COAX Stub notch 88-108 bandstop filter for stock antenna

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Jekko
Posts: 24
Joined: Wed Oct 17, 2018 9:39 am

COAX Stub notch 88-108 bandstop filter for stock antenna

Post by Jekko » Thu Oct 25, 2018 3:00 pm

Hi everybody,

i would like to try to make a coax 88 108 bandstop filter for the stock dipole antenna sold with the dongle RTL-SDR V3 (while i am waiting for the restock of RTLSDR filter :lol: ).
I will use a SMA "T" connector and the same cable of stock antenna.
I am interested mainly on get the frequency of about 70-80mhz with less interference possible.

How much should be long the open stub of cable?

Thank you very much!

snn47
Posts: 153
Joined: Tue Dec 27, 2016 11:00 pm

Re: COAX Stub notch 88-108 bandstop filter for stock antenna

Post by snn47 » Thu Oct 25, 2018 7:33 pm

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.

Jekko
Posts: 24
Joined: Wed Oct 17, 2018 9:39 am

Re: COAX Stub notch 88-108 bandstop filter for stock antenna

Post by Jekko » Fri Oct 26, 2018 3:20 pm

Thank you very much snn47, tomorrow i will try to put something together.
If in the future you will find the DUBUS hams project please send it. I am very curious.
Bye

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