Making coax stub filters
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nd5y
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You use the electrical length of the coax. That is the free space wavelength called for (usually 1/2 or 1/4 wave) multiplied by the velocity factor of the coax.
As an example say you want to notch out a strong FM broadcast station at 97.5MHz. To find a 1/4 wave in free space for 97.5MHz divide 234 by 97.5 and you get 2.4ft. If you were to use a particular brand of RG-58 coax for the 1/4 wave stub it might have a velocity factor of .66 so multiply the free space 1/4 wavelength of 2.4ft by .66 and you get 1.584ft. That is roughly how long a piece of RG-58 coax would be hanging off a T connector to act as a notch filter at 97.5MHz.
In the real world you would now have a target length that could be off a bit so you might cut 1.7ft of coax, put a connector on one side and use an antenna analyzer and cut a little off at a time until it shows 0 ohms impedance and a very high VSWR at 97.5MHz when attached to a T adapter and a 50 ohm resistive load on one side of the T adapter to simulate whatever circuit you will use this thing with.
In the real world you would now have a target length that could be off a bit so you might cut 1.7ft of coax, put a connector on one side and use an antenna analyzer and cut a little off at a time until it shows 0 ohms impedance and a very high VSWR at 97.5MHz when attached to a T adapter and a 50 ohm resistive load on one side of the T adapter to simulate whatever circuit you will use this thing with.
Search the brand and model of your coax for specs and the velocity factor will show up. I will be different between brands so you need to find the exact specs for the coax you are using.
Or if you have an antenna analyzer you could use a velocity factor around .8 for foam type dielectric and .66 for clear polyethylene then cut the cable extra long and tune with your analyzer. The point of 0 ohms on the antenna analyzer will come at a lower frequency when the cable is too long and you cut small sections off and walk it up to your desired frequency.
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nd5y
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If you want to actually measure velocity factor you need a TDR. Some antenna analyzers can do that.
Not sure why you need one, but I make them before I buy an expensive filter to be sure a filter will actually filter out what I’m having problems with. Whew!
They are usually too broad to leave in and have harmonics, but are a good troubleshooting tool, for me at least.
Does all of this introduce any loss?
Ok another question. Watched a video on determining velocity factor with a MFJ analyzer. Following his instructions I'm trying to get my first dip (he gets it to zero) mine only drops to three. Is this ok to work with or is something out of whack?
nd5y
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Yes and coax stub filters like that don't just magically disappear and not affect other frequencies than the one you are trying to notch out.
Outside the range of the notch no, not much if any. However the notch will repeat at every odd harmonic so a notch at VHF will also land in the middle of the UHF band like one made for 155MHz will also notch at 465Mhz then another at 775MHz and so on.
Ubbe
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The better the coax you use the narrower the notch will be. I think the velocity factor follows that, the higher the VF the narrower the notch.
Bandstop Filter versus Coaxial Stub
/Ubbe
Bandstop Filter versus Coaxial Stub
/Ubbe
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