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A question about radio recievers

  1. Jun 16, 2017 #1
    Could somebody explain me why it would not be sufficient for a radio receiver of an AM signal to simply consist in two elements:
    1. A very long antenna.
    2. A speaker/headphones.
    The set up would be as follows, the antenna is connected to the speaker and the other part of the speaker is grounded. My reasoning is as follows: the EM wave induces an alternating current in the wire which in its turn is connected to the speaker which produces the sound signal.
    Thanks in advance!
    -A learner
     
  2. jcsd
  3. Jun 16, 2017 #2

    marcusl

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    The frequency is too high for your approach to work. AM radio signals are impressed, or "modulated," onto a radio frequency (RF) carrier that ranges in frequency from 530 to 1600 kHz. For efficient reception, some sort of resonator tuned to the carrier frequency is required. This also selects the particular channel you are interested in while rejecting other channels. The selective function is often performed at an intermediate frequency (IF), such as 455 kHz, which is reached through a process called "mixing" or "down-conversion," but it is possible to perform this selectivity at RF without any down-conversion. This latter is done in a "crystal radio," for example. Finally, to lift off the information (sound) or modulation from the carrier requires a "detector" which is a non-linear electronic element. There are many devices that can perform detection; a diode followed by some sort of low-pass filtering is common. These are the minimum elements needed in addition to your antenna and headphones.

    Old-fashioned crystal radios (so-called because the detector diode was formed from a piece of mineral or crystal) contain little more than these elements. You can build a modern crystal radio and use it to listen to AM radio stations. Here are two examples:
    http://www.sciencebuddies.org/scien...ctricity-electronics/crystal-radio#background
    http://sci-toys.com/scitoys/scitoys/radio/ten_minute_radio.html
    I encourage you to do a web search and to try your hand at building a crystal radio. You will find lots of information about radio waves, modulation, and detection, as well as radio construction.
     
  4. Jun 17, 2017 #3

    sophiecentaur

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    There is another basic problem with this arrangement. The speaker would need to be vibrating at the same frequency as the 'Carrier Wave' that is picked up on the wire. You could not get that to happen without driving the speaker so hard (a high voltage) that it would melt. Whilst it is possible to produce an Ultrasonic wave (many Megahertz) in the air by using a suitable transducer, as the name suggests, the wave would be Ultrasonic - frequencies above our hearing range.

    Actually, the limitation of our hearing is good news, in this respect, because, if we could 'hear' signals with all the frequencies of the Radio Frequency Spectrum, we would be totally confused by all the different signals at different frequencies that are passing by (and through) us all the time. A basic Radio receiver rejects signals at unwanted frequencies and, by various methods, 'beats them down' to an audible range. The Cat's Whisker diode is the simplest method and a diode, along with a very few other components does the job if you place it between your two listed items.
     
  5. Jun 17, 2017 #4

    berkeman

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    My very first radio receiver when I was about 8 years old was an unpowered crystal radio receiver with a long wire antenna outside my bedroom window (I think it was about 30 meters long running out to a tree). The only thing missing from your list is the crystal or other rectifying diode mechanism to recreate the baseband audio signal from the RF received AM signal...

    https://en.wikipedia.org/wiki/Crystal_radio

    EDIT -- I see now that @marcusl beat me to this already in post #2... :smile:
     
  6. Jun 17, 2017 #5
    Pretty much the same for me. Playing about with the radio and other bits that you could get cheap from government surplus shops is what first got me interested in physics.:smile:
     
  7. Jun 18, 2017 #6

    berkeman

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    This thread has veered off topic, so I'll clean it up some and leave it closed. I think the OP's question has been answered pretty well.
     
  8. Jun 18, 2017 #7

    berkeman

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    I've had requests to re-open this thread so the physics of EM AM radio transmission can be discussed to help the OP.

    Thread is reopened. Please keep replies on-topic and accurate. Thank you.
     
  9. Jun 19, 2017 #8

    Merlin3189

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    The substantial answer to your query has (I think?) been given. The frequency of the EM wave is much higher than audio frequency. So the AC in the wire would drive a suitable loudspeaker at supersonic frequencies. The audio frequencies, which are "carried" by the radio wave, are "modulated" onto it and need to be "demodulated" in some way to produce an AF current which could produce sound in a speaker.
    As Marcusl nearly said, you don't need to add much to your design: a diode is enough, if your radio wave is simple AM. Selectivity, as Marcusl did say, is an issue, if you want to choose which station to listen to. If you have a strong local signal (as I do), it is like listening to someone near to you with others talking in the background, but you don't have much choice. If you try to listen to someone else, loudmouth just keeps talking on top!

    What I actually wanted to comment on, was the "very long antenna", picked up by Berkeman with his 30m "long wire antenna". Presumably your VLA was intended to capture as much of the EM wave as possible, to give enough power/current to drive the speaker. And indeed as boys (or girls) we were urged to use as long a piece of wire as we could for the antenna (or in my case, aerial) of our crystal sets.
    But your conclusion from this may not be accurate. The current from an antenna does not simply increase with length. A "short" antenna has a very high impedance and can supply little current. As it gets longer, the impedance falls to a minimum when it is 1/4 wavelength long. This is usually taken as the optimum length. As you further increase the length, the impedance rises again up to 1/2 λ, then falls to another minimum at 3/4 λ and so on.
    The principle, "go longer, young man" works for our crystal sets, because the medium wave AM band starts at about 180m wavelength. So any antenna less than about 45m is actually short of the optimum for any station. For my favourite station, then the BBC Home Service on 200kHz long wave band, the optimum length single wire antenna is 375m . Very few of us had anything near long enough to be an optimum antenna, so increasing length would improve it.

    If you do go on increasing the length well beyond 1/4 λ, then the effect on your received current will depend on the impedance of your detector circuit. If you don't adjust this to account for the variation of antenna impedance, you could decrease the signal by increasing the length.
    As length increases, the directional sensitivity also changes. So unless you change the orientation, you could increase the antenna length and make the antenna less sensitive to waves from the particular direction you need.

    =========================
    BTWs
    Nice to join in v.happy reminiscences about crystal sets (mine had a Ge diode rather than a crystal, but I think it was a point contact diode - OA90). They depended on AM and would not resolve the more complex modulation systems around today. UK is threatening to switch off AM now that the evil DAB is taking over. Then crystal sets will be no more.

    Sophiecentaur celebrates the limitation on the range of frequencies we can hear, but is this right? Am I to be grateful that I can't hear half the range of frequencies that a young person can? It does not seem to be an advantage to me. Perhaps if I could hear up to 100kHz, I'd be annoyed by all the bat clicks? If dogs can hear ultrasonic sounds (is that a self-contradictory term?) presumably there should be something to hear that makes these sounds. WikiP suggests small rodents make such sounds, but don't say how. So perhaps I'd be totally confused by hearing small rodents? But perhaps I'd be good at finding the punctures in my bike tyre?
    Now if my hearing were even better (or for SC, worse) and I could hear up to 200kHz or more, I don't know what else I'd hear. But I don't think it would be radio stations. They broadcast radio waves, not sound waves. I don't know what VLF and ULF transmissions are around, but I believe they happen and I've never heard one AFAIK. Since they are very penetrative and long range, I'd have thought we would hear them, if it were ONLY a question of frequency range.

    If, as SC mentions, potentially we could use the long "short" wire to drive an acoustic transducer and convert the radio wave into an acoustic wave (very distorted I suspect.) , I don't think we'd hear the AM sound. As has been discussed in other threads, AM is not like the beats produced by adding signals of different frequencies. The only frequencies present in a broadcast 198 kHz AM radio signal are in the range 189 kHz to 207 kHz, inaudible even to dogs and children. No audio frequency radio waves are present.
     
  10. Jun 19, 2017 #9

    davenn

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    being a long standing amateur radio operator , I suspect Berkeman is well aware of this :wink:

    D
     
  11. Jun 19, 2017 #10

    Merlin3189

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    I intended to address OP., who might not. Berkeman simply got caught up in my comment because he used the magic words.

    Here, long wire is used formally to mean multiple wavelength end fed wire, but informally to mean any end fed wire, generally not cut to a multiple of 1/2 λ (or maybe not n/4 λ?)
     
  12. Jun 20, 2017 #11
    Your set up will receive all frequencies. If you receive a million stations at the same time it will be noise. You need a coil, capacitor, 1N34 diode to make it work. Coil and capacitor together is a tuned circuit that must match the frequency your trying to receive. Antenna must match the same frequency too.
     
  13. Jun 21, 2017 #12

    davenn

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    well a reasonably wide bandwidth :wink:

    a bit of an exaggeration, but definitely a lot of closer ones ... without a tuned circuit it may well just be noise
    I haven't actually experiment with just a length of wire into a detector diode to see what happens
    maybe nothing ??

    that will help lots, but there will still be a few stations on top of each other, depending on how many are in the local region
    because the selectivity will still be very poor


    for receive, no ... it isn't essential, it helps but far from essential ...
    the proverbial "piece of wet string " ... some random length of will work well for receive


    anyway I'm pretty sure all your comments have been well addressed in previous posts :smile:


    Dave
     
  14. Jun 21, 2017 #13

    Merlin3189

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    Ok. So now I've done the obvious and tried it!
    First thing I noticed is, we'd all forgotten the earth. If electricity does come down the long wire, it has to go somewhere. You can also use two long wires, a counterpoise instead of earth, or attach to the centre of the long wire and call it a dipole.

    I chucked 100m roll of shielded twin audio flex (just handy!) out of the bedroom window and took it 70m or so to a tree and threw the rest of the reel over a low branch. It ended up between 2m and 3m off the ground*. For the other end of the circuit I ran about 20m of similar wire down to where the water pipe enters the house and attached it to the mains earthing point (probably why I'm getting so much hum.) (* pathetic for an aerial, but I just couldn't throw it any higher.)

    As expected, I can't hear much with just the wire and headphones (and earth), maybe a bit of 50Hz hum.
    When the diode was added (actually the BE junction of a small Ge transistor 2G345A) I can indeed hear Radio 4 ( presumably Droitwich, 500kW about 20 miles away) audible, but hardly intelligible. I can tell it is speech, but not understand what's said. I listened to R4 on a real radio and compared the speech/silence pattern to satisfy myself that it was in fact R4. The 50 Hz is very noticeable with the speech: possibly it is modulating the speech because the diode is operating very non-linearly.
    The millions of other radio sources I can't hear, but no doubt with a bit of amplification (or maybe younger ears) they would be noise in the background with maybe one or two stronger ones being identifiable as distinct sources. Even with a selective receiver one can't get away from received noise until well up in the HF band.

    As has been commented on above, any old bit of wire is still a tuned circuit. With my wire so low I guess the large capacitance favours low frequency and I'm already selecting for R4 on 198kHz over the MF stations which are much more common. (But 500kW must help!)
     
  15. Jun 21, 2017 #14

    davenn

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    not really

    those of us that know about radio receiver .... particularly crystal sets know that an earth helps lots
    but it will work well without a physical earth
    typical cct .......
    will work without the earth

    th?id=OIP.6cYEvvT6o6KdaouEfQs9xQEsC4&pid=15.jpg

    generally if you use 2 separate coils the primary will be connected to antenna and earth and the secondary wont
    need to have an earthed side, but it does help :smile:

    cct4.jpg


    there's so many variations out there!

    Dave
     
  16. Jun 21, 2017 #15

    davenn

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  17. Jun 21, 2017 #16

    Merlin3189

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    I'm not sure what you're trying to say here. All your circuits show an earth?
    Perhaps you mean they will work if modified by omitting the earth?
    xtalset.jpg
    But then, as I've indicated, I think you're simply using stray capacitance, mainly through the body of the user. Not as effective as a piece of copper wire to a piece of buried metal, but something. Maybe I haven't shown enough series capacitors - chassis to body, body to radiator, pipe to ground or whatever.
    If there is no earth around, then we'd have to make do with the free space capacitance of the set and the body capacitatively coupled to it.

    Whatever. But current always goes somewhere and with RF it's usually through stray capacitances if you haven't provided anything better (& often, even if you have!)
    So perhaps I'm wrong to say people have forgotten the earth, maybe they just ignored it or took it for granted.
     
  18. Jun 21, 2017 #17

    davenn

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    ohh dear, you really didn't read my post, huh

    yes


    if you had read my text .... YES ..... UNLESS they are the dual coil type


    pays to real all the text :wink:
     
    Last edited: Jun 21, 2017
  19. Jun 21, 2017 #18

    berkeman

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  20. Jun 21, 2017 #19

    jim hardy

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    My 2 cents.
    Since OP does not seem versed in "RF Detection" , here's the basic physics of it:

    the average of a sinewave is zero.

    The diaphragm in op's headphone can only move in audio frequency range, it's too massive to move at radio frequency. That's a Mechanical not an Electrical limitation.

    So the diaphragm will mechanically average the radio frequency sinewave received on the long wire antenna.
    It will move according to that average, which is zero, so no sound .

    That Merlin heard power line frequency hum proves that the diaphragm can reproduce radio waves provided they fall in the audio frequency range.
    Adding a rectifier, as he did , changes the picture.

    The average of a rectified sinewave is NOT zero. it's some DC value, and a quick search will show pictures.
    http://www.electronics-tutorials.ws/accircuits/average-voltage.html
    upload_2017-6-21_11-6-21.png


    A headphone diaphragm will displace under DC.
    Rectifying the radio frequency wave allows to the diaphragm to displace according to the short term average of that rectified radio wave, averaging being done by the diaphragm's mechanical mass - spring behavior .

    That's "RF Detection" at its simplest.
    I too remember my first crystal set. A local 50 kw station boomed in.

    old jim
     
    Last edited: Jun 21, 2017
  21. Jun 21, 2017 #20

    davenn

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    here in Oz, we have the winter VHF/UHF and up field day this weekend ... being mid winter, it takes some effort to get out there:biggrin:

    D
     
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