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Chat => Tech Chat => Topic started by: JGO on June 09, 2013, 11:24:04 AM

Title: 800 MHz notch
Post by: JGO on June 09, 2013, 11:24:04 AM
With 4G tests going on this, may be of use for anyone who needs a quick cheap  filter, even if it isn't perfect.

A co ax o/c stub across the TV feeder will put a notch about 30 dB deep at it's L /4 resonant frequency. It should be in front of any amplification i.e masthead preamps, but not necessarily right at the aerial.
 
 The wavelength for 800 MHz is 37.5 cm so the stub should be 9.375 cm reduced by the co-ax velocity factor, typically 85%  i.e about 8 cm.   The classic method is to is to cut 2 stubs slightly over length, connect one and prune gradually it for best results.  The second stub is to get spot on when over-pruning the first results in overshooting ( Murphy's Law !!)  Take care not to short circuit the cut end.

By coincidence the 3L /4 resonance is at 2400 MHz which would attenuate WiFi interference if this is a problem.

 
 
Title: Re: 800 MHz notch
Post by: guest on June 09, 2013, 11:46:36 AM
Alternatively you could get a filter free from https://at800.tv/guide-user/households/ as they have been given 180mill for sorting out households who have freeview issues after 4G is turned on.
Title: Re: 800 MHz notch
Post by: JGO on June 09, 2013, 12:53:15 PM
Alternatively you could get a filter free from https://at800.tv/guide-user/households/ as they have been given 180mill for sorting out households who have freeview issues after 4G is turned on.

Sure, but how long does that take ?  If someone asks for help at a weekend that may be important.