Active Antenna Circuit

Discussion in 'Wireless & RF Design' started by gccradioscience, Apr 21, 2009.

  1. gccradioscience

    Thread Starter New Member

    Apr 30, 2008
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    I need to know if there are any other active antenna circuits that take just a
    2N3904 transistor. I want to build a active antenna for indoor use for 150 kHz to 530 kHz, the longwave band, and also to see if there are better active antennas to build without damaging the front end. MPF102 transistor
    circuits are fine, but if I see something that I don't understand or is hard to
    construct, I will reply back.

    Adam E.
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Here's what you need to know about active antennas (voltage probe antennas)

    A very very short whip (in terms of wavelength) exhibits extremely low radiation resistance (a small fraction of an ohm) and extremely high capacitive reactance (several megohms).

    You need a device with an equivalent high resistance....approaching infinity, which can only be had with a mosfet, or a voltage follower. Currrent mode op amps in a voltage follower configuration are actually very good for this application.


    With an active antenna, no attempt is made to match the impedance of the antenna...it's actually used as a true voltage source....meaning essentially no power is transferred. This requires the active device to have essentially infinite power gain, which a voltage follower approaches.


    Hope this gives you some ideas. It won't work with a simple low impedance device.

    Eric
     
  3. PRS

    Well-Known Member

    Aug 24, 2008
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    35
    That's interesting, KL7AJ. So we should use a FET. But why in the follower mode? Why not some voltage gain?
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    If you can achieve voltage gain without lowering the input impedance you can...but it's nigh unto impossible to pull this off. If you need voltage gain, do it AFTER the voltage follower.

    Eric

    P.S. You also want a device with extremely low input capacitance, as well, since this will essentially shunt your input signal. There's definitely an art form to making active antennas work properly.

    :)
     
  5. PRS

    Well-Known Member

    Aug 24, 2008
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    Thanks. You know that actually makes sense. But doesn't static electricity pose a danger to the FET?
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Since the antenna is very short (and presumably indoors!) the danger of static damage is fairly minimal. You can also get gate-protected mosfets. Another option is to use a VERY high resistance "bleeder" (10 megohms or more) to ground. This won't eliminate sudden "zaps", but it will prevent any gradual buildup of static charge on the antenna. You can build a >10 megohm resistor by drawing a heavy line on a piece of paper with a pencil.

    Eric
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    A long time ago I made this simple Cmos active antenna.
    It sound a little bit strange but the Cmos inverter chips will amplify the antenna signal.

    [​IMG]

    Greetings,
    Bertus
     
  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Hi Bertus:

    I remember seeing this circuit ages ago. Thanks for the memories!

    Indeed, CMOS works in the unsaturated mode, if you're careful to bias it, which the feedback loop(s) do(es).


    Another VERY different approach is the AMRAD LF antenna:


    http://www.prc68.com/I/LF-Ant.shtml

    Check the link to the original QST article. The Amrad antenna uses a POWER mosfet in the front end to get really good dynamic range. This is really jarring to some people....a POWER transistor in the front end of a receiver>??!!

    Yes indeed! This concept was first developed by amateur contesters who went to extreme lengths to eliminate intermod distortion in close quarter mult-operator stations. It's very effective...but you have to do it right!

    Eric
     
  9. gccradioscience

    Thread Starter New Member

    Apr 30, 2008
    2
    0
    I understand now that a 2N3904 will not cut it. I would need a JFET for to meet the high impedance circuit. 2N3904 is only good for audio circuits. I remember seeing a FM active antenna in a Electronics Now magazine that actually took 2 transistors. It was good for the VHF-FM bands. So for lower frequencies I have no choice but to use JFET circuitry just for impedance reasons. Well, I better stock up on JFETS more often. MOSFET's are my next pieces, but which MOSFET's would you recommend?

    Adam E.
     
  10. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    You have GOT to be one of the better RF guys out there.

    And two are very good friends, KA8EQP and N8DCC. The four of us should have lunch some time - or hold a Field Day together!
     
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