WEBVTT NOTE Yagis, Parabolics, and Electrically Short Antennas 1 00:00:00.000 --> 00:00:02.698 What parasitic elements do. 2 00:00:02.698 --> 00:00:11.978 Question. What is the purpose of making a Yagi's parasitic elements either longer or shorter than resonance? 3 00:00:12.878 --> 00:00:16.288 Answer. Control of phase shift. 4 00:00:16.888 --> 00:00:36.654 That is the mechanism, and it is worth having precisely. A parasitic element re-radiates what it absorbs, with a phase that depends on its reactance: Longer than resonant → inductive → lags → acts as a reflector.. 5 00:00:36.654 --> 00:00:56.048 Shorter than resonant → capacitive → leads → acts as a director.. The lengths are not arbitrary and they are not about size. They set phase, and phase decides which direction the re-radiated energy reinforces. 6 00:00:56.048 --> 00:01:02.008 Question. Approximately how long is a Yagi's driven element? 7 00:01:02.908 --> 00:01:06.228 Answer. 1/2 wavelength. 8 00:01:06.828 --> 00:01:15.468 Question. Why do most two-element Yagis with normal spacing have a reflector instead of a director? 9 00:01:16.368 --> 00:01:18.943 Answer. Higher gain. 10 00:01:19.543 --> 00:01:27.953 At the spacings a two-element Yagi actually uses, a reflector gives more forward gain than a director does. 11 00:01:27.953 --> 00:01:41.943 A director wins on front-to-back ratio, and gain usually matters more, the gain-versus-pattern trade from the General track, resolved. Circular polarisation from linear antennas. 12 00:01:41.943 --> 00:01:50.222 Question. How can two linearly polarized Yagi antennas be used to produce circular polarization? 13 00:01:51.122 --> 00:02:04.162 Answer. Arrange two Yagis on the same axis and perpendicular to each other with the driven elements at the same point on the boom and fed 90 degrees out of phase. 14 00:02:04.762 --> 00:02:11.093 Two perpendicular linear antennas fed in quadrature produce a rotating field. 15 00:02:11.093 --> 00:02:27.950 That matters for satellite work, where the spacecraft's orientation is unknown and changing, circular polarisation avoids the deep fades that spin fading produces on a linear antenna. Parabolic reflectors. 16 00:02:27.950 --> 00:02:38.198 Question. How much does the gain of an ideal parabolic reflector antenna increase when the operating frequency is doubled? 17 00:02:39.098 --> 00:02:42.298 Answer. 6 dB. 18 00:02:42.898 --> 00:02:58.877 Gain scales with area, and doubling the diameter quadruples the area. Four times the power is 6 dB. The same relationship read the other way: 3 dB of extra gain needs a 1.4 times larger dish. 19 00:02:58.877 --> 00:03:13.433 Dish gain gets expensive fast, which is why microwave work uses the highest frequency it can, gain for a given diameter rises with frequency too. Electrically short antennas. 20 00:03:13.433 --> 00:03:24.058 An antenna much shorter than a quarter wavelength is capacitively reactive. Making it resonant means cancelling that reactance. 21 00:03:24.058 --> 00:03:31.177 Question. What is the function of a loading coil in an electrically short antenna? 22 00:03:32.077 --> 00:03:38.188 Answer. To resonate the antenna by cancelling the capacitive reactance. 23 00:03:38.788 --> 00:03:44.868 That is all resonance means here. And the placement question is the interesting one: 24 00:03:44.868 --> 00:03:52.536 Question. What is the most efficient location for a loading coil on an electrically short whip? 25 00:03:53.436 --> 00:03:57.981 Answer. Near the center of the vertical radiator. 26 00:03:58.581 --> 00:04:06.381 Question. What is an advantage of top loading an electrically short HF vertical antenna? 27 00:04:07.281 --> 00:04:11.721 Answer. Improved radiation efficiency. 28 00:04:12.321 --> 00:04:26.554 Both answers point the same way, and the reason is current distribution. Radiation is proportional to current, and current is highest at the base of a monopole and falls toward the top. 29 00:04:26.554 --> 00:04:42.017 A loading coil placed at the base sits where the current is greatest and therefore dissipates the most. Moving it up, centre, or better still top, puts it where the current is lower, so it wastes less. 30 00:04:42.017 --> 00:04:50.711 Top loading with a capacitance hat, from the General mobile lesson, does the same job with almost no coil at all. 31 00:04:50.711 --> 00:04:58.711 Question. Why should antenna loading coils have a high ratio of reactance to resistance? 32 00:04:59.611 --> 00:05:03.051 Answer. To maximize efficiency. 33 00:05:03.651 --> 00:05:20.615 Reactance over resistance is Q, from the electrical principles lesson. A high-Q coil provides the needed reactance with less loss resistance, so more of the power radiates instead of warming the coil. 34 00:05:20.615 --> 00:05:27.571 What short antennas cost. Three consequences, all asked, and all the same physics. 35 00:05:27.571 --> 00:05:36.411 Question. How does radiation resistance of a base-fed whip antenna change below its resonant frequency? 36 00:05:37.311 --> 00:05:49.431 Answer. Radiation resistance decreases. 37 00:05:50.031 --> 00:06:05.624 Shorter electrically means less radiation resistance, which means the same fixed loss resistance becomes a larger share, so efficiency falls. This is the mobile HF problem quantified. 38 00:06:05.624 --> 00:06:15.842 Question. What happens to SWR bandwidth when one or more loading coils are used to resonate an electrically short antenna? 39 00:06:16.742 --> 00:06:19.942 Answer. It is decreased. 40 00:06:20.542 --> 00:06:26.022 Question. What happens as the Q of an antenna increases? 41 00:06:26.922 --> 00:06:31.722 Answer. SWR bandwidth decreases. 42 00:06:32.322 --> 00:06:35.295 Loading raises Q, and BW = f 0/Q. 43 00:06:35.295 --> 00:06:53.402 A heavily loaded antenna is a high-Q circuit and therefore narrow, the same relationship as the matching network in the electrical principles lesson, and the reason a screwdriver antenna needs a motor. 44 00:06:53.402 --> 00:07:05.112 The chain worth holding: short antenna → loading needed → high Q → narrow bandwidth and low radiation resistance → low efficiency. 45 00:07:05.112 --> 00:07:23.850 One cause, four consequences, and no way to escape it other than making the antenna longer. Check yourself. A Yagi's parasitic element is cut 5% longer than the driven element. Reflector or director, and why? 46 00:07:23.850 --> 00:07:37.002 Your 3 metre dish gives 30 dBi. What would a 6 metre dish give? You are fitting a loading coil to a short 40 metre vertical. Base, centre, or top? 47 00:07:43.002 --> 00:08:00.956 Reflector. Longer than resonant is inductive, which lags, and a lagging re-radiation reinforces forward, putting the element behind the driven one. Doubling the diameter adds 6 dB: about 36 dBi. 48 00:08:00.956 --> 00:08:12.802 As high as practical, centre is better than base, top better still, because current is lower there and the coil dissipates less.