rf modulator / video broadcasting ?

Discussion in 'Wireless & RF Design' started by Mathematics!, Jan 29, 2010.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
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    Ok I bought a cheap radioshack rf modulator
    http://www.radioshack.com/product/index.jsp?productId=2103095

    I plugged the s-video of my pc into the s-video of this rf modulator and then plugged an antenna into the output.

    I have an old crt analog TV that I used some old bunny ear antenna that I had lieing around.

    I should be able to broadcast this pc screen or any signal I send to the rf modulator out to the antenna thru the air wirelessly to the old analog crt tv.

    But to much of my superize I didn't see any image not even a outline or blurry image. The only thing I can think of is that the signal needs to be amplified alot before I send it out an antenna....

    If this is all that it is then I will just get a signal amplifier and should be all set......

    Curious to know why it is not giving me any image at all???
    When I connect the output thru a coaxial cable instead of wirelessly be 2 antennas it works perfect nice and crisp clear... even if I touch the antennas together I get a good image....

    So my only thought is it needs some serious amplification or the antennas are really screwing it up.... but I think it must be amplifying more then anything because even with crappy antennas I should pick up something?

    Does anybody know how much I would have to amplify it to get it to cover a normal house size or at least a few rooms...


    Thanks for any help

    video/audio signal coming from pc , dvd ,...etc --- rf modulator onto VHF channel 3 or 4 ----- to antenna/amplifier.

    I would think this is all I need to make video broadcasts?
     
  2. bertus

    Administrator

    Apr 5, 2008
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    hello,

    The shown modulator has a very low output that allows direct connection to the TV set.
    It is not able to transmit the signal on its own.

    Greetings,
    Bertus
     
  3. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
    4
    So then it is just a question of amplifying it enough to get it to transmit wirelessly correct.

    So my only question is their any cheap radioshack or something signal amplifier that would make this possible?

    I not would a simple LM386 circuit work ?

    Thanks but their should be away to make small broadcasts with this rf modulator in someway...
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    A LM386 will not work, it is designed to amplify audio signals.
    You will need a RF transistor to amplify.

    Greetings,
    Bertus
     
  5. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
    4
    Well I am just curious .... if I amplify the rf signal out of the rf modulator will it beable to broadcast wirelessly to my TV analogly on channel 3 or 4. Because right now the only thing I can think of why it is not working is because it needs amplification?

    It does say 4 something watts on the rf modulator???

    Wondering if I just be a radioshack signal amplifier will it work then ???

    Thanks
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    At the first place, do you have a picture when you connect the modulator directly to the TV set?
    If so, you can proceed with the test to make it wireless.
    Canhhel 3 and 4 are pretty low in RF therms.
    A 2N3866 wil be a useable transistor for the amplifier.

    Greetings,
    Bertus
     
  7. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
    4
    Thanks but I would really not want to start building ampifiers components
    What I was thinking is getting radioshacks one of their signal amplifiers.
    Would this work???

    For the power or signal amplifier I am looking for how many watts it should be?
    Does anybody have a reference of how many watts will get me how far a range with the rf modulator

    Something at radioshack in the range of $40 to $60 if possible with really good range
    http://www.radioshack.com/search/in...RUE&s=D-StorePrice-RSK&origkw=amplifiers&pg=3
     
    Last edited: Jan 30, 2010
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Which amplifier did you have in mind?
    The page you showed are AUDIO amplifiers and not RF amplifiers.

    Greetings,
    Bertus
     
  9. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
    4
    I am just wondering
    If I get a rf or power amplifier how much watts will get me what distance?

    Like how much would 30 or 40 watts or 240watts ,...etc get me for distance...

    I know analog TV broadcasts used AM for the video which means it needs more amplifiication to get the same distance as an FM broadcast.

    I am just wondering if their is a formula or approximation formula that given the output wattage it will give you how much distance it is covered???

    Thanks
     
  10. bertus

    Administrator

    Apr 5, 2008
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  11. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
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    Well I guess if that is true my main question is how many watts do I need to get a certain distance for VHF channel 3 and 4 for an analog TV broadcasts?

    Channel 4 has a video carrier frequency of 67.25 MHz so for a omindirectional (straight up and down antenna the optimal would be
    1.1144701040892193308550185873606meters =
    3.65639798 feet

    So if I use a straight up and down wire for an antenna that is about 3.65 feet long it won't be the antennas fault so the only last thing it most be is amplification

    I will try the antenna tonight but in the meantime can anybody give me a range for that wattage needed to broadcast AM analog video on channel 3 or 4.
    Their must be an approximate formula for this?

    I know when I was making radio FM voice transmitters on the 88Mhz to 108Mhz band it was some inverse square law formula that worked pretty well for my power/range approximation. But that had to do partly by how sensitve the radio reciever (detector ...etc was ) But I was using the average radio as an approximation. As for TV tuning cards in a TV I don't know how sensitive this is typically?

    But I don't know if the same formula holds for AM or if it does if it holds for a different band like the VHF TV band?

    Curious as you go up in carrier frequencies do you need more power to get a given range? Or is it the other way around? I know as you go up in frequencies you are more prone to interference....? Which could be another factor in transmission but at least if I have a certain amplifier I should start to see the image as I move the transmitter closer to the TV.

    Thanks
     
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