Hi all,
Is it possible to eliminate a particular resonant frequency on a microstrip antenna? Say like I have this result, I like to eliminate 3.5 - 4 GHz with a simple RC bandstop filter. Am I in the right direction?
Yes you can use notch filters to eliminate any unwanted frequency. It can be in form of a series resonance circuit connected to ground. That is you can shortcircuit these frequencies to ground by the socalled traps. A trap is a series resonance circuit connected to ground. You can also use parallel resonance circuit in series with the feed line to stop the required frequency from passing to the ouput.
Best wishes
You could look at an electronic bandgap structure within the antenna to eliminate the resonance.
Thank you for the answers.
I chose this method as I require the filter's resonant frequency to be tunable. I have read about tunable microstrip antenna. Thought of implementing gaps while varying the antenna but it just introduces other resonant frequencies, which requires filtering as well. Help me out here Marvin.
I am still designing the LC notch filter (diagram) and have been doubting the method as I have yet to perform any sort of test. Feeling more confident now thanks to Zekry.
V1: input
R1: input SMA connector
C1: variable capacitor from 1p to 30p
R2: output SMA connector
As for the inductor L1, I am planning to coil my own air core inductor as the inductance is small.
Is there anyway I way I can improve this filter?
The selectivity of this filter will depend on the quality factor of your indcutor. As the quality factor increases the selectivity of the notch filter increases. You can increase the selectivity of the filter by increasing the filter order which can be realized by cascading another parallel resonance circuit. This is just a proposal it is required to simulate it and if succeeded it can be implemented.
Best wishes
A 1nH inductor is rather small, about 2mm of 0.3mm diameter wire. Your filter may be difficult to realise. A distributed circuit based design may be easier.
Yes I have realized the inductance is the major problem. The Q factor will definitely be impacted significantly for a self made inductor. The size is also an issue as most inductor in nH range are SMD components.
So Marvin, how can I convert this circuit to a distributed circuit? Are you using the air coil inductance formula to calculate the dimensions of the wire? Because I'm thinking of using 50 SWG (0.025mm) wire to coil the inductor.
I was using the formulation for the self inductance of a straight wire. See https://www.allaboutcircuits.com/tools/wire-self-inductance-calculator/
Dear Dik,
You can implement band stop filter as a defective ground structures:
For more information please see the link:https://www.researchgate.net/publication/224242129_Compact_bandstop_filter_using_defected_ground_structure_DGS
Best wishes
You can use CSRR, SRR, DGS, DMS, hair pin resonators, etc., to filter out the unwanted frequencies.
Thank you all for your answers.
The circuit is being fabricated. So if the filter has a resonance frequency at 3.7GHz and cut off at 3.5 to 4GHz, will the result of my S11 be a straight line at the stop band at 0dB, referring to my results here? Or do I need to test with S21?
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