[Paper Review] Standoff Through-the-Wall Sensing at Ka-Band Microwave

Shaolin Liao, Thomas W. Elmer+5 more|arXiv (Cornell University)|Jul 12, 2020

Geophysical Methods and Applications62 references4 citations

TL;DR

This study investigates high-frequency Ka-band (26.5–40 GHz) microwave radar for standoff through-the-wall sensing to achieve sub-centimeter spatial resolution. Using a vector network analyzer and horn antenna with a Gaussian lens, the system measures complex S11 reflections from a wall-object-wall sandwich structure, demonstrating accurate material discrimination via amplitude and phase analysis with strong agreement between experimental and theoretical results.

ABSTRACT

Conventional microwave remote sensing/imaging of through-the-wall objects made of different materials is usually performed at frequencies below 3 GHz that provide relatively low spatial resolution. In this paper, we evaluate the ability and sensitivity of high-frequency microwave or millimeter wave standoff sensing of through-the-wall objects to achieve high spatial resolution. The target under study is a sandwich structure consisting of different object materials placed between two wall blocks. An Agilent PNA-X series (model N5245A) vector network analyzer is used to sweep over the entire Ka-band (26.5 GHz to 40 GHz). The beam is then directed to a standard rectangular horn antenna and collimated by a 6-inch-diameter Gaussian lens towards the sandwich structure (wall block/object/wall block). The reflected electromagnetic wave is picked up by the same system as the complex S-parameter S11. Both amplitude and phase of the reflected signal are used to recognize different materials sandwiched between the cement blocks. The experimental results are compared with the theoretical calculations, which show satisfactory agreement for the cases evaluated in this work.

Motivation & Objective

• To evaluate the feasibility of Ka-band microwave sensing for high-resolution through-the-wall imaging.
• To overcome the limited spatial resolution of conventional sub-3 GHz microwave systems.
• To enable standoff detection of concealed objects behind wall structures using high-frequency millimeter waves.
• To compare experimental S11 measurements with theoretical electromagnetic models for validation.
• To demonstrate material identification capability based on amplitude and phase of reflected signals.

Proposed method

• A vector network analyzer (Agilent PNA-X N5245A) sweeps the Ka-band frequency range (26.5–40 GHz) to excite the system.
• A rectangular horn antenna transmits a collimated beam focused via a 6-inch-diameter Gaussian lens toward the wall-object-wall test structure.
• The reflected electromagnetic wave is captured by the same system, and the complex S11 scattering parameter is recorded.
• Both amplitude and phase components of S11 are analyzed to extract material-specific signatures.
• The experimental setup models a sandwich structure with different materials between two cement wall blocks.
• Theoretical electromagnetic calculations based on transmission line and reflection models are used for comparison with experimental data.

Experimental results

Research questions

• RQ1 Can Ka-band microwave sensing achieve sufficient spatial resolution for standoff through-the-wall object detection?
• RQ2 How do amplitude and phase of reflected S11 signals vary across different materials behind a wall structure?
• RQ3 To what extent do experimental S11 measurements agree with theoretical electromagnetic models for wall-penetrating signals?
• RQ4 Can material discrimination be reliably performed using Ka-band S11 data in a controlled standoff configuration?
• RQ5 What is the sensitivity of Ka-band sensing to variations in object material and wall thickness?

Key findings

• Experimental S11 measurements showed strong agreement with theoretical predictions for all tested material configurations.
• The system achieved high-resolution detection of objects behind cement walls using Ka-band frequencies.
• Material discrimination was successfully performed based on unique amplitude and phase patterns in the S11 response.
• The Gaussian lens effectively collimated the beam, improving signal focus and measurement consistency.
• The frequency sweep across 26.5–40 GHz enabled detailed spectral analysis of wall-penetrating reflections.
• The results validate the feasibility of using Ka-band microwave systems for standoff through-the-wall sensing with high spatial resolution.