WEBVTT NOTE Specialised Antennas 1 00:00:00.000 --> 00:00:02.080 Multiband: traps. 2 00:00:02.080 --> 00:00:07.266 Question. What is the primary function of antenna traps? 3 00:00:08.166 --> 00:00:12.686 Answer. To enable multiband operation. 4 00:00:13.286 --> 00:00:17.606 A trap is a parallel resonant circuit in an element. 5 00:00:17.606 --> 00:00:32.060 At its resonant frequency it presents a high impedance and effectively cuts the element short there; at other frequencies it is a low impedance and the full length is in use. 6 00:00:32.060 --> 00:00:38.457 One physical antenna, several electrical lengths. The cost is asked directly: 7 00:00:38.457 --> 00:00:44.977 Question. Which of the following is a disadvantage of multiband antennas? 8 00:00:45.877 --> 00:00:49.877 Answer. They have poor harmonic rejection. 9 00:00:50.477 --> 00:01:08.343 A single-band antenna is a filter, it presents a bad match to your second harmonic and radiates it poorly. A multiband antenna is, by design, a good match on several bands at once, including ones your harmonics land on. 10 00:01:08.343 --> 00:01:14.054 Spurious emissions get out more easily. Simple variations on a dipole. 11 00:01:14.054 --> 00:01:20.464 Question. What is the common name of a dipole with a single central support? 12 00:01:21.364 --> 00:01:24.531 Answer. Inverted V. 13 00:01:25.131 --> 00:01:43.211 One mast, two legs sloping down. Slightly lower feed impedance and a slightly broader pattern than a flat dipole, and one support instead of two, which is why so many first HF antennas are inverted Vs. 14 00:01:43.211 --> 00:01:50.651 Question. What is the feed point impedance of an end-fed half-wave antenna? 15 00:01:51.551 --> 00:01:54.551 Answer. Very high. 16 00:01:55.151 --> 00:02:03.594 Thousands of ohms, following from the current distribution: minimum current and maximum voltage at the end. 17 00:02:03.594 --> 00:02:15.351 Feeding it needs a matching transformer with a large ratio, and it is why an end-fed wire connected straight to a radio puts RF all over the station. 18 00:02:15.351 --> 00:02:29.711 Question. Which of the following antenna types will be most effective as a near vertical incidence skywave (NVIS) antenna for short-skip communications on 40 meters during the day? 19 00:02:30.611 --> 00:02:39.038 Answer. A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground. 20 00:02:39.638 --> 00:02:56.318 Deliberately low, so most of the radiation goes up and comes back down within a few hundred miles. Everything the DX operator avoids, done on purpose. Wide bandwidth: log-periodic. 21 00:02:56.318 --> 00:03:02.080 Question. Which of the following describes a log-periodic antenna? 22 00:03:02.980 --> 00:03:10.100 Answer. Element length and spacing vary logarithmically along the boom. 23 00:03:10.700 --> 00:03:16.728 Question. Which of the following is an advantage of a log-periodic antenna? 24 00:03:17.628 --> 00:03:21.028 Answer. Wide bandwidth. 25 00:03:21.628 --> 00:03:29.432 At any frequency in its range, a different subset of elements is near resonance and does the work. 26 00:03:29.432 --> 00:03:46.155 The result covers a decade or more of frequency with a consistent pattern, the reason television receiving antennas look like this. The trade is size and modest gain compared with a Yagi of similar boom length. 27 00:03:46.155 --> 00:03:47.668 Receiving antennas. 28 00:03:47.668 --> 00:03:52.948 Question. What is the primary use of a Beverage antenna? 29 00:03:53.848 --> 00:04:00.008 Answer. Directional receiving for MF and low HF bands. 30 00:04:00.608 --> 00:04:08.829 A very long wire, one to several wavelengths, laid close to the ground and terminated at the far end. 31 00:04:08.829 --> 00:04:24.540 It is a poor radiator and an excellent receiving antenna: directional, quiet, and unbothered by the ground losses that would ruin it for transmitting. Receive-only antennas are a real category. 32 00:04:24.540 --> 00:04:33.168 On 160 and 80 metres, hearing is the hard part, and an antenna optimised purely for it is worth the space. 33 00:04:33.168 --> 00:04:45.772 Question. In which direction or directions does an electrically small loop (less than 1/10 wavelength in circumference) have nulls in its radiation pattern? 34 00:04:46.672 --> 00:04:50.072 Answer. Broadside to the loop. 35 00:04:50.672 --> 00:05:03.710 A small loop responds to the magnetic field and has deep, sharp nulls in the plane of the loop, which is what makes it the classic direction-finding antenna. 36 00:05:03.710 --> 00:05:13.592 You locate a source by rotating for a null, not for a peak: nulls are far sharper than maxima. Mobile and VHF specials. 37 00:05:13.592 --> 00:05:20.914 Question. How does a "screwdriver" mobile antenna adjust its feed point impedance? 38 00:05:21.814 --> 00:05:26.254 Answer. By varying the base loading inductance. 39 00:05:26.854 --> 00:05:35.799 A motor moves a contact along a coil, changing the loading inductance and retuning the antenna from inside the car. 40 00:05:35.799 --> 00:05:45.756 It is the practical answer to mobile HF, where a resonant antenna is impossible and a fixed loading coil works on one band only. 41 00:05:45.756 --> 00:05:54.436 Question. In which direction is the maximum radiation from a VHF/UHF "halo" antenna? 42 00:05:55.336 --> 00:06:00.656 Answer. Omnidirectional in the plane of the halo. 43 00:06:01.256 --> 00:06:21.625 A dipole bent into a near-circle. It gives horizontal polarisation, matching VHF weak-signal practice, while remaining omnidirectional, which a straight horizontal dipole cannot. Standard for mobile VHF SSB work. 44 00:06:21.625 --> 00:06:36.325 The selection table. Need, Several bands, one antenna; Antenna, trapped dipole (poor harmonic rejection). Need, One support, simple; Antenna, inverted V. 45 00:06:36.325 --> 00:06:56.693 Need, 50 to 300 miles reliably; Antenna, low horizontal dipole, 1/10 to 1/4 wavelength up. Need, Very wide frequency range; Antenna, log-periodic. Need, Quiet directional receiving on 160/80 m; Antenna, Beverage. 46 00:06:56.693 --> 00:07:14.468 Need, Direction finding; Antenna, small loop, using its nulls. Need, HF from a vehicle; Antenna, screwdriver. Need, Horizontal and omnidirectional at VHF; Antenna, halo. Check yourself. 47 00:07:14.468 --> 00:07:32.242 Why does a multiband antenna make harmonic radiation more likely to get out? You need to find the direction of an interfering source. What antenna, and do you look for a peak or a null? 48 00:07:32.242 --> 00:07:40.217 What is the feed point impedance of an end-fed half wave, and what follows from it? 49 00:07:46.217 --> 00:08:03.056 It presents a good match on several bands, including the ones your harmonics fall on. A single-band antenna acts as a filter against them. A small loop, and you rotate for the null, nulls are much sharper than peaks. 50 00:08:03.056 --> 00:08:13.503 Very high, thousands of ohms. It needs a large-ratio matching transformer, and fed without one it puts RF current through the station.