A lot of article about 60 cm shielded loop antennas and how to measure antenna factor using antennas with already known antenna factor.

But for example if I have one shielded loop antenna (construction http://www.rfmw.cmi.cz/documents/deliverables/Traceable_loop_antenna_calibration_VNA.pdf ) with unknown antenna factor. Is it possible theoretically find Antenna factor if I know the current ant voltage throw antenna, radius and antenna inner conductor length.

Kind regards Francisco

  • $\begingroup$ That article is about calibrating an antenna for applications like EMC testing where the response of the antenna must be known accurately. Is that really what you need? Or do you just want an antenna for communication? $\endgroup$ – Phil Frost - W8II Oct 20 '20 at 19:14
  • $\begingroup$ Yes it about antenna calibration. It passive shielded loop antenna 60 cm diameter for frequency range below 30 MHz. But I was wondering if I can calculate AF without measurements because for real antenna calibration they use 2 or 3 antennas for it. But in my case I don't have such narrowband antennas with known antenna factor. $\endgroup$ – Francisco Oct 21 '20 at 7:42

Assumed the frequency range of interest is up to and including shortwave then the description below is the approach. A loop antenne can be used as passive antenna with or without tuning, or as active antenna with an amplifier. Most probably your question relates to passive wideband reception antennas. Then you can read further. Active antenna can be deduced from the info.

Conversion from H-field A/m (magnetic component) into induced voltage. Common radio use is expression of field strength in the E-field component V/m. Relation: E/H equals 376.73... Ohm. Depending on the environment (in-house or otherwise shielded, or in a wood) this relation deviates: in many practical cases the E-field is more attenuated than the H-field.

The induced voltage from a EM-field is now, metric non-US values:

EMF = 2 * PI * A * N / Lambda for 1 V/m where A is the area of the loop in square meters, N the number of turns and Lambda the wavelength in meters.

The source impedance can be measured and will be in the order of circumference (meters!) multiplied with 1 microHenry (per meter) and multiplied with N^2 in case of more than one winding. Do not forget the load impedance. A circuit simulator can help you to do the math.

Important to know: the short-circuit current of such a loop is a frequency-independent measure of the field strength.

Practical use: Wideband active loop antenna for use with receivers needs conversion factor (effective height since dimension follows from "volts per volt per meter") of factor 0.4 in load when receiver noise figure is 10 dB, to be sure that:

---> received noise floor on a quiet location converted in an output voltage for the antenna that corresponds to the input noise of the receiver.

Depending on the environmental noise and on the need for better large signal behavior the conversion factor can be (must be) adjusted for local situation.



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