CATV RF Notch Filter

Hi,

I never made a NF (Notch Filter), but I am thinking about making a "stub"
(I think that's the term) NF for CATV to "block" a single "channel" coming in
on the digital TV cable system. Note: This "channel" consists of ANNOYING
ads/promos that you can not avoid or "turn off" when you use the "On Demand"
feature to look at a list of "on demand" available movies, TV shows, music
videos, etc.

I know the frequency, which by the way is between 200 and 300Mhz.

If I have this correct, the "stub" is a 1/4 wave length of the offending
frequency. The 75 ohm coax is cut to that length and is shorted on the far
end. The other end is connected via a "T" connector.

Also, I don't know if the notch will be "deep" (high Q) enough to make
the digital data "un-usable" (can't decode). Another concern is the width of
the notch. If it is too wide, it may affect the adjacent channel.

Does anyone have any suggestions, or can you recommend a place
where I can order a good CATV NF (I am willing to pay)?

Thank You in advance, John

 

[Dave Platt]

In article <9lecead9k74vq9h3kie60a0jbak9ieb20t@4ax.com>,
<jaugustine@verizon.net> wrote:

I never made a NF (Notch Filter), but I am thinking about making a "stub"
(I think that's the term) NF for CATV to "block" a single "channel" coming in
on the digital TV cable system. Note: This "channel" consists of ANNOYING
ads/promos that you can not avoid or "turn off" when you use the "On Demand"
feature to look at a list of "on demand" available movies, TV shows, music
videos, etc.

I know the frequency, which by the way is between 200 and 300Mhz.

If I have this correct, the "stub" is a 1/4 wave length of the offending
frequency. The 75 ohm coax is cut to that length and is shorted on the far
end. The other end is connected via a "T" connector.

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.

Also, I don't know if the notch will be "deep" (high Q) enough to make
the digital data "un-usable" (can't decode). Another concern is the width of
the notch. If it is too wide, it may affect the adjacent channel.

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.

http://www.tinlee.com/CATV-ChannelEliminator_Single.php?active=1 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.

http://www.tinlee.com/NotchTraps.php?category=CATV&active=1 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.

Does anyone have any suggestions, or can you recommend a place
where I can order a good CATV NF (I am willing to pay)?

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).

 

[Allodoxaphobia]

On Thu, 19 Feb 2015 13:22:56 -0800, Dave Platt wrote:

jaugustine@verizon.net> wrote:

If I have this correct, the "stub" is a 1/4 wave length of the offending
frequency. The 75 ohm coax is cut to that length and is shorted on the far
end. The other end is connected via a "T" connector.

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.

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.

Also, I don't know if the notch will be "deep" (high Q) enough to make
the digital data "un-usable" (can't decode). Another concern is the width of
the notch. If it is too wide, it may affect the adjacent channel.

The latter is not at all unlikely.

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
--
Marvin L Jones | W3DHJ | W3DHJ | http://W3DHJ.net/
Pueblo, Colorado | @ | Jonesy | __
38.238N 104.547W | jonz.net | DM78rf | 73 SK

 

[Bruce Esquibel]

jaugustine@verizon.net wrote:

I never made a NF (Notch Filter), but I am thinking about making a "stub"
(I think that's the term) NF for CATV to "block" a single "channel" coming in
on the digital TV cable system. Note: This "channel" consists of ANNOYING
ads/promos that you can not avoid or "turn off" when you use the "On Demand"
feature to look at a list of "on demand" available movies, TV shows, music
videos, etc.

I know the frequency, which by the way is between 200 and 300Mhz.

I think you need to look up building a better tin-foil hat, the one you are
using isn't working.

I used to mess around with cable notch filters back when the systems were
all analog and I can't see where you came up with this idea to work on
digital systems. It's just not the same thing.

The analog systems used the same 6 mhz channel bandwidth as the terrestrial
broadcast systems so you narrowing down to the 200 to 300mhz range isn't
much better than saying "it's in the cable somewhere".

On digital systems the channels simply don't exist, unless they are analog.
It's data like ethernet. Your cable box is more akin to a computer than a
cable converter. What's worst is, some systems like fios and uverse, the
"cable" ends at that box out front or in the alley and your converter is
just telling what to send down the copper from the box to your house. Is how
they feed the 500 channel universe into a pair of 50 year old abestos
covered wires.

Even if it is an analog channel that you can pin down exactly (i.e.
63-69mhz), the filter is quite a bitch to work properly. If you look at (I'm
sure there is some still for sale on ebay, I think the big one was ARCOR)
one professionally made, those things are a precision made, highly shielded
peice of hardware.

The easiest thing to mess with to experiment is something that may be
difficult to get because radio shack sold it. The device was an FM radio
trap for TV use. Besides coming in a nice metal box that was easily opened,
it had both an in/out F connector and a circuit board with non-waxed coils
you could spread open and close, changing the frequency and depth of the
notch. If you were trying to get rid of something below 150mhz, it probably
could be beat into submission.

Of course having a $20,000 Wavetek rf sweep generator with the 75 ohm option
and a scope with a decent pre-scaler to see where and how the notch is
working is somewhat mandatory for that $4.99 fm trap. Luckily I had that
stuff laying around.

But the bigger problem is, besides getting your notch deep enough and on the
right freq, the major challenge is getting everything above and below it to
pass though. This was a major problem back when most cable systems were
limited to 450 or 550mhz total bandwidth but being 2ghz quality cable is
being used and channels/carriers up in the nose-bleed section above 1 gig, I
doubt if it could be done correctly.

Even those high dollar arcor filters the cable companies used to manually
block channels "leaked". Maybe they did a good job scrubbing out channel 18
or something, but at least 2 or 3 others up the way from it were also
weaken. Snow, herrbones, colored snow.

But the point is, if it's a digital system, whatever you think is a
"channel", isn't. It's like trying to come up with a notch filter for your
internet ethernet to filter out all the porn.

It just doesn't work like that.

I dunno why you find that advertising so annoying, if it's the one I'm
thinking of on the selection screen, but a better solution that is more
practical is just get a black peice of paper and tape it over the PIP box,
mute the audio and you are all set. When you find something to watch, take
the paper off and unmute the audio.

-bruce
bje@ripco.com

 

[Jeroni Paul]

On some more traditional cable services the channels are transmitted in fixed frequency channels (MUX) usually 8Mhz wide just like air DTV. If the OP has such service it makes sense to add a filter to block that channel (but it may block other data that may cause the selection list to not work). It is not an ethernet like system but a DVB-C system.

 

<SNIP>

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.

http://www.tinlee.com/CATV-ChannelEliminator_Single.php?active=1 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.

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 sneva@tinlee.com with the frequency of the channel I
want to "block". I am waiting for a response.

Again Thanks, John

 

[Jeff Liebermann]

On Thu, 19 Feb 2015 15:19:26 -0500, jaugustine@verizon.net wrote:

> I know the frequency, which by the way is between 200 and 300Mhz.

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.

If I have this correct, the "stub" is a 1/4 wave length of the offending
frequency. The 75 ohm coax is cut to that length and is shorted on the far
end. The other end is connected via a "T" connector.

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.
<http://802.11junk.com/jeffl/SCCARC-talk-2013-09-20/LTspice-FM-Notch/>

Also, I don't know if the notch will be "deep" (high Q) enough to make
the digital data "un-usable" (can't decode).

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.

Does anyone have any suggestions, or can you recommend a place
where I can order a good CATV NF (I am willing to pay)?

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

--
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

 

I used to mess around with cable notch filters back when the systems were
all analog and I can't see where you came up with this idea to work on
digital systems. It's just not the same thing.

The analog systems used the same 6 mhz channel bandwidth as the terrestrial
broadcast systems so you narrowing down to the 200 to 300mhz range isn't
much better than saying "it's in the cable somewhere".

On digital systems the channels simply don't exist, unless they are analog.
It's data like ethernet. Your cable box is more akin to a computer than a
cable converter. What's worst is, some systems like fios and uverse, the
"cable" ends at that box out front or in the alley and your converter is
just telling what to send down the copper from the box to your house. Is how
they feed the 500 channel universe into a pair of 50 year old abestos
covered wires.

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

 

[Phil Allison]

Jeff Liebermann wrote:

I know the frequency, which by the way is between 200 and 300Mhz.

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.

** 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 Dave,

Thank you for this very useful information.

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

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

Again Thanks, John

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

As I said a while ago (02-20-15), I sent an email to sneva@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

 

Here's an update on the email results.

As I said a while ago (02-20-15), I sent an email to sneva@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.

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

 

[Bruce Esquibel]

jaugustine@verizon.net wrote:

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

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
bje@ripco.com

 

On Friday, February 20, 2015 at 8:26:01 AM UTC-5, Bruce Esquibel wrote:

jaugustine@verizon.net wrote:

I never made a NF (Notch Filter), but I am thinking about making a "stub"
(I think that's the term) NF for CATV to "block" a single "channel" coming in
on the digital TV cable system. Note: This "channel" consists of ANNOYING
ads/promos that you can not avoid or "turn off" when you use the "On Demand"
feature to look at a list of "on demand" available movies, TV shows, music
videos, etc.

I know the frequency, which by the way is between 200 and 300Mhz.

I think you need to look up building a better tin-foil hat, the one you are
using isn't working.

I used to mess around with cable notch filters back when the systems were
all analog and I can't see where you came up with this idea to work on
digital systems. It's just not the same thing.

In other words hardwire or air broadcasts visible on today's High Definition Cable/Dish TV are in digital broadcasts only. And that is in:

A.) High speed online

or

B.) Satellite Dish.

Even that may be wrong, because you can hook your phone up to TV or computer and get internet that way. Internet now gives you radio, TV, etc.

So only a software program (now called an app) can code, decode or recode these broadcasts. Their app vs yours. Only the providers' help desks can help you - and that's if they're allowed to.

 

Here's an update on the email results.

As I said a while ago (02-20-15), I sent an email to sneva@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.

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.

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