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Re: [loopantennas] Tuned loop better than untuned? How to match, what amplifier to use?

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  • Richard (Rick) Karlquist
    ... My experience with the NCJ loop is that no preamp is needed beyond the built in preamp in the radio. The loop as described in NCJ has more noise output
    Message 1 of 3 , Feb 3, 2013
      On 2/3/2013 2:20 AM, Vojtech wrote:

      > I wonder whether matching a resonated loop with a balanced JFET high
      > impedance source follower is a good design choice as there is a rumor
      > that such an amplifier is prone to electric imbalance of the loop, which
      > makes the loop act as an active monopole. Rick K6RK in his paper "An
      > Improved Feed Network for Loop-Type Receiving Antennas" describes high
      > impedance matching of a resonated loop by a transformer on an Amidon
      > ferrite material 61 toroidal core. I measured two of these transformers
      > back to back and the losses are very low indeed, which makes me thinking
      > of using a W7IUV 20dB preamp just after the transformer to bring the
      > loop signal power up to the normal levels. Is it a good idea, or will
      > the JFET source follower work better?

      My experience with the NCJ loop is that no preamp is needed beyond
      the built in preamp in the radio. The loop as described in NCJ has
      more noise output than the noise floor of my Yaesu FT1000. The matching
      network described transforms ohmic losses in the coax to 50 ohms at
      the output connector. The effective noise temperature is therefore
      room temperature (300K). If you want to use a preamp, it can just
      be a conventional 50 ohm amplifier, where it is easy to get a NF
      of a few dB. Alternately, you can double the size of the loop and
      pick up about 9 dB, but it will now be 160 meters only.

      Bring "the loop signal power up to the normal levels" will not increase
      the sensitivity if the external noise already is dominating. It will
      make the AGC perform better. My QTH is only 6 miles from a 50 kW
      BCB station, so I avoid preamps, and if I do use them, I have to
      preceed them by a high pass BCB reject filter.

      Rick N6RK
    • Vojtech
      Hello Rick. ... I am doing an exercise in simplicity. Currently the only HAM HF transceiver in house is a tiny kit fitted in the Altoids tin - the ATS-3b by
      Message 2 of 3 , Feb 8, 2013
        Hello Rick.

        > My experience with the NCJ loop is that no preamp is needed beyond
        > the built in preamp in the radio. The loop as described in NCJ has
        > more noise output than the noise floor of my Yaesu FT1000.

        I am doing an exercise in simplicity. Currently the only HAM HF transceiver in house is a tiny kit fitted in the Altoids tin - the ATS-3b by KD1JV. The receiver is a common 2x SA612 based superhetodyne. I matched a 37cm diameter loop wound with 1cm diameter aluminium tube to 50 Ohm coax using a transformer wound with 50t:3t on FT50-61 toroid core. The matching was verified with a VNA, matched impedance is around 40 Ohm on 40m, 80 Ohm on 20m, those are my primary bands of interest. Because the original band pass filters of the radio kit are lossy, I fitted the radio with a replacement bandpass filter with single resonated transformer wound on a FT37-6 core, matching the input 50 Ohm to the 1.5 kOhm input impedance of the SA612. This is my 2nd prototype, the first matching circuit just connected the loop to the input of the SA612 with a 4:1 Guanella transformer wound on FT50-43. The 4:1 matching provides a bit stronger signal, but I am sticking to the solution with the 50 Ohm coax and the 2nd resonator to ease the strain on the SA612. Yes, I know the theory sais the Guanella transformer will be lossy at high impedances because the transmission line impedance is low, but it works quite well in my situation.

        I am doing this exercise as a learning oportunity and to have a portable receiving equipment for hotel stays. The loop has to fit into my backpack and has to be small enough to be easy to rotate and to fit on the window sill. The loop with the simple matching circuit to the SA612 performs reasonably at the outskirt of Prague, where the power line noise is very strong. I am able to significantly reduce the powerline noise by positioning the loop parallel to the powerline direction. The loop noise over receiver noise on 40m is quite marginal though and I will have to do more tests in the radio quite environment to asses whether the SA612 will provide enough amplification and be quiet enough to bring the background noise over the receiver noise. If not, then either an external amplifier or bigger loop will help, but I would rather avoid the 2nd.

        My original question really was, what amplifier and amplification levels one really needs for such a tiny loop to overcome receiver noise (receiver without preamp) and whether it is better to amplify at low impedances or at the high impedances with the FET followers. I think I will have to try both.

        To fit a bigger loop into a backpack means likely to build a wooden cross and using a coax for the loop, but then it would be difficult to rotate indoors and the symmetry will be compromised meaning worse chance to null out indoor noise.

        With the 1 foot dmr loop on the windowsill with no preamp I hear NZ, US and JA stations on 40 and 20m regularly, so the loop certainly works.

        73, Vojtech OK1IAK
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