WEBVTT NOTE Wire Antennas, Arrays, and Ground Effects 1 00:00:00.000 --> 00:00:11.371 Two verticals, three patterns. Feed two quarter-wave verticals a half wavelength apart and the phasing between them decides the pattern. 2 00:00:11.371 --> 00:00:21.238 Three cases, all asked: Spacing, 1/2 wavelength; Phasing, in phase; Pattern, figure-eight broadside to the array axis. 3 00:00:21.238 --> 00:00:36.372 Spacing, 1/2 wavelength; Phasing, 180 degrees out of phase; Pattern, figure-eight along the array axis. Spacing, 1/4 wavelength; Phasing, 90 degrees out of phase; Pattern, cardioid. 4 00:00:36.372 --> 00:00:53.513 The reasoning, so you can reconstruct rather than memorise: In phase, half-wave spacing. Broadside, the path lengths are equal, so the signals add. End-on, one is half a wavelength further, so they cancel. 5 00:00:53.513 --> 00:01:10.904 Maximum broadside. 180 degrees out of phase. Now broadside cancels (equal paths, opposite phase) and end-on adds (half-wave path difference plus half-wave phase difference = in phase). Maximum along the axis. 6 00:01:10.904 --> 00:01:14.750 Quarter-wave spacing, 90 degrees out of phase. 7 00:01:14.750 --> 00:01:29.550 In one direction the path difference and the phase difference cancel exactly, giving full addition; in the opposite direction they add to a half wavelength, giving cancellation. 8 00:01:29.550 --> 00:01:47.610 The result is unidirectional, a cardioid, heart-shaped, with a deep null off the back. The cardioid is the one worth building: two cheap verticals and a phasing line give a directional low-band antenna with no tower. 9 00:01:47.610 --> 00:01:50.704 Making a wire antenna unidirectional. 10 00:01:50.704 --> 00:01:58.664 Question. What is the effect of adding a terminating resistor to a rhombic or long-wire antenna? 11 00:01:59.564 --> 00:02:06.061 Answer. It changes the radiation pattern from bidirectional to unidirectional. 12 00:02:06.661 --> 00:02:19.519 Without a termination, energy reaching the far end reflects and travels back, producing a bidirectional pattern. A resistor absorbs it, so only the forward wave remains. 13 00:02:19.519 --> 00:02:31.541 You lose the power dissipated in the resistor and gain a front-to-back ratio, which is exactly the Beverage receiving antenna's design from the General track. 14 00:02:31.541 --> 00:02:40.759 Question. What happens to the radiation pattern of an unterminated long wire antenna as the wire length is increased? 15 00:02:41.659 --> 00:02:49.658 Answer. Additional lobes form with major lobes increasingly aligned with the axis of the antenna. 16 00:02:50.258 --> 00:03:03.114 Longer wire, more lobes, and the main ones swing round toward the wire's direction. A long wire is not a broadside radiator. The named wire antennas. 17 00:03:03.114 --> 00:03:19.336 Antenna, Folded dipole; What it is, a half-wave dipole with an additional parallel wire connecting its ends. Antenna, Two-wire folded dipole feed point; What it is, approximately 300 ohms. 18 00:03:19.336 --> 00:03:31.502 Antenna, Zepp; What it is, an end-fed half-wavelength dipole. Antenna, Extended double Zepp; What it is, a centre-fed 1.25-wavelength dipole. 19 00:03:31.502 --> 00:03:40.735 Antenna, G5RV; What it is, a wire antenna centre-fed through a specific length of open-wire or ladder line. 20 00:03:40.735 --> 00:03:52.038 Antenna, Off-centre-fed dipole (OCFD); What it is, fed away from centre to create a similar feed point impedance on multiple bands. 21 00:03:52.038 --> 00:04:12.660 The folded dipole's 300 ohms is worth understanding rather than memorising: the second wire halves the current at the feed point for the same power, and impedance is voltage over current, so it rises by a factor of four from about 75 ohms. 22 00:04:12.660 --> 00:04:21.116 That is also why a 4:1 balun feeds one from 75-ohm coax. The OCFD trick generalises the same idea. 23 00:04:21.116 --> 00:04:32.937 A dipole's impedance varies along its length, General's "steadily increases toward the ends", and it varies differently on each harmonic. 24 00:04:32.937 --> 00:04:47.864 Choose a feed point where several bands happen to present similar impedances, and one antenna feeds them all through one matching arrangement. Height, and the takeoff angle. 25 00:04:47.864 --> 00:04:53.818 This is the material that decides whether an HF antenna works for DX. 26 00:04:53.818 --> 00:05:03.607 Question. How does the radiation pattern of a horizontally polarized antenna vary with increasing height above ground? 27 00:05:04.507 --> 00:05:10.277 Answer. The takeoff angle of the lowest elevation lobe decreases. 28 00:05:10.877 --> 00:05:25.677 Higher antenna, lower takeoff angle, better DX. It is the single most reliable improvement available on HF, and it is why "get it higher" is always good advice. 29 00:05:25.677 --> 00:05:36.695 Question. How is the far-field elevation pattern of a vertically polarized antenna affected by being mounted over seawater versus soil? 30 00:05:37.595 --> 00:05:42.555 Answer. Radiation at low angles increases. 31 00:05:43.155 --> 00:05:51.959 Seawater is a far better conductor than soil, so the ground reflection is stronger and the low-angle reinforcement larger. 32 00:05:51.959 --> 00:06:03.795 This is why coastal and shipboard verticals outperform inland ones, and it is the same soil-conductivity effect as the efficiency question in the parameters lesson. 33 00:06:03.795 --> 00:06:16.230 Question. How does the radiation pattern of a horizontally-polarized antenna mounted above a long slope compare with the same antenna mounted above flat ground? 34 00:06:17.130 --> 00:06:23.450 Answer. The main lobe takeoff angle decreases in the downhill direction. 35 00:06:24.050 --> 00:06:41.986 A slope tilts the effective ground plane, which tilts the reflection, which lowers the takeoff angle downhill. A hillside is a free antenna improvement in one direction, and a penalty in the other. Check yourself. 36 00:06:41.986 --> 00:06:56.470 Two quarter-wave verticals, quarter-wave spacing, fed 90 degrees out of phase. Pattern? Your folded dipole reads about 300 ohms. What balun ratio feeds it from 75-ohm coax? 37 00:06:56.470 --> 00:07:08.765 You have a choice of two sites for a 20 metre dipole: 10 metres up on flat ground, or 10 metres up on a slope facing your target direction. Which? 38 00:07:14.765 --> 00:07:33.044 Cardioid, unidirectional, with a deep null off the back. 4:1, 300 / 75. (A 4:1 balun is a 2:1 turns ratio, from the General transformer lesson.). The slope, facing the target. 39 00:07:33.044 --> 00:07:42.445 Sloping ground lowers the takeoff angle in the downhill direction, which is what DX wants.