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Idiot Question About RF burn

Discussion in 'Radio and Wireless' started by dietermoreno, Mar 31, 2013.

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  1. dietermoreno


    Dec 30, 2012
    Hello, the village idiot has returned.

    Time for stupid questions round 2.

    To save you the time of answering my questions in my "wireless electric current" thread, I watched YouTube tutorials about spark gap transmitters and AM transmitters for the past 3 hours.

    I found out that a transformer is NOT used in a spark gap transmitter to create radio waves, rather the transformer is used to STEP UP THE VOLTAGE.

    The high voltage is connected to a tank circuit consisting of an inductor and a capacitor (of course it is tuned before you start transmitting so you don't electrocute yourself!).

    Early spark gap transmitters did not have oscillators. Later spark gap transmitters used tank circuits to create the oscillator as the radio spectrum became more crowded with transmissions and also since resonance greatly amplifies a signal to be picked up by a reciever at the same resonance so the transmission distance is much further than with unmodulated spark gap transmitters.

    The spark gap is used because it was the first practical way to transmit before amplification components were invented since the gas breakdown makes an audible sound.

    I wonder what would happen if I touched the antenna of a spark gap transmitter?

    I think a spark gap transmitter looks dangerous to build and it unfortunately only works for on-off morse code and neither CW nor AM.

    I wonder if I could build an AM transmitter using spark gap transmitter principles, with the major change of there being no spark gap so AM is possible, and the transformer used to step up the voltage and current is a 500 W PA output transformer.

    So a 500 W transmitter?

    The audio signal is connected to the PA, amplified by the transistors in the PA, and the voltage is stepped up by the output transformer (the PA is solid state, so that means it doesn't actually have an output transformer, but it still has something to step up the voltage performing the same function) from the low line level (about 0.3 volts for consumer audio gear) to 8 ohm speaker level for a 500 watt amp (63.2 volts).

    P=(I squared)(R)
    500 watts = (I squared)(8 ohms)
    62.5 amperes = (I squared)
    square root (62.5 amperes) = I
    7.9 ampere = I
    V= (7.9 ampere)(8 ohm)
    V= 63.2 volts

    Increasing the load of the speaker increases the voltage, but decreases the current.

    Decreasing the load of the speaker decreases the voltage, but increases the current.

    Power can be increased by either increasing current or increasing voltage.

    Since my threads were locked, I went to this forum and found this thread about what happens if I touch an antenna.

    The thread linked to this page.

    So if I touch the end of a speaker cable with my finger connected to the output transformer of that PA, will that really give me RF burn, since we have already discussed that current in a wire creates a magnetic field?

    Or does RF burn only apply at radio frequencies and does not apply at audio frequencies?

    I don't want to try at home, because I don't want to burn a hole in my finger to see if RF burn applies to audio frequencies.

    Well if I connected a tank circuit consisting of a loop with a vari-cap and an inductor to the speaker cable and connected an antenna wire to the other end of the tank circuit, would 500 W of RF give me an RF burn from touching the antenna wire?

    Edit: Well actually, that wouldn't be AM RF either, that would just be base band at radio frequency, which is used for the spark gap transmitter, but is not used for the AM transmitter. but it would still be RF because the frequency is higher than audio frequency and lower than microwave frequency. just the RF has the base band at RF carrier wave that can be turned on and off by a morse key so it only works for morse code (which the morse code can be made to be heard as CW with my regenerative detector feedback loop).

    The AM transmitter uses a voltage controlled oscillator adding some resistors into the tank circuit so the base band at radio frequency carrier wave has the signal modulated into its amplitude peaks at a small voltage difference.

    Here I found a good link for FM transmitter that explains base band at radio frequency (this is how FM can be stereo and mono on the same carrier wave frequency for backwards compatibility with mono FM receivers) and explains voltage controlled oscillators (VCO):
    Last edited: Mar 31, 2013
  2. davenn

    davenn Moderator

    Sep 5, 2009
    what do you think ? do you really think RF = AF ??

    CW IS Morse code transmission

  3. BobK


    Jan 5, 2010
    No, such a transformer would respond very poorly to RF frequencies.

  4. dietermoreno


    Dec 30, 2012
    I didn't think AF would cause RF burns, even though it is electro magnetic waves, just electro magnetic waves at AF instead of RF.

    Lol that would be funny if AF electro magnetic waves caused RF burns because then every time a vocalist connected a speaker to the PA while the PA was on he or she would get RF burn.:D

    So its safe for me to try this experiment at home, as long as its not RF.

    I presume that if I attach a tank circuit to make the 500 W AF into 500 W RF then I shouldn't touch the antenna.

    So its safe for me at home to try adding a tank circuit, just don't touch the antenna wire when the transformer has power to it!
  5. duke37


    Jan 9, 2011
    If you carry on playing with high voltages, then this thread will become much less interesting!

    A DC or low frequency AC voltage will give a shock with the current passing through the flesh and damaging the nerves (see electric chair). A high frequency voltage will constrain the current to the surface (see skin effect) and burn the skin with all the problems that burns have.
  6. dietermoreno


    Dec 30, 2012
    Well I just verified that there is indeed a nasty spark formed when I connect the PA output speaker cable to my guitar which is connected to my guitar amp which is grounded as the signal sparks the gap jumping to ground.

    This big fat spark was formed with the PA on only master volume 1 (out of 10) and a few sparks turned part of my guitar string black.

    Even with my guitar amp gain turned to zero, the gas breakdown of the spark excited my guitar amp to amplify a high pitched buzz that hurt my ears.

    So if I touch an antenna wire connected to the cable I will get a nasty shock if I touch a wall outlet plate or touch a pipe!

    So I should NEVER touch the antenna wire even if it is only AF because of the high voltages I am working with now to obtain transmission level.

    Good thing I only turned the PA gain to one to start out with or I might have electrocuted myself...

    Unfortunately the PA amp has the speaker connected to it even when a cable is connected to the speaker output jack.

    So to generate a high enough voltage and not blow out my ear drums I should use my tube amp head (which tubes can handle high voltage I think) using a stereo to mono adapter to connect the audio signal to the tube amp head. Also that would be truer to the spark gap transmitter way because tube amps actually use an output transformer.

    Of course I should NEVER touch the high voltage speaker cable end while the tube amp is on.

    Also just in case, I probably should NEVER touch the high voltage speaker cable at all when the amp is on because a spark could arc through the insulating material to my body if I accidently ground myself by accidently stepping on my guitar strings which are connected to my other guitar amp which is grounded.

    My tube amp is 100 W.

    Well here's a video of my spark gap transmitter. I thought I'd just put it here instead of spamming you with a new thread.
    Last edited: Mar 31, 2013
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010

    Will you show us a video when you manage to finally destroy wither your guitar or amp?

    All you've managed to show is that electrical interference can be picked up by sensitive circuits.

    Despite all the care that the manufacturers of these things go to in order to make this stuff idiot proof, there are three things to consider:

    • "It is impossible to make anything foolproof, because fools are so ingenious." --RobertHeinlein
    • "Those who try to build idiot-proof systems always underestimate the persistence and ingenuity of idiots." --anon
    • "If you make your system more idiot-proof the idiots will build a bigger idiot" --anon
  8. dietermoreno


    Dec 30, 2012
    Okay, so maybe using my other guitar amp as ground of my spark gap transmitter is not a good idea if I don't want to (1) blow up my amp, (2) electrocute myself, and or (3) start my house on fire.

    So much safer would be to use the wall outlet screw.

    The wall outlet screw I have heard is grounded.

    It is safe to use my screw driver to lossen the screw and wind a wire around it and then tighten the screw to create a ground wire without turning off the power at the circuit breaker to that circuit, correct?

    Or do you always turn off the power at the circuit breaker to the circuit containing the wall outlet whose plate you are unscrewing?

    I've seen it done in YouTube videos of home made radios that connect the ground wire to a wall outlet screw.
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    We're not going to be complicit in your attempts to kill yourself.
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