capacitor value (wifi)

Discussion in 'Wireless & RF Design' started by mik3ca, Mar 26, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    I was playing around with my wifi card and antenna.

    When I hooked up the antenna to the card through a 3.9uH inductor, I picked up no signal.

    When I hooked up the antenna to the card through a 0.1uF capacitor, I picked up a signal, and Linux reports a strength of 15.

    When I hooked up the antenna directly to the card, I picked up a signal, and Linux reports a strength of about 8.

    this makes me think that I am on the right track connecting a capacitor between the antenna and the wifi card.

    I think I should lower the capacitor value.

    What should the capacitor value be and why?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Picking up any RF signal is done by grabbing some of the broadcast signal with a tuned circuit. The circuit is resonant at the frequency of interest, and tends to capture some of the RF energy.

    One common method of tuning radios is to use a variable capacitor with the antenna. The two elements form a tank circuit that is resonant throughout the broadcast band, and also tunes sharply enough to minimized adjacent station interference.

    At a guess, the capacitor may be improving the characteristic of the front end in your WiFi card. The frequency is high enough that the capacitor is also somewhat inductive, so it's hard to guess about a better value. I would tend to guess that picofarad values would be more reasonable to experiment with. Something like a 5 - 25 pF trimmer should show some effect.
     
  3. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    the frequency I'm picking up is around 2.4Ghz.

    I'll lower the capacitance.

    Thanks.
     
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