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FM notch/band stop filter

Discussion in 'Electronics & Electrics' started by goldpenis, Nov 19, 2014.

  1. goldpenis

    goldpenis Member

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    VIC
    I want to block a specific radio station (gold FM 104.3 here in Melbourne) in an attempt to stick it to the radio Nazi at my work. Would this be easily enough done by a basic filter implemented into one of the stages of the receiver or as part of the antenna input? Can anyone tell me where I can get some more info on filter design specifically for this purpose?

    Any help appreciated. Thanks
     
  2. starkers92

    starkers92 Member

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    Might be easier to build a 104Mhz oscillator, power it with a 9v battery, and hide it in your draw.

    Hypothetically, of course..
     
    Last edited: Nov 19, 2014
  3. RobRoySyd

    RobRoySyd Member

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    Rebroadcast a different station on 104MHz to really confound :)
     
  4. @rt

    @rt Member

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    Lol, that's great, get it done!! :D

    If you get to mess around inside the radio it would probably be simple to make one of the FM ipod transmitter
    kits and power it from the radio and mounted inside the radio.
     
    Last edited: Nov 19, 2014
  5. OP
    OP
    goldpenis

    goldpenis Member

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    Thanks for the suggestions. I really need this to be discreet and maintain the original functionality of the radio (all buttons, etc need to function but gold is filtered or severely attenuated). Re broadcasting on the same frequency could be interesting... maybe some really hard core rap or heavy metal! I'll have to look into this a little more. They should be purchasable through Jaycrap/Altronics correct?
     
  6. RobRoySyd

    RobRoySyd Member

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    Yes, they have several FM transmitter kits.
    If this is inside a building and KMs away from the offending transmitter then such a very low powered transmitter will probably swamp the real one if it's close by.

    Because of their low power, them being inside a building etc. there's no problem legally with them as you're not "broadcasting". At such low power at worst you'd get a frown from the authorities. On the other hand you are doing something inside your place of employment where your employer is judge, jury and executioner.
     
  7. 2xCPU

    2xCPU Member

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    The only filter with narrow bandwidth and deep enough notch (high Q) is a cavity resonator. The trouble is the size is dependent on frequency and at 100MHz, it'd be bigger than the radio.

    I've been in the same situation working in a quiet lab with six other people, then some twat walks in and tunes his phone to a radio station. I happened to be sitting within arms reach of a sig. gen.
    I found the radio could cope with a CW signal fine, but I adjusted the output power quickly (zero to a few mW), I could defeat the AGC and blank the audio.
    The radios output from full volume to nothing as fast as I could turn the attenuator - the result was very pleasing!

    2.
     
  8. Whoreof Babylon

    Whoreof Babylon Member

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    I was talking to an ACMA faults/interference investigator about this and he suggested that someone should not do something like this. i.e. Being a nucence by interfering with a FM radio station not technicly illegal but dont do it anyway.
     
  9. @rt

    @rt Member

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    It sounds like none of the ACMA's business.
    If the radio has an external wallwart to deliver low voltage supply to the appliance it's fine.
    If the radio has a straight mains cord and supply inside the radio then it's
    probably not legit to modify the appliance.
    If the radio is not yours then there's another problem if you tamper with it.

    If you can build the device into the radio the transmitter antenna can be connected via a pot to the radio antenna internally.
     

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