Can the Q of a loop antenna be changed?
If so what are some common techniques? (Formulae appreciated).

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    Hi! Welcome here :) . As on any other SE site, we do appreciate very much when askers explain what they've researched so far, so that we don't tell them things that they already know. So, have you researched how the Q of a loop antenna "happens"? If so, what were your thoughts on changing it? What problems did you see, and what exactly do you need info on :)? – Marcus Müller Jun 22 at 8:16
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    There are many types of loop antennas. Based on the question, you mean a small loop HF antenna of a meter or two in diameter? – Glenn W9IQ Jun 22 at 10:36
  • The antenna is composed of multiple loops about 4 cm in diameter. It is for an rfid tag at 13 MHZ – monzie Jun 22 at 13:47
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    I'm not even sure that what you're describing is technically an antenna (although it's usually called antenna, for sure); it's more of the secondary side of an air-core transformer (there's a magnetic field between reader and tag, but the coil's purpose is not to extract power or radiate power in shape of an electromagnetic wave) . So, not even sure it has a Q; can you point us to some kind of description of the loop? – Marcus Müller Jun 23 at 8:12
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    Also, if this really is about RFID, I'd argue it's very specific to a field of electrical engineering that is neither amateur radio nor radio theory in general (as applicable esp. to ham radio), so I think it would find much better answers on electronics.stackexchange.com – Marcus Müller Jun 23 at 8:13
up vote 1 down vote accepted

An RF tag system is designed to function in the near field of the antennas. The frequencies involved are in the 13 MHz range and the antennas used in these applications are defined as electrically short antennas (<0.1 $\lambda$).

The near field region of the reader antenna has reactive, evanescent currents that easily couple to nearby conducting materials. It is this phenomenon that the passive tag uses to easily harvest energy via its antenna in order to send its response. Any reduction of near field currents or near field distance will result in a shortened read range due to insufficient coupling in order to power the tag.

The OP specifically asked about altering the Q of the antenna system. This can be done by altering the physical aperture of the antenna or by the matching network.

It is important to understand that the Q of the antenna system determines its bandwidth. If the Q become too high, the narrower bandwidth may not allow proper reading of the tag. As the Q is lower through its matching network, the read range will be reduced.

You can alter the Q in an existing configuration by changing the value of the resistor in the matching network of the reader. This will also require adjustment of the other matching elements. As the resistance increases, the Q of the antenna system increases and vice versa. Removing the resistor entirely will maximize the Q for a given antenna. Since the resistor effectively dissipates power in order to lower the Q, the higher the value of the resistor, the greater the currents in the near field of the antenna and vice versa.

The mathematical approach to analyzing the matching network is an exercise in simple parallel/series reduction to determine the effect of the dissipative effect of the resistor. A blanket formula is not possible due to the various topologies of matching networks.

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