WEBVTT NOTE Receiving Antennas and Direction Finding 1 00:00:00.000 --> 00:00:03.578 Why receiving antennas are judged differently. 2 00:00:03.578 --> 00:00:10.994 Question. Which is generally true for 160- and 80-meter receiving antennas? 3 00:00:11.894 --> 00:00:20.073 Answer. Atmospheric noise is so high that directivity is much more important than losses. 4 00:00:20.673 --> 00:00:26.277 This is the insight that makes receiving antennas a separate discipline. 5 00:00:26.277 --> 00:00:40.443 On the low bands the noise arriving from the sky vastly exceeds the receiver's own noise, so making the antenna more efficient raises signal and noise together and gains you nothing. 6 00:00:40.443 --> 00:00:47.370 What helps is rejecting noise from directions you do not want. Hence the figure of merit: 7 00:00:47.370 --> 00:00:53.490 Question. What is receiving directivity factor (RDF)? 8 00:00:54.390 --> 00:01:03.107 Answer. Peak antenna gain compared to average gain over the hemisphere around and above the antenna. 9 00:01:03.707 --> 00:01:17.305 Not gain over a dipole, and not front-to-back ratio. RDF compares the peak with the average over the whole sky, because noise arrives from the whole sky. 10 00:01:17.305 --> 00:01:31.615 An antenna with modest gain and a genuinely quiet pattern has a high RDF and hears better than a high-gain antenna that also hears everything else. The Beverage. 11 00:01:31.615 --> 00:01:38.547 A long low wire, terminated, and one of the great low-band receiving antennas. 12 00:01:38.547 --> 00:01:50.154 Question. When constructing a Beverage antenna, which of the following factors should be included in the design to achieve good performance at the desired frequency? 13 00:01:51.054 --> 00:01:55.695 Answer. It should be at least one wavelength long. 14 00:01:56.295 --> 00:02:10.952 At 160 metres that is over 160 metres of wire, which is why Beverages belong to people with fields. Longer is better; directivity improves with length. 15 00:02:10.952 --> 00:02:17.291 Question. What is the function of a Beverage antenna's termination resistor? 16 00:02:18.191 --> 00:02:22.564 Answer. Absorb signals from the reverse direction. 17 00:02:23.164 --> 00:02:34.227 Without it the antenna is bidirectional; with it, unidirectional. Same mechanism as the terminated rhombic from the wire-antennas lesson. 18 00:02:34.227 --> 00:02:41.310 Question. What indicates the correct value of terminating resistance for a Beverage antenna? 19 00:02:42.210 --> 00:02:49.210 Answer. Minimum variation in SWR over the desired frequency range. 20 00:02:49.810 --> 00:03:06.664 An elegant test. Correctly terminated, the wire behaves as a matched transmission line, so its input impedance is nearly constant with frequency. Sweep it and adjust the resistor for the flattest response. 21 00:03:06.664 --> 00:03:18.010 Optimising for lowest SWR at one frequency gets the wrong answer; flatness across the range is the criterion. Loops for direction finding. 22 00:03:18.010 --> 00:03:24.941 Question. What challenge is presented by a small wire-loop antenna for direction finding? 23 00:03:25.841 --> 00:03:30.801 Answer. It has a bidirectional null pattern. 24 00:03:31.401 --> 00:03:44.422 A small loop has two nulls, 180 degrees apart, so a bearing is ambiguous, the source is in one of two opposite directions and the loop cannot say which. 25 00:03:44.422 --> 00:03:51.704 Resolving that ambiguity is the central problem of loop direction finding. Two fixes: 26 00:03:51.704 --> 00:03:56.264 Question. What is the function of a sense antenna? 27 00:03:57.164 --> 00:04:05.684 Answer. It modifies the pattern of a DF antenna to provide a null in only one direction. 28 00:04:06.284 --> 00:04:21.915 A small vertical whip added to the loop, combined in the right phase and amplitude. The loop's figure-eight plus the whip's omnidirectional response gives a cardioid, one null instead of two. 29 00:04:21.915 --> 00:04:30.915 Question. What type of radiation pattern is created by a single-turn, terminated loop such as a pennant antenna? 30 00:04:31.815 --> 00:04:34.343 Answer. Cardioid. 31 00:04:34.943 --> 00:04:43.325 Question. What feature of a cardioid pattern antenna makes it useful for direction-finding antennas? 32 00:04:44.225 --> 00:04:47.785 Answer. A single null. 33 00:04:48.385 --> 00:05:03.209 Unambiguous. And note again that DF works on nulls rather than peaks, a null is sharp, a peak is broad, so the bearing is far more precise. Loop construction. 34 00:05:03.209 --> 00:05:11.668 Question. What is the purpose of placing an electrostatic shield around a small-loop direction-finding antenna? 35 00:05:12.568 --> 00:05:22.528 Answer. It eliminates unbalanced capacitive coupling to the antenna's surroundings, improving the depth of its nulls. 36 00:05:23.128 --> 00:05:40.937 A small loop responds to the magnetic field. Any electric-field coupling to nearby objects, a hand, a body, a wall, unbalances the loop and fills in the null, which destroys the only thing the antenna is for. 37 00:05:40.937 --> 00:05:51.382 A shield (split, so it does not form a closed turn) removes the electric coupling and leaves the magnetic response intact. 38 00:05:51.382 --> 00:05:58.924 Question. How can the output voltage of a multiple-turn receiving loop antenna be increased? 39 00:05:59.824 --> 00:06:07.716 Answer. By increasing the number of turns and/or the area enclosed by the loop. 40 00:06:08.316 --> 00:06:24.439 Voltage is proportional to turns times area times rate of change of flux. Both are yours to choose, and a multi-turn loop is how you get usable output from a physically small antenna. Check yourself. 41 00:06:24.439 --> 00:06:37.646 Why does making a 160 metre receiving antenna more efficient often not help? You take a bearing with a small loop and get two possible directions. What do you add? 42 00:06:37.646 --> 00:06:43.236 How do you know a Beverage's termination resistor is the right value? 43 00:06:49.236 --> 00:07:03.523 Atmospheric noise dominates on the low bands, so more efficiency raises signal and noise together. Directivity, rejecting noise from unwanted directions, is what helps. 44 00:07:03.523 --> 00:07:20.956 A sense antenna, which converts the bidirectional figure-eight into a cardioid with a single null. Minimum variation in SWR across the frequency range of interest, flatness, not a minimum at one frequency.