loop antennas pdf
Hi,
You have to specify whether the loop antenna's circumference (of one turn) is around one wavelength or much smaller than one wavelength?
In case the circumference is around one wavelength, the one turn resonant loop (either a square with quarter wave sides or a full circle) has got about 110-130 Ohm real input impedance at its resonant frequency. How to match this symmetric output (or input in case of a transmitter loop) to 50 Ohm (unbalanced) line is here:
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In case the circumference of your loop is much smaller than one wavelength, the input impedance is reactive (inductive) with very small real Ohmic value and this latter continuously increases as the frequency increases till the one wavelength value is reached. So the problem is matching a very small real Ohmic value with a high value inductive reactance in series with it to a 50 Ohm assymetric line at the receive or transmit frequency. For Bluetooth frequency for instance see this tabelle on input real and inductive impedances in case of one or more turn circular small loops:
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Links for different practical small loop antennas with matching shown are here:
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And here is a useful link on small loops when the circumference of the loop is about 1/4 wavelength, how the input impedance changes at the different locations alongside the loop: **broken link removed**
rgds, unkarc