Q factor coaxial cable structure

[lambdaanttech]

Hi Everyone ,

I'm struggling how to calculate the Q factor of a coaxial structure .
I want to make a notch filter that attenuates a frequency (220MHz) with a 3dB bandwidth of 4 MHz .
The notch filter would be made very simple by using a quarter wave resonator and a T piece .
Simple coaxial cable doesn't work since it is to widebanded , therefore I wanted to calculate a coaxial structure with a very high Q factor .

According to this equations : https://www.rfcafe.com/references/electrical/coaxial-resonator.htm
I putted them into a spreadsheet but it doesn't looks right .

Can anyone help me further how to calculate a Q factor of a coaxial line and how to further calculate the bandwidth according to the Q factor .
Any tips and hints are welcome .

Thanks in advance !
Kenny

 

[biff44]

you will shortly find that, at the least, you will need a small value series capacitor, the coax line in a Tee junction, with the stub end shorted out half wave away, and another small value series capacitor. You adjust the two capacitor values until you have the right stopband width.

 

[vfone]

To get a 220 MHz notch with 1.5% bandwidth you need a Q factor of the resonator greater than about 2500, which is almost impossible to be done with standard coax cables.
You need a cavity notch, or a Heliax Notcher circuit as is described below:

**broken link removed**

 

[biff44]

ignore my first post, I had the topology wrong. Here is what u want, using RG214:



btw, that simulation is for a capacitor Q of 200