wideand active antenna circuit schematic

Wideband Active Antenna Circuit

A 30 to 50 cm whip antenna provides reception from 10 MHz to over 220 MHz. T1 BF981, a dual-gate MOSFET, provides low noise, high-input impedance and high gain. The circuit is powered via the coaxial cable used to connect the antenna to a receiver.

Wideband active antenna amplifier circuit

wideand active antenna circuit schematic


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  • ali

    I’m a novice to this game!
    my signal is like a RF signal.
    I found a balun transformer that supports 4Khz to 300Mhz:
    I think it’s a good choice but the ratio of Balun is 1:1 so the input impedance of LNA should be 100 Ohm!
    totally I want a low Noise figure and Linear LNA with the mentioned specification… I don’t have more knowledge in RF engineering 🙂
    Please help me…

  • G3TSA

    You are being very secretive , or are a novice to this game.
    We need lots more information.
    You say the signal is a few microvolts , that is pretty weak.
    What other undesired signals are around , and what level?
    Has the freq response to be reasonably flat (say + /- 3 Db)
    with error corrected in software?
    What is the impedance looking back into the source ?
    What accuracy over time / temp is required ?

    You are not going to get any ferrite transformer to work 1-200 megs. 50-200 maybe. 10kc/s to 200 megs not a chance.

    Perhaps operate the front end and digitize by battery power , enclosed in a metal box along with your source.
    Opto couple the output.

  • ali

    I’m working on my project.
    I need a LNA to amplify the input signal that should be sampled.
    the signal is in the microvolt range
    the frequency range is DC(or 100 or 1/10/100Khz) to 100Mhz or 200Mhz
    it’s better to isolate the LNA using a balun transformer from the primary side but the LNA is so important to me…
    I would be so grateful if you could help me…
    thank you.

  • G3TSA

    Would be worth adding a 22 ohm in the drain of BF981 , and a ferrite bead on base of 2N5109.
    Both to prevent vhf oscillations.
    If C3 ground end connected to BF981 source , the low (50Kc/s)freq performance would be much better.
    I would guess 2N5109 runs at 50ma, and the two pots are set for minimum intermod.
    I played with similar circuits but all suffered intermod from medium wave tx about six miles away.
    The best ariel I used was about a foot long pcb, broken into 2 inch sections coupled by tiny twisted coilwire capacitors.

  • dj_dx

    I’m afraid this won’t work properly.
    Fet has about 1200 ohms input impedance so you need some matching transformer for 50 or 75 ohms antenna.

    Also the noise figure will be poor without LC resonant circuit in the gate.

    • OutOfSight

      This is designed for a whip antenna which will NOT be 50 or 75 ohms, its likely to be in the tens of hundreds.

      Furthermore the noise figure will be much more related to the devices used than the input configuration. Selectivity is (should be) provided by the receiver front end, this is meant to be an active antenna, not a tuner!

      However it wasn’t designed by the people running this site so they’ll unlikely be able to offer help

  • Aren van Waarde

    Dear author,

    Regarding the VLF-VHF Wideband Active Antenna: you write that it allows reception in the 10 MHz – 220 MHz range. I think this is a slip of the wordprocessor, and text should read: 10 kHz to 220 Mhz range. Since, otherwise, it would not allow reception at VLF frequencies.

    Best regards,