modulation and use of a capacitor before an antenna to block dc voltage

Discussion in 'General Electronics Chat' started by PG1995, Apr 27, 2012.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Hi

    I was watching this UTube video to learn about modulation. At 02:31 it is said, "By the way you always want a capacitor before your antenna to protect your circuit and block any DC voltages the antenna may accidentally touch".

    Okay. The capacitor would block DC voltage but it can also touch an AC source too. Above all lightning could also strike the antenna and this can produce AC voltage inside the antenna which would definitely damage the circuit. So, in my view, one should be more concerned about AC damage than the DC. Do I have it right and how do we protect the circuit from AC transients? Please let me know. Thank you.

    Regards
    PG
     
  2. K7GUH

    Member

    Jan 28, 2011
    191
    23
    If lightning is a credible threat where you live, you install a lightning arrestor. There are different varieties, but a simple one consists of a spark gap -- one side hooked to the antenna driven element, the other to earth ground. This might not divert a direct hit, but will suffice for "ordinary" protection.
    If lightning is not much of a threat, many people omit the arrestor, but it is still a good idea, and cheap insurance. Much cheaper than replacing a transceiver melted into slag.
     
  3. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thank you for the reply, K7GUH.

    Forget about the lightning. Most of the wires running around a place contain AC, not DC. So, the probability of an antenna coming in contact with AC is very high than with DC. The capacitor won't block any AC which means we are not secured against real danger and DC danger is just 'phantom' danger, in my humble opinion. What do you say? Please let me know. Thank you.

    Regards
    PG
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You need to select a capacitor that is low impedance to RF, but high impedance to mains frequency. This should be easy, since the frequencies are separated by several orders of magnitude.
    Where are you contemplating putting an antenna that might come in contact with AC?
     
  5. MBVet05

    New Member

    Jul 21, 2011
    27
    1
    You could select a DC Block. I have used them in my GPS set.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,002
    3,229
    Which is likely just a capacitor. ;)
     
  7. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Hi Ron

    Actually I'm not contemplating to put an antenna anywhere. :) While watching that linked video that point about the AC came to mind and I though I must ask about it.

    Regards
    PG
     
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