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I've noticed that increasing load on magloop reduces it's Q-factor, it becomes less selective. Traditional way to approach this is reducing size of pickup loop, or alternatively, using transformer.

But is it possible/known approach to build FET high-Z preamp, which is connected to pickup loop? Being high-Z it should allow to reach maximum possible selectivity for magloop, as only minimal current will flow due to capacitance of the gate. One might even make it resonant.

I understand that high-Z would not be impedance-matched to 50-Ohm, but in this case it's not the goal. Goal is to let main loop oscillate with high-Q. This will increase field strength, and might allow good reception even with mismatched pickup loop.

Magloop example: This one is from KR1ST, but mine is similar. Depending on size of the small, low-Z coupling loop - looses in main loop increase and it becomes less selective. enter image description here

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    $\begingroup$ What do you mean by oscillate? And on my old low-Z magnetic loop, I used a separate coupling loop and a low-Z preamp. Can you please edit your question and insert a picture or drawing of your magloop? $\endgroup$
    – Mike Waters
    Feb 1 at 17:55
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    $\begingroup$ @MikeWaters I updated the question. I am also using separate coupling loop, and noticed that when coupling loop is larger - system becomes less selective. $\endgroup$ Feb 2 at 14:54
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Certainly it is possible to build a high-impedance amplifier. If you resonate the loop with a parallel capacitor (which perhaps includes the input capacitance of the amplifier), you've made a parallel LC circuit which at resonance has minimum current and maximum voltage. A high-impedance amplifier is what you'd want to best extract that signal.

But if your objective is to use the loop on a wide frequency range without retuning, this is the opposite of what you want. Instead, you want a low-impedance amplifier, the lower the better. You can think of the loop as a current source by Faraday's law of induction. The lower the input impedance of the amplifier, less impedance to this current. LZ1AQ has an example of such an amplifier:

enter image description here

In this design there is a passive filter at the input to attenuate the AM broadcast band and avoid overloading the amplifier. The low input impedance is achieved through the common-base Q3 and Q4, then Q1 and Q2 further buffer the signal and are designed to drive a twisted pair.

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    $\begingroup$ Thanks for detailed answer! I am focused on tunable, high-Q resonant loop, it will require re-tuning on any frequency change, but hopefully will offer superior filtering of out-of-band signals. $\endgroup$ Feb 1 at 1:24
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    $\begingroup$ @BarsMonster selectivity of modern receivers is so good, there's nothing to be gained from a high-Q loop. So unless you're building an ultra-simple crystal set or something like that, I would not expect any measurable performance improvement from a loop -- just a lot of inconvenience in tuning it. $\endgroup$ Feb 1 at 14:30
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    $\begingroup$ @PhilFrost-W8II Unless he is using to to null out local RFI. $\endgroup$
    – Mike Waters
    Feb 1 at 17:51
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    $\begingroup$ @MikeWaters with the directivity of the loop? Does the directivity change significantly with Q? For that matter, is the directivity of a loop substantially different from that of a dipole? $\endgroup$ Feb 1 at 23:53
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    $\begingroup$ @MikeWaters I am using SDR receivers: rspdx (it has some band filters, and tracking filter - but as far as I understand filtering performance is less than on high-end transcievers). Redpitaya SDRlab 122-16 - has no filters at all, it expects me to do all the filtering for more customized solutions. $\endgroup$ Feb 2 at 14:58
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If you prefer to purchase rather than build your own, high-input impedance preamps from Hi-Z Antennas sold by DX Engineering may work for you. Because they are often used to drive multi-element phased array systems at long wavelengths, they are designed to work with inexpensive 75-ohm coax, but should work fine with 50-ohm cable in your application.

Two bullet points from the manual may be important considerations for use with a small receiving loop:

  • The amplifier input Resistance is approximately 54KOhms in parallel with 12 Picofarads
  • The Antenna Ground input terminal is AC coupled to eliminate any ground related DC current

Many other features that you might value. I have no affiliation with Hi-Z Antennas or with DX Engineering.

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John Kaufmann, W1FV, published a two-part article in the September/October and November/December 2011 issues of the ARRL's National Contest Journal titled, "A Compact Dual-Band, 9 Circle Receiving Array." The phased-array system uses a simple amplifier with high input impedance:

enter image description here

The AD8055 op amp has a gain-bandwidth product of 300-MHz, so there's only about 10-dB of gain on the 10-meter band. Note that this "amplifier" is meant primarily to provide a wideband match between an electrically short antenna and a feedline. Its overall gain, as evidenced by the two 470-ohm resistors on the op amp inputs, is close to 0-dB.

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