Coil resistance

[Sergey_P]

The RX resistance matters very little.

The electromagnetic characteristic that distinguishes one substance from another is phase. If the phase of the detector's receiver impedance is indistinguishable from the phase of the surrounding soil impedance, they have no effect on each other. To reduce phase noise in the detector's receiver when searching on the ground, it is necessary to approximate the phase of the detector's receiver impedance with the phase of the surrounding soil impedance. In this case, the coil resistance plays a significant role.

[Carl-NC]

The electromagnetic characteristic that distinguishes one substance from another is phase. If the phase of the detector's receiver impedance is indistinguishable from the phase of the surrounding soil impedance, they have no effect on each other. To reduce phase noise in the detector's receiver when searching on the ground, it is necessary to approximate the phase of the detector's receiver impedance with the phase of the surrounding soil impedance. In this case, the coil resistance plays a significant role.

You'll have to explain more because I don't understand. In a circuit where the RX coil is loaded by a simple resistor (say, 10k) then the phase shift of the RX coil is practically zero even with a coil resistance of, say, 100Ω. Even if you lowered the load resistance enough to create a phase error, it would still be reasonably constant and would simply be removed by the ground balance calibration, along with the phase shift of the preamp.

Ground permeability can cause a shift in the inductance value but it is typically a very slight change and, for a resistively loaded RX coil, still won't make any difference. The IDX RX coil is capacitively loaded but I assume it is set considerably off-resonance of the TX frequency precisely to avoid issues with the Q causing variations in the phase shift. In the end, I can see how the RX coil resistance can matter where the RX coil is resonated at the TX frequency, but otherwise not so much.

[Sergey_P]

Ground permeability can cause a shift in the inductance value but it is typically a very slight change and, for a resistively loaded RX coil, still won't make any difference.

The detector measures two parameters simultaneously: amplitude and phase. Changing the sensor's impedance in the presence of soil changes the measured values simply by changing the distance to the ground surface. This introduces additional noise, reducing target sensitivity.
To reduce noise in the presence of soil, the sensor's impedance must be phase- and impedance-matched to the ground.
The transmitter (TX) is phase-matched, and its active resistance is selected based on the required magnitude of the generated field. A non-resonant LR oscillator or a detuned LRC oscillator with external excitation is used (an extreme case is a low-Q oscillator).
The receiver (RX) is phase-matched, and its active resistance is selected based on the conductivity of the surrounding soil in the search field. (An example calculation for the sensor described above is shown in the image.)

[Carl-NC]

Sergey_P, I've moved this to a new topic because I'd like to discuss it more in-depth. What you are saying (especially in your last post) goes against everything I've understood about the effects of coil impedance. I'd like to start with the TX coil:

The transmitter (TX) is phase-matched, and its active resistance is selected based on the required magnitude of the generated field. A non-resonant LR oscillator or a detuned LRC oscillator with external excitation is used (an extreme case is a low-Q oscillator).

The best TX coil is an ideal inductor (R_TX = 0) such that the TX drive voltage and TX current are exactly 90° apart. That way the phase reference for the demod clocks has no error due to wire resistance that might change with temperature. It also minimizes power loss. The high Q may result in phase noise but if the demod phase rotators are reasonably fast then the demods will track the TX phase and it should not matter much.

But we never have R_TX = 0, there is always some wire resistance and the design trade-off is between the resistance and the weight of the coil. I have never seen anyone use TX resistance as a way to control the magnitude of the generated field -- that is done with the power supply. Intentionally adding resistance just wastes power.

The receiver (RX) is phase-matched, and its active resistance is selected based on the conductivity of the surrounding soil in the search field. (An example calculation for the sensor described above is shown in the image.)

On the RX side, I agree that RX coil resistance creates thermal noise which is detrimental. But 50Ω adds less than 1nV/rtHz which is lower than most of the preamps used in VLF designs. The effect of RX-R on the phase depends on whether the coil is loaded with a simple resistor, parallel resonated with a cap (either on-resonance or off-resonance), or is series resonated. As long as the self-resonant frequency of the coil is well above the operating frequency, the only situation I see where the series resistance makes any difference is for parallel on-resonance. Again, I've never heard of anyone designing the RX coil to match the ground's loss angle. Finally, the example calculation is for a series RLC circuit, so I don't understand the relationship.