19 Aug 2012

LF/MF Loop antennas on transmit

For the last few winters I've used a wire loop antenna on 136 and 500kHz transmit with moderately successful results. Well, considering the extremely low ERP it is better to say excellent results. My wire is not that thick: around 1mm PVC covered multistrand wire and the loop area around 80m square, with the lower part of the wire either very close to, or actually on, the ground.  Using the TX loop calculator at http://www.66pacific.com/calculators/small_tx_loop_calc.aspx the efficiency is around -30dB at 500kHz, so that 5W from the PA would result in around 5mW ERP radiated. My calculations and measurements, with less than ideal loop geometry, suggest the ERP is some 3dB lower at around 2-3mW. At 137kHz the loop is considerably less efficient at around -50dB.

For a small plot with nearby trees I am not sure how well this compares with a small Marconi with a short vertical section (around 6m max) and a horizontal section of around 6m. Anyone know the answer?
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4 comments:

  1. ik1wvqSun Feb 01, 06:27:00 pm GMT

    Hi Roger,
    can you please give me more infos about this antenna for 470 kHz ??
    - how do you put the antenna to resonance ??
    - how do you couple the tx power ??? (small coupling loop or toroid transformer??
    I live on the sixth and last floor, and I can access a large terrace (20 * 40 meters), but I can not put high poles.. then no "Marconi" antennas at all ,,,

    thanks in advance.

    73 de Mauro IK1WVQ

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  2. Roger G3XBMSun Feb 01, 08:39:00 pm GMT

    I resonated the loop with large value, high voltage, capacitors in series. I matched this to 50 ohms using a 42mm diameter T90 toroid. As the loop at resonance is a low impedance you need a step DOWN transformer. It is some time since I used this (old QTH and I have been very ill for 16 months now - stroke)) but I seem to recall the secondary had about 2-5 turns and the primary about 18 turns. The ratio should be adjusted to match from 50 ohs down to the RF resistance of the loop at resonance. My loop was vertical i.e one wire above the other, with the lower wire a few metres over ground.

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  3. ik1wvqSun Feb 01, 09:09:00 pm GMT

    Hi,
    see please this picture: www.stmb.it/ik1wvq.jpg
    I understand you correctly?

    thanks for your valuable informations.

    73 de Mauro IK1WVQ

    ReplyDelete
  4. Roger G3XBMSun Feb 01, 09:39:00 pm GMT

    Yes Mauro, exactly as your diagram.

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