WEBVTT NOTE Antennas: Polarisation, Length, and Gain 1 00:00:00.000 --> 00:00:01.427 Polarisation. 2 00:00:01.427 --> 00:00:06.709 Question. How is the polarization of an antenna described? 3 00:00:07.609 --> 00:00:12.209 Answer. By the orientation of the electric field. 4 00:00:12.809 --> 00:00:27.639 A vertical antenna radiates a vertical electric field; a horizontal dipole, a horizontal one. The magnetic field is perpendicular to it, and by convention we name the electric one. 5 00:00:27.639 --> 00:00:45.024 It matters because mismatched polarisation costs you signal, typically 20 dB or more between vertical and horizontal at VHF over a line-of-sight path. Which is why: FM repeater operation is vertically polarised. 6 00:00:45.024 --> 00:00:58.948 Handhelds and mobile whips are vertical, so everything else is too. Weak-signal SSB and CW work on VHF is horizontally polarised. Yagis are usually mounted horizontally. 7 00:00:58.948 --> 00:01:14.108 Two stations doing the right thing for their mode, on the same band, can be nearly deaf to each other. That is not a fault; it is polarisation. Length, resonance, and the 19-inch whip. 8 00:01:14.108 --> 00:01:29.515 An antenna is resonant when its physical length matches a convenient fraction of the wavelength. Start from the formula you already have: At 146 MHz, wavelength = 300 / 146 = 2.05 metres. 9 00:01:29.515 --> 00:01:34.294 A quarter of that is about 0.51 metres, roughly 20 inches. 10 00:01:34.294 --> 00:01:41.551 Question. Why is a 19-inch-long vertical antenna often used on 2 meters? 11 00:01:42.451 --> 00:01:46.891 Answer. It is a resonant quarter-wave. 12 00:01:47.491 --> 00:02:03.390 The 19 rather than 20 is the end effect: a real antenna behaves as slightly longer than its physical length, so the practical cut is a few percent short. Which direction does length move resonance? 13 00:02:03.390 --> 00:02:17.756 Shortening a dipole increases its resonant frequency. Shorter antenna, shorter wavelength, higher frequency, the same inverse relationship as always. Loading: cheating on length. 14 00:02:17.756 --> 00:02:28.571 Sometimes you cannot fit a resonant antenna, and a mobile HF whip is the obvious case: a quarter wave on 40 metres is ten metres long. 15 00:02:28.571 --> 00:02:33.744 Question. Which of the following describes a type of antenna loading? 16 00:02:34.644 --> 00:02:41.758 Answer. Electrically lengthening by inserting inductors in radiating elements. 17 00:02:42.358 --> 00:02:58.830 A loading coil adds inductive reactance, cancelling the capacitive reactance of a too-short antenna and bringing it to resonance at a physical length it has no business being resonant at. It works, and it costs you. 18 00:02:58.830 --> 00:03:08.177 A heavily loaded antenna has low radiation resistance and high losses, so it is resonant and inefficient at the same time. 19 00:03:08.177 --> 00:03:21.202 Resonance and efficiency are different properties, and confusing them is why people are disappointed by short mobile antennas. Handheld antennas, and why they disappoint. 20 00:03:21.202 --> 00:03:35.276 Question. What is a disadvantage of a handheld radio transceiver's short flexible antenna compared to a full-sized quarter-wave antenna? 21 00:03:36.176 --> 00:03:39.856 Answer. It has low efficiency. 22 00:03:40.456 --> 00:03:50.092 The stubby "rubber duck" is a helically loaded antenna, shortened the same way a mobile whip is, and paying the same price. 23 00:03:50.092 --> 00:04:03.096 Swapping it for a proper quarter-wave whip is usually the single biggest improvement available to a handheld, more than any amount of extra power. And a related trap: 24 00:04:03.096 --> 00:04:14.336 Question. What is a potential drawback of using a handheld VHF transceiver inside a vehicle that lacks an externally mounted antenna? 25 00:04:15.236 --> 00:04:21.228 Answer. Signal strength is reduced due to the shielding effect of the vehicle. 26 00:04:21.828 --> 00:04:40.253 A car body is a metal box. Sitting inside one with a handheld is operating inside a partial Faraday cage. An external antenna, even a magnetic-mount whip on the roof, is transformative in a way more power is not. 27 00:04:40.253 --> 00:04:53.028 Directivity and gain. A beam antenna concentrates signals in one direction. That is the whole definition, and it leads straight to what gain means: 28 00:04:53.028 --> 00:04:56.554 Question. What is antenna gain? 29 00:04:57.454 --> 00:05:05.121 Answer. The increase in signal strength in a specified direction compared to a reference antenna. 30 00:05:05.721 --> 00:05:23.070 Read that carefully. In a specified direction, and compared to a reference. A passive antenna adds no energy, it takes radiation away from directions you do not care about and puts it where you do. 31 00:05:23.070 --> 00:05:39.098 Gain is redistribution, not amplification. The reference matters: dBi compares with an isotropic radiator, dBd with a half-wave dipole, and they differ by 2.15 dB (dBi = dBd + 2.15). 32 00:05:39.098 --> 00:05:56.271 Manufacturers naturally quote the larger number. Of the common types, a Yagi offers the greatest gain, a driven element plus a reflector and one or more directors, all of which shape the pattern. 33 00:05:56.271 --> 00:05:59.001 How a dipole actually radiates. 34 00:05:59.001 --> 00:06:06.608 Question. In which direction does a half-wave dipole antenna radiate the strongest signal? 35 00:06:07.508 --> 00:06:10.971 Answer. Broadside to the antenna. 36 00:06:11.571 --> 00:06:23.847 Perpendicular to the wire, in both directions, in a shape like a doughnut with the antenna through the hole. Off the ends it radiates almost nothing. 37 00:06:23.847 --> 00:06:35.630 This is immediately practical: a horizontal dipole strung east-west favours north and south. Which way you hang the wire decides who hears you. 38 00:06:35.630 --> 00:06:52.273 And a 5/8-wavelength whip, popular for VHF and UHF mobile, has more gain than a quarter-wave, it compresses the pattern toward the horizon, which is exactly where the other stations are. Check yourself. 39 00:06:52.273 --> 00:07:08.998 Your handheld cannot reach a repeater from inside your car. Name the two cheapest fixes. A dipole is resonant at 7.2 MHz and you need 7.15 MHz. Longer or shorter? An antenna is advertised at "9 dB gain". 40 00:07:08.998 --> 00:07:11.635 What is missing from that claim? 41 00:07:17.635 --> 00:07:37.601 An external antenna on the vehicle, the body is shielding you, and replacing the rubber duck with a full-size quarter-wave whip. Longer. Shortening raises resonant frequency, so lowering it means adding length. 42 00:07:37.601 --> 00:07:48.155 The reference. dBi or dBd? They differ by 2.15 dB, and gain is meaningless without saying what it is gain over.