High-Z RF amplifier for magloop RX vs Q-factor

Question

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.

Answer 1

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:

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.

Answer 2

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.

Answer 3

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:

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.

Answer 4

Whoops, the idea of using a low impedance diff amp for loop antennas, used below resonance, is wrong advice. Recall that the equivalent circuit of a loop antenna consists of three resistances in series: radiation resistance, skin resistance, and inductive reactance. Radiation resistance drops as frequency is lowered. But inductive reactance is still there. And it is higher than the radiation resistance. So, to connect a receiver to a broadband LF loop you need a differential, high-Z amp, with a single-ended (unbalanced) output to connect via coax to that receiver. The idea of the high-Z amp input is to create a RF voltage divider connection where the antenna Z and the preamp Z cut the received voltage by half or less. That means that you may need megohms of input Z at the amp input. ...Conversely if you connect a LOW-Z preamp (say, 50 ohms or less) to a loop being used below first resonance, or between the first and second and third, etc., resonances, you are going to hear only at or near those resonances. Which is what you get from those cheap kits on Ebay. JOhn, K6dBi

Answer 5

Understanding the Q Factor in Magnetic Loop Antennas

For an SDR receiver system without a built-in pre-selector, maintaining a somewhat high Q factor can be beneficial for rejecting out-of-band interference, but there are different trade-offs, and high Q is not automatically desirable. That is because Q is not a metric for antenna performance... quite the opposite.

The confusion begins because the "Q-factor" is the language of resonators and imposing that frame on antennas, which have distinctly different objectives. The Q factor is proportional to the ratio of the energy stored to the energy dissipated per cycle at the operating frequency. We'll have to face this mess for a moment to make sense of the design tradeoffs.

Variables Governing the Q Factor

Several variables directly influence the Q factor of magnetic loop antennas:

1. Conductor Quality: Using high-quality conductors with low internal losses.
2. Loop Size: Smaller loops generally have higher Q factors due to reduced coupling with external fields (assuming the conductors are large enough to keep the ohmic loss small).
3. Shielding: Enclosing the antenna in a shielded case can increase the Q factor by minimizing external field interactions.

Note: the antenna coupling with the external field is considered a loss component if using a resonator analogy, quite confusing, because of the differences in the semantic framing.

Benefits and Trade-offs of High Q Factor

While a high Q factor may be desirable for sharp preselector action, it comes with trade-offs: Coupling Efficiency. Higher Q factors often mean less effective coupling with external fields, reducing the antenna's overall sensitivity.

Feedline Coupling Mechanisms

Several approaches can be used to couple the magnetic loop to the receiver:

1. Coupling Loop: Coupling through a second loop.
2. Transformer: Using a transformer for impedance matching.
3. Matching Network: Involving an extra variable capacitor and possibly other components.
4. Gamma match: a gamma match is also quite effective and adjustable.
5. High Impedance Amplifier: For scenarios requiring additional gain.

Matching mechanisms may impose an additional degree of flexibility (which complicates operation when changing frequency) and may also have a small impact on the Q. The impact on the Q is not large if designed well.

The Role of Amplifiers

Amplifiers can be beneficial but should be used judiciously:

• Gain Requirements: If the SDR system already has sufficient gain, adding an amplifier may not improve signal reception at all, but may make more vulnerable to out-of-band interference.
• JFET Amplifier: For specific gain requirements (e.g., up to about 10 dB), a differential amplifier using jFETs can be constructed. The loop can be modified to feed the differential inputs effectively. Diff amps are effective in minimizing even-order distortion.

Practical Considerations

For the OP's SDR system, consider the following:

1. Antenna Size: Choose an antenna size that balances convenience and performance for the desired frequency range.
2. Coupling Mechanism: Select a coupling mechanism based on construction ease, noise immunity, and necessary gain.
3. Amplifier Use: Only add an amplifier if additional gain is required. Otherwise, consider alternative coupling methods like pickup loops or gamma matches.

Focus on overall system performance rather than maximizing the Q factor alone. Design trade-offs based on reasonable requirements will yield better results.