I can provide some general insights and resources that might help you further investigate the sensitivity of the S11 parameter to environmental changes and frequency.

Factors affecting S11 sensitivity:

• Material properties: The material being measured and its sensitivity to environmental changes (e.g., temperature, humidity, pressure) are crucial factors. Some materials exhibit significant changes in their electrical properties with environmental variations, leading to stronger effects on S11.
• Structure and geometry: The structure and geometry of the device or antenna under test also play a role. Complex geometries might be more susceptible to environmental variations due to resonances or higher-order modes being affected.
• Measurement setup: The specific calibration, reference plane, and measurement technique can influence the observed sensitivity. Careful calibration and consideration of parasitic effects are crucial for accurate interpretation.
• Frequency range: Depending on the device, the frequency range of interest can impact the sensitivity. Resonant frequencies might exhibit larger sensitivity due to amplified changes in reactance.

General observations:

• Temperature: Temperature changes often affect permittivity and conductivity, impacting S11 through changes in impedance. The extent of sensitivity depends on the material properties and measurement setup.
• Humidity: For hygroscopic materials (absorbing water), humidity can alter permittivity and lead to S11 shifts.
• Pressure: In certain applications like acoustic sensors or vacuum electronics, pressure changes can significantly impact S11 due to variations in acoustic or dielectric properties.

Resources:

• Microwave Engineering textbooks: Most advanced microwave engineering books discuss S-parameters and their sensitivity to various factors.
• Application notes from RF component manufacturers: Specific component manufacturers often provide application notes discussing the performance of their devices under various environmental conditions.
• Technical papers: Search for research papers exploring the sensitivity of S11 parameters to specific environmental changes in your application area.
• Simulation tools: Electromagnetic simulation software like HFSS or ADS can be used to model the behavior of your device under different environmental conditions and analyze the S11 response.

Remember, the best way to assess the sensitivity of S11 in your specific case is to perform actual measurements or simulations considering the factors mentioned above. Consulting relevant resources and experts in your field can further enhance your understanding and analysis.

This is an antenna. It will transmit and receive. Reflections from surrounding objects will be received and will add to S11 measurements. If the objects move, the reflection will change and S11 will change.

S11 of -45 dB is very low. If it drops to -65 dB then the extra reflected signal coming back nearly cancels it entirely so is very close to -45 dB, which is 0.003% of the transmitted signal. Signals at this level do change the resonant frequency, but not by any amount you can notice. It is similar to frequency pulling of oscillators.

I fully agree with Malcolm values in the ball park of -50 dB are very low and you can easily "see" changes when someone is moving around nearby. This kind of observation was the very early beginning of radar more than 100 years ago; keep in mind that the very first operational radar was 1904 in Cologone/Germany (Hülsmeyer); recently IEEE Milestone plate installed there.

One should remember that S11 is measured in Log scale. The S11 range of -45 to -65 relates to extremally low values around measuring tools' noise level. Any small fluctuation in the environment will be measurable.