Detecting Radio Waves

Discussion in 'Wireless & RF Design' started by Sparky49, Sep 10, 2012.

  1. Sparky49

    Thread Starter Well-Known Member

    Jul 16, 2011
    ***N00b alert***

    Hahaha, so we got that out of the way. :) This thread may make you pros facepalm, but please bear with me. :)

    I've sneakily built an antenna in my back garden, and as I don't have any radio licence (yet), all I can do is listen to or for signals.

    It's a simple 9m long antenna, however I haven't got any components to create a specific reciever quite yet. I thought that if I hooked up a sine wave to it at a resonant frequency, I might be able to detect any changes in voltage along the wave on oscilloscope - to see my basic setup, please look at the attachment.

    Now if I'm feeding this resonant signal into the antenna, (graph 1) shouldn't I be able to see where any radio waves picked up by the antenna have either added or subtracted from the original wave? (graph 2)

    I'm sorry if this makes you guys cringe, but I'd really like to get a grasp of radio, and I reckon the best way is to experiment. :)

  2. Papabravo


    Feb 24, 2006
    Not likely. The incoming RF signals will be in the microvolt range. The signal generator and the oscilloscope probably can't go that low. What the generator will see is the combination of outgoing signals and reflected signals depending on the configuration of the antenna. I'm assuming it is configured as a 9 meter "long-wire".

    It might be more interesting to look at the antenna with a spectrum analyzer.
    Sparky49 likes this.
  3. Sparky49

    Thread Starter Well-Known Member

    Jul 16, 2011
    Let me just dig out my spare £1500... ;-)

    Is there anyway that I could see something cool?
  4. K7GUH


    Jan 28, 2011
    Experimenting is good, but you need some basic book learning to go with it.
    An antenna 9 meters long is going to be resonant at or about 16 MHz for a quarter wave antenna. Any signals you may receive at that frequency are likely to be very weak, and undetectable by a simple circuit.

    I'd suggest you build an old fashioned crystal set, with a view to receiving signals at or about 1000 KHz. There are a gazillion sites on the web with info on how to do this.
    Sparky49 likes this.
  5. MrChips


    Oct 2, 2009
    Every budding electronics hobbyist should build a crystal radio at some point in their lives. It is simple and a lot of fun.

    Here is something you can begin with. The hard part is finding the right components.
    Find a real "cat's whisker" point contact diode. If not find a point contact Ge diode.
    Next, find a high impedance (10kΩ or higher) headset. Look for old army surplus receivers.

    Connect the diode between the long wire antenna and ground.
    Connect the headset across the diode.
    Enjoy! (Assuming you still have AM Radio Stations operating in your town).
    Last edited: Sep 10, 2012
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  6. Papabravo


    Feb 24, 2006
    I suppose that depends on your definition of something cool. There is stuff to see but you might have some trouble recognizing it for what it is. A crystal set is a pretty good idea and it will be hard to go wrong with that approach.

    Knowing where the activity is will also be helpful so having a receiver for any collection of bands is probably a good idea. If you happen to live near an international shortwave station like VOA or WWV you could also try building a special purpose receiver for one of those stations. Adding a one transistor amplifier to a crystal set for any HF frequency makes a world of difference.
    Sparky49 likes this.
  7. Sparky49

    Thread Starter Well-Known Member

    Jul 16, 2011
    I will give it a shot - thanks guys!

    My definition of cool is just about anything in electronics! I was getting geared up to see some 'cool' flickers on the sine wave, but alas it was not to be!

    Just seeing (or even hearing!) stuff working, I think, is incredible.
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    I saw something on TV documentary about soldiers in the trenches making crystal radio point contact diodes with a pencil lead touching onto a razor blade.

    Now that is "old school" electronics! :)
  9. Swifty01

    New Member

    Aug 30, 2011
    I made my first radio out of a toilet tube and a OA91 diode connected at one end of the coil wound on the tube and the other end to ground. To tune it I used two pieces of copper sheet one that moved with a screw and knob. That worked very well. I was about 8 then. I'm 56 now and still love experimenting with radio.