Improving matching by adding a stub

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yefj
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Hello, I have a gap transmission line resonator the gaps are sighned in arrows.The matching is -5dB.
Is there a way to increase the matching by adding an opencircuit stube to the transmission line resonator?
It will be great to have some physical intuition.
Thanks.
Line graph of S-parameters magnitude in dB against frequency from 4 to 7.5 GHz, showing a single sharp dip to about -5.5 dB near 6.2 GHz and near 0 dB elsewhere.


3D CAD render of a cyan plate with a thin yellow gap slot running diagonally across it, a red plate at the left edge, and red hand-drawn arrows pointing at the slot ends, with a u, v, w coordinate triad in the centre.
 
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We live in a world of mismatched impedance. Where there is no change in impedance, we cannot see the boundary. If you match the resonator to the line, it will no longer resonate. The greater the end mismatch, the greater the Q of the resonator.

Do you want a deep rejection notch or a BPF?
You are plotting S22, so what do you connect to port 1 on the line passing through the resonator? How will the be used in a circuit?


Why use gaps when you can overlap λ/2 dipoles, en echelon, which reduces the area of the PCB. There are several filters etched on PCBs shown here.
https://en.wikipedia.org/wiki/Distributed-element_filter
 
Hello Baluncore, Its not a filter but a resonator, I am supposed to connect a capacitor in one gap to shift the resonance frequency as shown below, however that dip of S22 gets small ,I thought to use a stube to incread the dip is it possible?
Thanks.
"Do you want a deep rejection notch"
Simulation screenshot of a cyan rectangular planar structure with a yellow strip across the middle, marked with red handwritten labels 1 and 2 at the ends and a small coordinate triad showing axes w, u and v.

Line graph of S-Parameters magnitude in dB against frequency from 4 to 7.5 GHz, showing S2,2 dips for six capacitor values; the dip near 5.4 to 5.6 GHz is shallow for the larger capacitances, and the 1e-15 curve has its dip near 6.2 GHz at about -5.2 dB.

Smith chart titled S-Parameters [Impedance View] showing two S22 traces, red for cap3 = 1e-15 and green for cap3 = 1e-12, looping near the lower-left of the chart around the -1 and -2 reactance circles.
 
I cannot see how a stub can help you.

You have a dipole resonator in series with a transmission line. That resonator will act as a deep notch filter for signals travelling in the line. The signal voltage will appear across the resonator, not at the output port.

If the dipole resonator was connected between one port, or a signal node, and ground, then the resonator would enhance signals of that frequency, while all others would be attenuated.

I cannot guess what you are trying to do, so it is difficult for me to suggest how or where to change the system. Can you produce a block diagram of the wanted circuit?
 
Can you tell us the capacitor values, the Zo of the line and the length of the resonator. Thank you.
 
The Smith Chart shows a reasonable conductance value but a large negative susceptance. If you shunt a positive susceptance across the line at the generator side that will cancel this out. An open circuit line shorter than a quarter of a wavelength will provide the correct compensation. X = Zo / tan (theta), or Y = tan (theta) / Zo.
 
yefj said:
I am supposed to connect a capacitor in one gap to shift the resonance frequency as shown below
Wait, is this question for your schoolwork?
 
Hello Tech99,I am trying to understand the words you said.

I have a microstrip with two gaps ,I am having problem visualising where do I need to put the open circuit stub.
Could you please tell me on drawing below where should the stub be located?
Thanks.
The Smith Chart shows a reasonable conductance value but a large negative susceptance. If you shunt a positive susceptance across the line at the generator side that will cancel this out. An open circuit line shorter than a quarter of a wavelength will provide the correct compensation. X = Zo / tan (theta), or Y = tan (theta) / Zo.
Hand-drawn annotation on a simulation screenshot of a long horizontal microstrip line, with a green arrow labelled v and red arrows pointing to two gap positions along the line.

Smith chart in impedance view titled 5-Parameters, with a red S1,1 variable reference impedance curve forming a small loop near the lower right, and markers labelled 4 (3.09, -577 Ohm) and 8 (1.93, -286 Ohm).

Smith chart in impedance view titled S-Parameters (Impedance View), showing four S1,1 curves for line lengths 0.1, 0.3, 0.65 and 1, with the bottom-right region around -3i to -5i marked.

Cropped simulation view of a microstrip line: a dark red vertical strip on the left meets a bright yellow horizontal trace with a small gap, and a small yellow square stub projects upward from the trace, marked by a red hand-drawn arrow.
 
yefj said:
Hello Tech99,I am trying to understand the words you said.

I have a microstrip with two gaps ,I am having problem visualising where do I need to put the open circuit stub.
Could you please tell me on drawing below where should the stub be located?
Thanks.
The Smith Chart shows a reasonable conductance value but a large negative susceptance. If you shunt a positive susceptance across the line at the generator side that will cancel this out. An open circuit line shorter than a quarter of a wavelength will provide the correct compensation. X = Zo / tan (theta), or Y = tan (theta) / Zo.
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The stub needs to be positioned on the transmission line not on the resonator. It needs to go on the sending end. It should be placed at the point where you measured your Smith Chart.
 
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