LF/MF Amplifiers

LF and MF operations typically require higher than normal transmit power, due to the inefficiency of LF/MF antennas.

Lower-power amplifiers.  These amplifiers can be used either as final output stages or intermediate amplifier stages.

Higher-power amplifiers.  In order to overcome high antenna losses on LF and MF, high-power amplifiers are often used.  Anywhere from 100W to 1kW amplifiers are common.  Most of these amps are solid state, usually using FETs originally designed for power supply of other uses.

  • NJD Tecnologies 100W 474 kHz amplifier.  A 630 meter amplifier that is prebuilt and tested for under $200.  Input is 1mW, runs off 24-28V.  Not tested by AMRAD at this time.
  • G0MRF 100W amplifier.  A smaller 136 kHz amplifier with an interesting design.  He uses two FETs, driven by a buffer amp, and a 4013 flip-flop, which divides the input frequency by two to create two 180-degree phased signals for the FETs.  Therefore, the input frequency must be twice the output frequency.
  • G0MRF 4-FET 250W-500W amplifier.  This 136 kHz amplifier design has been around for a while, David Bowman actually worked with AMRAD’s Andre, N4ICK on the output design.  There are also some modifications to this amplifier for different bands.  It uses four IFRP450 FETs, and can operate from about 60 kHz to 900 kHz.  N4TLF has one of these mostly built, but not yet tested.

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