Thank you guys.
Just to give some info in case you have anything to add. Operation frequency: 10M to 2 GHz. wire length: 8 cm (2cm x 2cm rectangle loop). Inductance: on the order of nH. And WimRFP, you're right. I am attempting to calculate the field strength, mutual inductance between two loops, etc. FvM: thank you for th advice. I think that makes lots of sense.
Of course, a H-sensor is far from an impedance matched antenna, the same with the E-sensors used for field strength calibration in an EMI lab.But to make it work at 2 GHz, the loop will be so small that it will be extremely inefficient at 10 MHz.
so it would be a non-resonant loop.
Thank you. Do you assume the input impedance to the antenna is 50 Ohm? Or you meant the input to the matching network+antenna which will indeed be 50 Ohm, but in that case, the voltage measured would at the input of the matching network instead of the loop itself. Isn't it? Or did I misunderstand anything?I would measure using the usual instruments (network analyzer or signal generator + spectrum analyzer/power meter) with some high impedance (kOhm) voltage divider at the loop antenna. This gives you (relatively) high impedance to measure the voltage. Calculating the attenuation caused by the voltage divider to the 50 ohm input is easy.
A single turn loop gives the lowest inductance (but also relative low output voltage). Depending on thickness of wire or strip, inductance will be in the 50 nH range.
When you know the inductance and the load impedance, you can determine the EMF of the coil based on the loop's impedance and (50 Ohms) load. The –3 dB corner frequency will be in the 150 MHz range.
You may add a balun function between loop and coaxial cable to get better rejection from E-field components. Because of this balance, the wire length halves increasing the useful frequency range where transmission line effects can be omitted.
Instead of a separate balun function, you may construct a loop with natural balun (like the coaxial loop with cut in the shield opposite to the feed point). Regarding balun function, a 0.01 to 2 GHz passive balun with high input impedance is hard to impossible to design.
If you really need high input impedance, you can insert a very small (chip) unbalanced amplifier into a loop with natural balun function (and feed it via the coaxial cable). The natural balun function makes the amplifier to float. Depending on input capacitance, you may need resistive loading to avoid resonance. This route will introduce more inaccuracy with respect to a passive approach. Of course you can go for a passive attenuator to reduce the load to the coil, but this also requries you to know its attenuation.
Can you explain the two voltage issue?
Assuming a balanced setup, the wire length is 0.02m and for 2 GHz lambda = 0.15m. you will have some deviation from simple induced voltage math due to transmission line effect. If you have access to an EM simulator, you may simulate the loop with 50 Ohms load and driven with a plane wave field.
If you measure close to structures that may affect the inductance, (for example close to a ground plane or tuned circuit), loading the coil will affect accuracy.
If you are unsure whether the input VSWR of your measuring instrument (spectrum analyzer, measurement receiver, oscilloscope, etc) is low, you may insert 3..6 dB attenuation so that the loop "sees" 50 Ohms (or other known reference impedance).
If you are in the far field of your structure (for the higher part of the frequency range), you may use simple dipole or other antenna with known gain. From the output of the antenna you can calculate the plane wave power, hence the magnetic field component.
It is highly unlikely that you will be able to get a loop antenna to work over that wide range of frequencies. There are 2 problems: 1) how do you get elements of the antenna to resonate at all those frequencies without nulls, and 2) how do you couple into the antenna.
Here is an example of the complexity needed to make such an antenna:
https://www.ahsystems.com/catalog/SAS-521F-2.php
As far as measuring, AFTER you have chosen a method to couple to the antenna, you would use a spectrum analyzer to measure output power and convert that to voltage.
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