Antenna Pulls In AM Stations

While we can’t argue that FM has superior audio quality and digital streaming allows even higher quality in addition to worldwide access, there’s still something magic about hearing a weak and fading AM signal from thousands of miles away with nothing between the broadcaster’s antenna and yours. If you can’t have a big antenna — or even if you can — a loop antenna can help your big antenna fit in less space. In the video after the break, [TheOffsetVolt] covers an AM loop and shows how it can pull in distant AM stations.

Continuing with the educational radio he’s talked about before,  he adds a loop antenna that is two feet on each side of a square, making it four square feet in area. Although he calls it an amplifier, it’s really just a passive tuned circuit that couples to the radio’s built-in antenna. There’s no actual connection between the antenna and the radio.

We aren’t sure if the reradiation explanation is really what’s happening, or if it is just transformer coupled to the main antenna. But either way, it seems to work well. You can think of this as adding a preselector to the existing radio. Loop antennas are directional, so this design could work as a direction finder.

We have seen many loop antennas, some with novel construction methods.  Some even tune themselves.

26 thoughts on “Antenna Pulls In AM Stations

  1. You can’t argue that FM has superior audio quality? That’d be with the implication that it doesn’t, right? But, with all other things being equal, why does FM have superior or inferior audio quality to AM? Thanks.

    1. In AM the amplitude is carrying the signal so noise added on top is all treated as a signal as well, but in FM the frequency carries the signal, so for noise to make it through it must be close in power to the carrier, eliminating a lot of it as a result. At least that’s the way I remember it working. This also has the effect that if two stations are transmitting over each other, you will only hear the stronger of the two with FM.

    2. As alluded to in a comment below, there is a major difference in the bandwidth of the communication channels.

      Broadcast band AM is typically broadcast on stations spaced 9kz apart, band limiting the higher frequency components being transmitted to not a great deal more than 4.5kHz, if interference between adjacent stations is to be avoided. These higher frequencies, although not crucial for voice transmission, add a lot of timbre and character to music, harmony and vocals.

      Broadcast band FM (a.k.a. WBFM = wideband FM) on the other hand, has stations spaced 150kHz or more apart, greatly increasing the ability to transmit a wider bandwidth signal, which, after leaving enough room to avoid excessive splatter causing interference between stations, allows frequencies between 30Hz and 15kHz of the baseband audio per left and per right channel to be transmitted with broadcast band FM, which is not far short of audio CD channel bandwidth.

      Amateur radio and CB radio typically use narrowband FM (a.k.a. NBFM), where channel spacings are very tight and only allow frequency deviations of around 5kHz, imposing a ceiling on the bandwidth of the audio frequencies transmitted not dissimilar to that of broadcast band AM.

      1. Ironically, though, CB radio in AM is often softer than in FM, not only because of the missing FM “noise”. So you don’t need a squelch that in turn blocks weak signals. And AM allows two stations to be heard simultaneously. FM signals can’t coexist, which is not only questionable in terms of moral/ethics (coexistence), but also dangerous in serious applications. In aviation, AM is still be used because of this. SSB and FM are simply not trustworthy. ;)

        PS: For international readers, it would be nice to make clear which bands are meant. AM and FM are types of modulation simply.
        Traditionaly, in broadcast, VHF or ultra shortwave used FM, while mediumwave used AM. But longwave and shortwave used AM, too. :)

    3. Most of the AM sections of today’s consumer radios seem to be designed to provide AM reception at the lowest price, hence the lousy sound quality of AM. Take a step up to an older AM receiver design like the older GE SuperRadio or Sony synchronous detection tuners and you might be surprised. Some IBOC (HD Radio) receivers also do a better job with AM signals, even when IBOC is not being broadcast than the typical clock radio.

    4. Because AM stations are spaced 10 KHz apart, the FCC limits the audio range they can broadcast to limit interference. FM stations are spaced 100 KHz apart, allowing for a higher audio frequency range to be broadcast. Also, as another poster noted, the stronger FM signal will ‘capture’ the receiver and the other nearby station won’t even be heard. Compare listening to FM anytime with listening to AM at night to hear the difference.

      1. Bandwidth of modulating signal (the audio signal) about 15kHz for FM versus <10KHz for AM.
        Bandwidth of modulated signal (the transmitter RF signal) about, 150 kHz for broadcast FM programs versus 15 kHz (9kHz in Europe) for AM broadcast programs.
        Communications channels, AM, FM or whatever M, are much narrower band to allow max number of available channels, same argument for 9kHz RF bandwidth for AM in Europe.

  2. Anyone remember tuning analogue AM receivers slightly off of signal center ( for music stations) to get a perceived bit of brighter tone?
    40 years have passed by and now I can’t recall what explanation I read for why that happened.

      1. All receivers are selective, so that they receive the wanted channel and not other channels. Unfortunately the selective circuits tend to attenuate the upper and lower edges of the wanted channel as well (because cheaper to do that than not to do that). Tuning the receiver slightly to one side of centre will attenuate more of one side of the signal but reduce the unwanted attenuation of the other side of the signal. The result is that you do receive the whole of one side of the signal and that gives you the the brighter sound, but you also get higher distortion.
        The solution – get a well spec’d, well engineered (and thus expensive) receiver.
        All the above only applies to AM broadcast transmissions, not FM.

    1. it was probably a cheap way of adding headroom to the signal. sometimes asymmetric modulation is intentional in am transmitters, one side band is compressed so there’s headroom for the other to be boosted beyond 100%.

    2. We have WBAA Purdue’s educational AM station now 99 years old. It’s bandwidth went out to 20k after the AM revamp of the eighty’s it went just out to 10k, still hifi compared to all the other stations on the AM dial. I have recordings of comparison reception of 920AM and the much newer FM when simulcasting.

      Oh yeah in the those glory days we knew how to delta-tune a radio to get the full sound that was there. We didn’t like the new digital tuning in our cars because it left you with dead on muffled speech. I heard 920 once on a Heathkit tube receiver with 2 dial tuning and the narrow-wide IF selector switch. Wow! I had what I called the 920 special (a jazz tune) a tuner with the IF coils adjusted to wide bandwidth. A tuned loop would eliminate the 120Hz hum that often got into the pristine signal.

      In 2007 Halloween when WBAA 920 went to IBOC I removed the tuner, presets, and all other use of the station. They got a FM translator going in 2016 finally. I still have to use my loop antenna at work to listen late night to Coast to Coast AM. I have fired up the one wide adjusted tuner on a Saturday night 650AM to listen to the Grand Ole Opry.

  3. a “few” years ago when I lived in Mile End my FM radio “solution” was a twin cassette ghetto blaster thing that had AM/FM/SW
    I had lamp on a post with the cord right next to the telescopic antenna

    the radio would only work well on FM when the lamp was on, what the hell was that about?

    It also made a good “intruder alert” when people walked up the hallway, which is how I almost put an arrow through some coppers…

  4. I’ve made several of these back in the day and they worked great. I want to attempt one tuned for ~60kHz so my “atomic” clock can receive the WWV signal and set itself correctly. Many of these clocks can’t pickup the signal inside a building.

    1. A former cow-orker made a 60KHz loop antenna to beat WWV against our rubidium standard.
      I don’t know how many turns he made, but he made it by winding the wire around a wastebasket. (~14″ diameter?)

  5. Yes. AM as a mode has limited capabilities. But medium wave (MW) as a band still has enormous potential for broadcasting over a large region. O how I wish that something open like DRM30 were common on MW. FM-like quality squeezed into a 10kHz wide signal

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