https://www.johndcook.com/blog/2022/03/30/portable-am-radio/ John D. Cook Skip to content * MATH + PROBABILITY + NUMERICAL ANALYSIS + SIGNAL PROCESSING + SEE ALL ... * STATS + EXPERT TESTIMONY + BAYESIAN ANALYSIS + RANDOMIZATION + SEE ALL ... * PRIVACY + HIPAA + CRYPTOGRAPHY + DIFFERENTIAL PRIVACY * WRITING + BLOG + TWITTER + ARTICLES + NEWSLETTER + TECH NOTES + SUBSCRIBE * ABOUT + CLIENTS + ENDORSEMENTS + TEAM + SERVICES (832) 422-8646 Contact How is portable AM radio possible? Posted on 30 March 2022 by John The length of antenna you need to receive a radio signal is proportional to the signal's wavelength, typically 1/2 or 1/4 of the wavelength. Cell phones operate at gigahertz frequencies, and so the antennas are small enough to hide inside the phone. But AM radio stations operate at much lower frequencies. For example, there's a local station, KPRC, that broadcasts at 950 kHz, roughly one megahertz. That means the wavelength of their carrier is around 300 meters. An antenna as long as a quarter of a wavelength would be roughly as long as a football field, and yet people listen to AM on portable radios. How is that possible? looking inside a portable radio There are two things going on. First, for reasons I don't understand very well yet, transmitting is very different than receiving. People are not transmitting AM signals from portable radios. Second, the electrical length of an antenna can be longer than its physical length, i.e. an antenna can function as if it were longer than it actually is. When you tune into a radio station, you're not physically making your antenna longer or shorter, but you're adjusting electronic components that make it behave as if you were making it longer or shorter. In the case of an AM radio, the electrical length is orders of magnitude less than the physical length. Electrical length and physical length are closer together for transmitting antennas. Here's what a friend of mine, Rick Troth, said when I asked him about AM antennas. If you pop open the case of a portable AM radio, you'll see a "loop stick". That's the AM antenna. (FM broadcast on most portables uses a telescoping antenna.) The loop is tuned by two things: a ferrite core and the tuning capacitor. The core makes the coiled wiring of the antenna resonate close to AM broadcast frequencies. The "multi gang" variable capacitor coupled with the coil forms an LC circuit, for finer tuning. (Other capacitors in the "gang" tune other parts of the radio.) The loop is small, but is tuned for frequencies from 530KHz to 1.7MHz. Loops are not new. When I was a kid, I took apart so many radios. Most of the older (tube type, and AM only) radios had a loop inside the back panel. Quite different from the loop stick, but similar electrical properties. Car antennas don't match the wavelengths for AM broadcast. Never have. That's a case where matching matters less for receivers. (Probably matters more for satellite frequencies because they're so weak.) Car antennas, whether whip from decades ago or embedded in the glass, probably match FM broadcast. (About 28 inches per side of a dipole, or a 28 inch quarter wave vertical.) But again, it does matter a little less for receive than for transmit. In the photo above, courtesy Rick, the AM antenna is the copper coil on the far right. The telescoping antenna outside the case extends to be much longer physically than the AM antenna, even though AM radio waves are two orders of magnitude longer than FM radio waves. Categories : Science Tags : Radio Bookmark the permalink Post navigation Previous PostApplications of continued fractions 3 thoughts on "How is portable AM radio possible?" 1. Andrew 30 March 2022 at 12:49 The reason why transmitting is different from receiving: you want a transmitting antenna to be *efficient*, but a receiving antenna can often be very inefficient and still do its job well. If you connect an antenna system with 0.0001% efficiency to a transmitter that generates 1000 watts, then that means that 1 milliwatt goes out into the world as RF and 999.999 watts become heat somewhere in the antenna system. Usually this is considered a bad result, and for the sake of effective communication we want to keep transmit antenna efficiency high. But if you connect an antenna system with 0.0001% efficiency to a receiver, which is 10km from a 1000-watt AM broadcast station, then that means that the antenna is delivering around a nanowatt (-60dBm) to the receiver rather than the milliwatt or so (0dBm) you could have gotten from a 100% efficient full-size antenna. A nanowatt is *plenty* of power for any decent receiver (it's still considered a very strong signal), and an entire milliwatt is actually in the range where you have to start worrying about *damaging* the solid-state amplifiers that go into most radios these days. An inefficient receive antenna reduces the strength of the signal that comes in, but it reduces the strength of any noise that's received over the air as well, so the signal-to-noise ratio (which is what really matters) remains virtually unchanged by antenna efficiency as long as the "received noise" stays well above the noise generated in the receiver's own electronics (including thermal noise). On frequencies below around 100MHz, this is almost always true, and down around 1MHz it's very very true. When you get to higher frequencies where the naturally-occurring noise is less and the efficiency of amplifiers is also less, *then* you start worrying about getting enough signal strength to satisfy the receiver's sensitivity. 2. Andrew 30 March 2022 at 12:59 The sensitivity of a good medium-wave receiver (the received signal strength at which the signal-to-noise ratio would start to degrade noticeably if there was no noise coming from the outside world) is around -120 to -110 dBm (1 to 10 femtowatts). 3. waldo 30 March 2022 at 15:04 antennas can operate down to 0.1 lambda ( wavelength ) before degradation ( using an L to make it longer ). a better question is: how does the antenna system for AM work with a 2 inch ground ?( the metal power return inside the radio ) ? ( which might make as a dipole in creative thought ) Leave a Reply Your email address will not be published. Required fields are marked * [ ] [ ] [ ] [ ] [ ] [ ] [ ] Comment * [ ] Name * [ ] Email * [ ] Website [ ] [Post Comment] [ ] [ ] [ ] [ ] [ ] [ ] [ ] D[ ] Search for: [ ] [Search] John D. Cook John D. 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