Depends on what Parameter you need if a stub is the solution for you, since a coax stub would also attenuate the 70 to 80 MHz MHz segment somewhat to, varying on design. I'm not specific because it depends on many factors.
In short a Notch filter or coax stub is a piece of cable of n * 1/4 wavelength
- The physical length of the cable has to be shorter than the calculated wavelength and varies with cable properties due to varying propagation in different cables.
-The calculated wave length is multiplied by a shortening factor which is cable specific which you have to look up
What you can(not) expect from a coax stub notch filter
- The notch depth (higher W narrower) and width of notch (higher Q deeper notch) depend on the Q(uality) of the cable.
- The notches will also occur on multiples of the lowest frequency the notch filter is designed for
- DUBUS Hams found for a project that even Teflon cables were not providing narrow enough Notches, so they experimented successfully with a loose coupling via a capacitor, however while width improved the looser the stub was coupled the less depth the Notch provided. Sorry I did not find a link describing their tests.
How to built one
- Start with a bit longer cable and shorten it as, since you cannot glue a cutoff piece on again, for some variations in design look here
http://dg7ybn.de/technical/Filters.htm#surge look for Notch Filter
Without means to measure it will be trial and error, since receive and stop band are close to each other.
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