Modifying AM receiver design for Shortwave.

Discussion in 'Wireless & RF Design' started by atldave, Aug 17, 2010.

  1. atldave

    Thread Starter New Member

    Jul 26, 2010
    13
    0
    OK. I need to build something, now (I got my grab bag and I want to apply what I've learned so far.). I've been reading the books, but unfortunately, when it comes to certain things, like circuit details, they gloss over them or I'm missing something really basic.

    I want to take this receiver and convert it to a commercial broadcast shortwave receiver: 4MHz to 20MHz - maybe higher.

    I will use a toilet paper roll tube (4.4cm X 10cm ) for a coil base. I will wrap it with 26 gauge magnet wire with tails every 10 turns or so for a total of 60 turns based on the equations here.

    I got that variable capacitor to de-solder from a circuit board - I don't have anything to measure capacitance so I was going to (trial and error) adjust the coil to make it work.

    As far as the diode is concerned, I have a bunch of 5245BP signal diodes that I would like to use, I have a few NPN transistors floating around and here's where I'm stuck - the capacitor 'C2' It shows 100p in the diagram. How do I calculate the time constant that will be needed for proper demodulation and filtering of the carrier? The time constant calculation is here but how to relate it to the frequencies of the radio? And will I need more than one?

    Or for that matter, how does one choose the proper diode?

    Any other thoughts, suggestions or comments would be greatly appreciated. :D
     
    Last edited: Aug 17, 2010
  2. atldave

    Thread Starter New Member

    Jul 26, 2010
    13
    0
    The more I'm reading, it looks like just a basic low-pass filter would work.
     
  3. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    You're right - the detector circuit just has to cut off everything above audio, so it should be the same for any RF band.

    But remember, the only selectivity to pick one station from a nearby station is the Q of the tuned circuit. You want the tuned circuit bandwidth wide enough to pass the audio sidebands but narrow enough to reject other nearby stations.

    My guess would be that the average home-built tuned circuit would be too low of a Q, meaning too wide of a bandwidth, so my suggestion would be to work on increasing the Q. You could do that by keeping the coil diameter close to its height, using thicker wire and using a large variable cap, if you have one. Also, by using space between the coil windings, you can reduce the capacitance between each winding to help the tuned circuit reach higher frequencies.

    But all the same, any coil could work just fine to pick up a powerful station having no nearby stations, so my best suggestion is to build anything as a start, try it out, and have fun!
     
    atldave likes this.
  4. atldave

    Thread Starter New Member

    Jul 26, 2010
    13
    0
    I'm trying to figure out my variable cap right now. It looks as though it's one of those VCs with trimmers built in or maybe I can adjust the capacitance of the VC?

    Or maybe it'll be the 'C1' in the diagram....
     
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