Looking for design for e-probe antenna for use with lfengineering L-400B upconverter

Discussion in 'The Low Bands - 630/2200 Meters - VLF' started by WB0GAZ, Oct 28, 2021.

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  1. WB0GAZ

    WB0GAZ Ham Member QRZ Page

    Have the L-400B converter unit (part of the product below), which is part of lfengineering's L-400B antenna product.


    I'm looking for design for a suitable e-probe antenna to go with the upconverter that I can build.

    The manual at https://www.lfengineering.com/files/pdf/L-400B_inst.pdf shows the antenna element as 2' long, with high impedance low-pass filter followed by a gain stage, but doesn't elaborate further.

    Any pointers/ideas?

  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Last edited: Oct 28, 2021
  3. WB0GAZ

    WB0GAZ Ham Member QRZ Page

    Thanks KA0HCP - that's very helpful! I had found a couple of the resources you posted, but several new to me. I'll read/study.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page


    Start by buying one of these off Ebay

    Put it out out in the back yard as far from house and powerlines as possible. Mount it on top of a 10ft stick of EMT conduit. Make sure the conduit is earth grounded at the bottom (using a ground rod). Feed it with IckyPick-flooded, quad-shield, outdoor RG6. Make sure that the ground side of the preamp PCB and the shield of the RG6 is bonded to the conduit at the top. Use a high quality coax Common Mode Choke on the RG6 (make sure that the choke works at VLF) placed ~2ft laterally out from the base of the conduit at ground level.

    The "e-field antenna" that you have just built is really a 10.1ft tall conductive monopole standing-on/grounded-to earth (the last 0.1ft is inside the preamp radome). Arriving signals induce a current along the conduit. The PA0RDT knock-off preamp mounted on the conduit is just a clever way of locating the feedpoint near the top end of the monopole, and since the impedance at that feedpoint is very high (>50K Ohms), the pre-amp does the matching to 75 Ohm coax.

    I bought one of those, and tried it various ways. The formula above is what worked with the best snr for VLF signals.
    Last edited: Jan 2, 2022
  5. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    Let's see what we have here. The L-400-B active probe specifications are:

    26" probe x 1" diameter
    10 kHz - 530 kHz
    E-field sensitivity -6 dBuV/m typical (in what E-field?)
    Filter >40 dB above 600 kHz
    Output impedance 50-100 ohms


    Th manual states -3 dB and this makes more sense than -6 dBuV/m. For now let's go with - 3 dB. Terminated into an infinite impedance a 26" monopole produces 330 mV in a 1 V/m E-field. Its sensitivity is -10 dB. So, the filter and amplifier gain is 7 dB. The amplifier gain is specified as 20 dB and that leaves 14 dB for filter loss. The 50-100 ohm output impedance points to a source follower rather than an IC op amp output stage.

    The probe (monopole) capacitance is ~10 pF and this is what it looks like as a source; A signal generator coupled thru 10 pF.

    Generator --> 10 pF series --> LP filter --> 20 dB amplifier/buffer --> ~75 ohm output

    The LP filter spec could be:
    Input driven by 10 pF
    Output drives a FET gate (~3 pF for a FET follower)
    Passband (10 kHz - 500 kHz) attenuation -14 dB
    Filter rejection >600 kHz 40 dB

    We will fix my mathematical errors on the next go around.
    Last edited: Jan 7, 2022

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