How to calculate loaded and unloaded quality factor of an antenna while it is placed within human body?
he loaded and unloaded quality factor (Q) of an implantable antenna are important parameters that determine its efficiency and performance in the human body. Here's a breakdown of each:
Loaded Q:
- Represents the antenna's efficiency when it's implanted within the body.
- Takes into account the electrical and dielectric properties of the surrounding tissue, which can significantly affect the antenna's resonance frequency and radiation pattern.
- A lower loaded Q compared to the unloaded Q indicates higher energy loss due to tissue interaction.
- Ideally, you want a high loaded Q for efficient power transfer and communication within the implant.
Unloaded Q:
- Represents the antenna's efficiency in free space, without any surrounding tissue.
- Measured in an anechoic chamber or simulated numerically.
- Provides a baseline for comparison with the loaded Q and helps assess the impact of tissue interaction.
- A higher unloaded Q generally indicates a more efficient antenna design.
Here's how these factors relate to each other:
- Loaded Q / Unloaded Q: This ratio gives an indication of the antenna's efficiency degradation due to tissue loading. A value closer to 1 signifies minimal impact, while lower values indicate higher energy loss.
- Design considerations: Antenna designers strive to optimize the geometry and materials to achieve a high unloaded Q and minimize the decrease in loaded Q when implanted. Techniques like using biocompatible materials and matching the antenna impedance to the tissue can help improve both factors.
It's important to note that the optimal values for loaded and unloaded Q depend on the specific application and desired operating frequency of the implantable antenna.
For further information, you can specify the type of implantable antenna you're interested in (e.g., near-field vs. far-field communication, specific body location) or provide any additional details about your application. This will allow me to provide a more tailored response with relevant research and design considerations.
If you can measure the S11 over the bandwidth, and the plot on the smith chart is a circle touching the outside, then the loaded and unloaded Q can be measured from the bandwidths where the circle crosses some easily found curves and lines. Q = (centre frequency)/bandwidth
Set the reference plane so it is close to the antenna (the loop makes a circle, and the ends cross at the edge of the smith chart). Adjust the reference plane slightly until the circle touches the edge at 3 o'clock (1, j0).
The loaded Q (QL) is given by the frequency difference between the frequencies where the circle crosses the diagonal lines from 3 o'clock (1, j0) to 12 o'clock (0, j), and from 3 o'clock (1, j0) to 6 o'clock (0, -j).
The unloaded Q (Q0) is given by the frequency difference between the frequencies where the circle crosses the two arcs from 3 o'clock (1, j0) to 9 o'clock (-1, j0) centred at 12 o'clock (0, j) and 6 o'clock (0, -j).
If the circle looks better rotated to another position on the chart, then just use the same lines rotated by the same amount from 3 o'clock.
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