of peak response in the antenna radiation pattern shown in Figure E9-2? 11 00:02:04.074 --> 00:02:07.654 Answer. 7.5 degrees. 12 00:02:08.254 --> 00:02:24.446 That is the number that matters for HF DX. A pattern peaking at 7.5 degrees is a DX antenna; one peaking at 45 degrees is a regional antenna, whatever its gain figure says. 13 00:02:24.446 --> 00:02:33.326 Question. What is the front-to-back ratio of the radiation pattern shown in Figure E9-2? 14 00:02:34.226 --> 00:02:37.198 Answer. 28 dB. 15 00:02:37.798 --> 00:02:43.158 Same method, better antenna. The far field. 16 00:02:43.158 --> 00:02:47.456 Question. What is the far field of an antenna? 17 00:02:48.356 --> 00:02:55.163 Answer. The region where the shape of the radiation pattern no longer varies with distance. 18 00:02:55.763 --> 00:03:11.546 Close to an antenna the field is still sorting itself out, and its shape changes as you move away. Beyond a few wavelengths it settles into the pattern the antenna "has", and from then on only the amplitude falls. 19 00:03:11.546 --> 00:03:30.071 Two practical consequences: pattern measurements must be made in the far field, and RF exposure calculations that assume far-field conditions are wrong very close to an antenna, which is exactly the indoor-antenna case from the General safety lesson. 20 00:03:30.071 --> 00:03:30.812 Modelling. 21 00:03:30.812 --> 00:03:37.252 Question. What type of analysis is commonly used for modeling antennas? 22 00:03:38.152 --> 00:03:41.272 Answer. Method of Moments. 23 00:03:41.872 --> 00:03:47.776 Question. What is the principle of a Method of Moments analysis? 24 00:03:48.676 --> 00:03:56.577 Answer. A wire is modeled as a series of segments, each having a uniform value of current. 25 00:03:57.177 --> 00:04:10.656 Divide every wire into short segments, assume the current is constant within each, and solve the resulting system for the currents that satisfy the boundary conditions. 26 00:04:10.656 --> 00:04:25.179 From those currents everything else follows: pattern, gain, feed point impedance. This is what NEC, EZNEC, and 4nec2 do, and it is why amateur antenna design became a desk activity. 27 00:04:25.179 --> 00:04:36.619 Question. What is a disadvantage of decreasing the number of wire segments in an antenna model below 10 segments per half-wavelength? 28 00:04:37.519 --> 00:04:43.287 Answer. The computed feed point impedance may be incorrect. 29 00:04:43.887 --> 00:04:59.568 Segment too coarsely and the model still produces a plausible pattern while the feed point impedance, the number you would actually build to, goes wrong. That is the dangerous kind of error: the output looks fine. 30 00:04:59.568 --> 00:05:11.127 Ten segments per half wavelength is the guideline worth remembering, and more near the feed point and near any sharp bend, where the current changes fastest. 31 00:05:11.127 --> 00:05:30.773 Check yourself. A pattern's peak is at the outer ring and the response 180 degrees round is 3 rings in, at 5 dB per ring. Front-to-back ratio? You are shown a pattern that covers a full 360 degrees. Azimuth or elevation? 32 00:05:30.773 --> 00:05:42.292 Your model gives a sensible-looking pattern but the feed point impedance is nothing like the built antenna. First thing to check? 33 00:05:48.292 --> 00:05:58.677 15 dB, three rings at 5 dB each. Azimuth. An elevation plot only spans 180 degrees, because ground is below. 34 00:05:58.677 --> 00:06:12.621 Segment count, below about 10 segments per half wavelength the computed feed point impedance goes wrong while the pattern still looks reasonable.