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Thread: AM reciever (simple)

  1. #1 AM reciever (simple) 
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    I have tried to make an AM receiver but always failed. I found a circuit, around forums here.
    I would be pleased if anyone of you would take a look at it and say me if there is any mistakes and if there is any way that I don't have to connect the radio to the ground, instead use a battery or something. So it would be more portable. Also what type of capaciator does the one used in the circuit have to be. Shortened words:
    Cap - capacitor.Circuit AM.jpg


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  3. #2  
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    I tested it and it didn't work.
    Also tested some other ways but the only one that works is like this:
    symbol : for nothing
    \|/ Antenna
    :|
    :|
    :--Earphones-|
    ::::::::::::::::::|
    ------------------|
    |
    |
    --- Ground


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  4. #3  
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    Am receiver cropped.jpgThe new circuit, exactly same but made with Eagle
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  5. #4  
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    Quote Originally Posted by Silver Est View Post
    Am receiver cropped.jpgThe new circuit, exactly same but made with Eagle
    I'm not surprised that you are having difficulty. For one thing, the transistor has no possibility of turning on unless there's a strong enough voltage to forward-bias two series-connected diodes (the separate diode + the base-emitter junction of the transistor). In this case, that means that the received signal has to induce volts (battery voltage + 2 diode drops). That's a tall order! Basically, the transistor could never, ever turn on in practice. The upside is that the battery will last a good long time.

    A second fatal problem is that there's no source of positive voltage for the collector. So even if we did have a strong enough signal to turn on the base-emitter junction, no current would flow through the collector circuit. It's twice-dead.

    There are many alternatives. I suggest doing an online search for "simple one-transistor radio" circuits or something like that. You won't be able to drive a speaker (you'll likely want to use special, high-impedance headphones), but you'll be able to hear several AM stations if you live in a typical urban area, or have a good antenna (and you'll likely need a long one).
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  6. #5  
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    Thx, well twice dead is a bit too good said, I accually thought that there was no need for antenna and left it out. I have that weird personality to think things unnecesary.
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  7. #6  
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    How long is a long antenna?
    Is it possible to make a circuit like this without ground?
    Would it take a lot of amplification and why?
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  8. #7  
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    Quote Originally Posted by Silver Est View Post
    How long is a long antenna?
    Is it possible to make a circuit like this without ground?
    Would it take a lot of amplification and why?
    You will need a long antenna because the wavelength of AM radio is long (order of hundreds of meters). The radiation resistance of an antenna is unreasonably small for lengths much shorter than a wavelength, meaning that it will be difficult to couple the received energy efficiently into your receiver (you will effectively encounter a big power divider going the wrong way, thanks to ever-present losses). The shorter the antenna, the worse the loss, and the greater the amplification needed.

    The good news is that AM radio transmitters can be ludicrously powerful. This situation is by conscious design, to accommodate inexpensive, insensitive receivers. But that still requires you to provide substantial amplification.

    If you are trying to build a very simple radio, then that implies that you will be working in a regime of low amplification, and therefore you can't afford to throw away signal power in an insensitive antenna. That includes the need for a good ground. If you have lots of gain, you can get away with a small antenna and no explicit ground (as in all those cheap portable AM radios, which used 4-10 transistors, typically, in classic designs from decades past). But a one- or zero-transistor radio? Nope, sorry. You'll typically need an antenna of tens of meters in length, as well as a good ground.

    Now for some numbers: It's hard to say what a "typical" field strength is, but I measured millivolts/m where I grew up (in S. California). Some stations were a lot stronger than that (one was about one volt per meter!), others were weaker. An antenna of tens of meters in length would thus typically induce an open-circuit voltage of tens of millivolts (the product of length and electric field strength; this computation yields a good estimate when the antenna is much shorter than a wavelength), relative to a good ground connection. If you work very carefully, you can get a reasonable fraction of that to drive your circuit, whether that's the input of a transistor, or a simple diode-based detector and headphone (as in the classic "crystal" radio). In a quiet room with good high-impedance headphones, you get readily audible signals when the headphones are driven with millivolt levels.

    Why is the ground connection important? Remember that only voltage differences matter (there's no such thing as an absolute voltage). Your circuit will operate on the difference between the antenna voltage and whatever terminal your circuit calls ground. If that ground terminal is free-floating, its voltage will tend to track the antenna voltage, reducing -- sometimes greatly -- the available voltage difference for your circuit to operate with. Again, if you have huge excess gain to spare, you can overcome a bad ground to a great extent. But if you're trying to build a radio out of Star Trek's famous "stone knives and bear skins," you need to work much harder.

    In old handbooks, the canonical ground connection was made to a cold water pipe, or some other thing. With the advent of three-wire AC outlets, another option has become available: Use the third terminal (the round terminal, often called the "green-wire ground" terminal in the US). It's important to know which is the actual ground terminal; hooking your radio up to one of the power terminals can lead to very bad outcomes.
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  9. #8  
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    Quote Originally Posted by tk421 View Post
    There are many alternatives. I suggest doing an online search for "simple one-transistor radio" circuits or something like that. You won't be able to drive a speaker (you'll likely want to use special, high-impedance headphones), but you'll be able to hear several AM stations if you live in a typical urban area, or have a good antenna (and you'll likely need a long one).
    I'll piggyback off of tk421.

    Start with the most basic receiver: an aerial, a ground connection, a diode, and high-impedance headphones. You cannot tune its frequency or adjust its volume, but you will hear a few stations at the same time, depending on which ones are closer and stronger.

    Then add tuning: a capacitor and coil, with one or both being adjustable (preferably only one, to make it easier to use). These components must have certain values for the particular frequencies you want to receive. I'm assuming you want to listen to short wave (about 2 MHz to 30 MHz).

    Then add volume control: a simple transistor amplifier powered by a battery(s).

    Finally, you might want to have an amplifier powered by mains electricity and driving a loudspeaker and some sort of tuning dial (and encase the entire radio in a box to protect people from being electrocuted).
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