CATV RF Notch Filter

If it's a quarter-wavelength long at frequency of interest, then shorting the far end will cause it to behave very much like an open circuit at the "T" at that frequency. A quarter-wavelength transmission line results in the maximum transformation of the impedance from one end to the other.

If what you're attempting to do is "short out" the frequency of that one channel, then you'd want either a quarter-wavelength stub which is open at the far end (and these can be tricky due to parasitic capacitance, and could also radiate signals from other channels) or a half-wavelength stub which is shorted at the end.

Be aware that any shorted stub is going to look like a short at DC, which might be an issue on some cable systems.

The latter is not at all unlikely.

Single-channel filters, deep enough to be useful and shallow enough not to affect adjacent channels significantly, tend to be fairly big and heavy and complex affairs.

is one source for them - these are the sort of "big guns" used by CATV companies if they want to knock out one channel, and insert their own content on that frequency.

might be more what you'd need. The CR7-(Fo)-HQ line seems appropriate for your need, if the notching is deep enough. I don't think these are designed to have enough attenuation to allow re-use of the same signal (e.g. injecting new content) without interference... they're just designed to attenuate an unwanted signal.

I bought one of TinLee's single-channel units some years ago, when I wanted to notch out Channel 11 from my main over-the-air antenna (pointed north) and insert a separate Channel 11 feed from a Yagi I built (pointed south). It worked very nicely and it survived outdoor weather for as long as I needed it (several years).

And, be aware that the _open_ quarter wave stub will also demonstrate 'zero' impeadance at all the odd 1/4 wave frequencies -- IOW, at 3X your "200 to 300 MHz" target, and at 5X, etc.

I have made 1/4 wave stubs of High Q out of 1/2" and 3/4" cable TV hardline to eliminate harmonic interference in TVs from nearby (non-TV) transmitters. WFM.

HTH Jonesy

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  Marvin L Jones     | W3DHJ      | W3DHJ  | http://W3DHJ.net/ 
   Pueblo, Colorado  |  @         | Jonesy |     __ 
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Hi Dave,

Thank you for this very useful information.

I decided to abandon the idea of making my own NF.

I sent an email to snipped-for-privacy@tinlee.com with the frequency of the channel I want to "block". I am waiting for a response.

Again Thanks, John

Do you know the bandwidth needed? If it's fairly wide band, you'll probably end up with a hipass and lopass filters in series.

Stubs will work, but I prefer LC. Here's an FM band (100 MHz) notch filter I threw together to keep the FM junk out of a local VHF ham repeater.

What digital data? Make my life easy and supply:

1. What are you trying to accomplish? What problem are you trying to solve?
2. What have you got to work with? Ability, test equipment, materials, etc.

There are plenty of custom filters around. Google. I gotta run immediately....

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Hi Bruce,

Digital broadcasts over the air uses specific frequencies. However, cable uses coax instead of "air". Those digital "packets" require RF to travel over the air or on cable.

I injected a 276Mhz signal into the cable and it "knocked out" those ANNOYING ads/promos you see when you use the "On Demand" feature to see a list of movies, TV shows, music videos, etc. With the ads "blocked", I was able to select and view (on demand) a music video because it uses a different frequency.

John

** Not likely.

It's not an analogue signal so you do not need to remove it all - just a part of it will do, enough to prevent decoding.

.... Phil

Hi, Here's an update on the email results.

As I said a while ago (02-20-15), I sent an email to snipped-for-privacy@tinlee.com. Later that same day, I received a reply!!!

In the reply I was asked for more details regarding what I want (specs) in the filter. I told Sneva the frequency (276Mhz), etc. I have not received another reply. I sent another email to Sneva, but still no reply.

I asked Sneva if he (assumed) can't make the filter, please let me know so I can look for another source. Still no reply.

Can someone recommend another CATV RF notch filter source?

Thank You in advance, John

Hi, UPDATE:

I finally received a quote, $185.00 plus $20.00 shipping. Since this filter has to be custom made, the price is high.

John

If you have more time than money, probably your only alternative is to look under rocks at surplus sites and see if anyone has a cable notch filter for HRC channel 33 (channel T). Those have a cf of 276 mhz.

But from memory, most common single channel notch filters only went up to channel J. The rest above that usually blocked an entire band.

-bruce snipped-for-privacy@ripco.com

Hi, FINAL UPDATE:

I received the filter from Tin Lee Electronics. This filter removed the streaming promos that ANNOY me. The results, with the promos "removed", when I go to "On Demand" (a small delay occurs), now I see whatever channel I was viewing, instead of promos. This can be handy when commercials come on, and I can check what is "on demand" during commercials of a program I am watching.

With my signal generator, I corrupted the streaming video to the degree it could not be decoded. The filter removed the signal.

John