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.

ad: L-HROutlet
ad: l-rl
ad: L-Geochron
ad: Left-2
ad: L-MFJ
ad: Left-3
ad: abrind-2
ad: l-BCInc
  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.

    https://www.lfengineering.com/products.cfm#Components

    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?

    Thanks!
     
  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

    @WB0GAZ,

    Start by buying one of these off Ebay
    https://www.ebay.com/itm/332868745351?hash=item4d8085f887:g:~RoAAOSwRjxb4Kho.

    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

    https://www.lfengineering.com/products.cfm#Components

    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

Share This Page