Design of Stub Loaded Transmission Line Matching Circuit for Series FED Patch Array

- Modification of the matching circuit for the mmWave antenna

In order to feed the power to the antenna with minimum loss, the matching circuit is required. By achieving this, the gain of the antenna can be maintained and desired result can be obtained.

The mmWave frequency circuits are very small in size because of the small wavelength of these waves. So, designing the realizable (the transmission lines of width greater than or equal to 1mm) matching circuit with very high impedance is challenging for thin substrates. In order to solve this problem, the stub loaded transmission line technique is used.

In this technique the high impedance transmission line can be converted to low impedance lines and a stub. So, it can be made realizable in thin substrates. 

Previously complex design techniques were used for the matching circuit.

The series fed patch is design based on Dolph-Tschebyshev distribution amplitude tampering. 

If it is matched by quarter wave transformer, the antenna becomes unrealizable because of the transmission line of very small width.

                                                              Series fed antenna matched by quarter wave transformer

This transmission line is replaced using stub loaded transmission line. 

                                           Conventional transmission line                                    Stub loaded transmission line

The ABCD matrix of the conventional transmission line is 

ABCD matrix of the loaded transmission line is

Simplifying (2), 

Equating (1) and (3), 

Substituting (5) in (6) and solving for B, it can be found out as

From (6),

Substituting (8) in (4)

Using (8) and (9) the matching circuit can be designed.

Structure of short circuited stub loaded matching circuit for the series fed patch array  w₁=0.72mm,l₁=0.98mm,w₂=1.25mm,l₂=0.92mm,w_s=0.5mm and l_s=0.27mm

Photograph of the fabricated antenna with matching circuit

                S parameter response

             Simulated radiation pattern

This type of mmWave antenna and the matching circuit can be used for the applications like vital sign detection, people counting etc.

M. P Mohan, L Zhao, J. Jimeno, A Alphones, M. Y. Siyal, M Faeyz Karim, “Design of stub loaded transmission line matching circuit for series fed patch array”, 2021 International Symposium on Antennas and propagation and USNC-URSI Radio Science Meeting, December 2021, Singapore. (Link)