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Looking for TV notch filter with a low number of coils

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Member level 2
Sep 17, 2001
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Hello :lol:

I want to build TV NOTCH FILTER for "descramling" (filtering out) unwanted carrier in TV signal for particular channel of my cable provider. I like the folowing design:


But the problem I encountered in this design is a LOT OF COILS. I'm very week in coil building and they are very hard to get in my country.

Does anbody have a notch filter design with a lot less coils, if possible none...I don't know if such kind of filter can be acheaved without coils. If it is, I would realy like to have that plan. I wouldn't mind if it would be an active (powered) component.

Thanx in advance for all your possible answers. :wink:


homemade notch filter

Hi Rostfrei,

I do not think that making an air wound coil at the kitchen table is very difficult...
All you need is 1-2 meter long bare or enameled copper wire of the suitable diameter (this is obtainable in electric hardware shops in most countries, aren't they?) and a cylinder form of an outside diameter that more or less fits to the would-be coil inside diameter. Such forms come out of containers of medicins or of a handle of broom or the surface of a ball-point pen etc.

Once you obtained the above two things, you can start winding the wire turn by turn on the form, each turn goes to as close as your fingers can control it. When you have reach the needed number of turns, you are ready, the coil may need some stretch.
I know it is difficult for the first time but practice makes perfect..

If you are definitely against any coil in your notch filter, you may order quartz crystals from a good crystal manufacturer either in your country or from abroad : it will definetely cost you much more than making the coils, it is sure. Ordering a crystal for a particular frequency (which frequency is unique and there is no off-the-shelf) usually involves at least US$10-15 per quantity + post/package!

If you can afford it and you know a crystal manufacturer to turn to, then I can tell you a notch filter made of two identical crystals, it is an American patent. It may need two wide band RF impedance matching transformers (like Mini-Circuits T6:1) which add to the cost of course!

By the way, what is the frequency of the disturbing signal on your particular TV channel? You need to know that precisely if you order crystals! Usually the good manufacturers make crystals up to 250MHz or even 300MHz.

So if you notch filter should work under 200-250MHz, I recommend making friends with the coils.... :D
On the web page you mentioned the tuning order of the coils is also included which would make life easier for you to make your filter really work.

regards, unkarc

how to build notch filter

Hello unkarc :)

Firs of all thanx for your reply.

An air coil wouldn't be such a problem for me to make, but as i can se from design, there are coils L1, L2 and L4 that are adjustable and have alum (does that stand for aluminium?) core. These are the coils I'm worried about. Can I make adjustable coil at home ? Where do I get the core ? What is a diameter of a core ? How to get the core to slide in and out (that's the way to adjust coils right?).

Waiting for your reply :roll:

how to build a notch filter

I took a quick look at the notch schematics and I would **not** use this for "descrambling". Years ago the true schematics on how the cable-networks notch filter are designed were published. This ueses 3 coils and a handful of capacitors.

You can use ready made coils from Toko and get a notch deeper than 60 dB at 70 MHz. At frequencies above 150 MHz temperature drift thends to make it slightly troublesome.

If you let us know what frequency you want to remove I could do a quick simulation and post the exact components value
This should work for 50-100 MHz
L2 is small 50 nH and c aprox 2-4 pf
L1 and L3 100-200 nH and C chosen for resonance. All inductors should be tunable.
in>----L1-C-------> out
        |           |
        |           |
        c           c
        |           |
       |           |
       L3         C
       |            |
The schematic looks no good but I could send proper drawing:oops:


cable tv notch filter

Hello webdog :p

I'm wondering why do you think I should not use the schematics on the page I provided?
Is this the true schematics on how the cable-networks notch filter are designed, that you replied? on more specific questions. In my country (Slovenia) there is almost impossible to get ready made coils at the electronic store, so there is almost no way arround making a coils by myself. To get adjustable ready made coils IS impossible. The problem with home made coils is, when you give me the walue for example 50 nH, I don't know what kind of wire, on what diameter and how many turns to use, plus I don't know how to make adjustable coil at home.

I hear you mentioned TOKO coils. I have a few old radio PCB's with litle metal cubes on it and on them is written the name Toko RLC and it is possible to adjust them with screwdriver (I think the ferite core is inside). I gues thats the thing you are talking about.
If it can I know witch Toko to use? Is there a way to read a inductance walue like the resistance on the resistor for example?

I don't know the exact frequency of the disturbing signal. All I know is that I have a PAL system and that the "coded" channel is on S9 (161 MHz). I read channel frequency from the grafic user interface of the Samsung TV for that channel.

Waiting for your reply. :-?

crystal notch filter

the reason for not using "your" notch is that the Q-value is low hence a wide and not very deep notch is acheived. The drawbacks are your picture may show anything from severe to light interference from the jamming signal, and the sharpness of the picture will suffer due to that you have a wide notch in the channel.

The jamming signal is located a.f.a.i.k. in the middle betwwen the sound carrier and the vision, 2.75 MHz off.

If you get a good RF analyze/simulator program i.e. Genesys (see other posts on this board) you should be able to simulate and then build a working prototype. It sounds to me that you do not have access to any test or measurement instruments and this really complicate the matter!

I doubt you can use the RLC-inductors as they are likely to be in the IF (455 kHz or 10 MHz) range and you want to go higher in frequency.

Sorry to be such a drag, but it will be pretty hard for you to get the stuff working, especially if you have no or very little experience in the art of RF.

What you could do just for fun, is to wind a coil say 3-4 turns on a 3 mm drillbit, remove it and put it in series with at 2-10 pF multi-turn capacitor. Put it across the TVcoax and start to tune the cap. You should see that you can affect the picutre and by this have the inductor/capacitor in the "ballpark"..

Good Luck and sorry for not beeing able to give any real help

wideband notch calculator

Dear Rostfrei,

I uploaded the file to my unkarc directory in FM 1.
It includes two air-wound coil calculator programs run under DOS (from the Windows's DOS windows is fine).
I prefer using the coil.exe program which includes everything you may desire to know on a particular air-core coil.

Because it gives desired inductance within a few percent, I do not think you would need a tuning core inside the coil. The alum indeed means aluminium core (a half to one cm long piece of alu screw or alu wire with at least 5-6mm diameter would do it BUT it has an effect of DECREASING the inductivity as if you were stretching out the coil. So it has just the opposite effect than a ferrit core does. This also means you can use such piece of alu to make sure about the direction of the tuning needed for a particular coil when you approach it INTO the coil and can watch the result on the screen!! I.e. when you experience that putting into the coil the alu piece the picture improves then you simply have to pull the turns a little apart from each other to DECREASE the inductivity to the same degree the alu piece did OR vice versa of course.
I agree with Webdog that your notch circuit would be good to simulate in a good software and besides Genesys I recommend the Student version of Serenade (what you can download from h**p:// and look for SerenadeH.exe , about 30MB self-executing file). Sorry but I do not know any place at the moment where you could find the FULL version of Serenade but for your present notch filter project the student version is perfectly enough. Its capacitor and inductor models include the quality factor, Q as well and from the coil.exe you can receive the air-wound copper coil's Q (about between 200-500) and for the capacitors' Q I suggest to choose a Q of min 1000-1500. Around your needed S9 channel where the picture carrier is 161.25MHz the above capacitor Qs are reasonable.

You can experiment with the diameter/coil lenght in the coil.exe program to see which one of them makes the Q higher at a particular wire diameter (for self-supporting coils use at least 0.7-08mm diameter copper wire; silver coating would be even better as you will see from the coil.exe but getting silver coated wire is difficult of course, you may turn for help to local amateur radio experimenters).

Your webpage with that notch filter will be good for your purposes (the success depends on the coils' Q, the higher Q you can make, the better, and on the tuning of your coils).
What Webdog showed you is also a good notch filter (bridged-T type) but its tuning is different from the above where you already know which coil when to tune.

regards, unkarc

fm notch filter schematic

Neat to upload Coils.exe, seems like a handy program for calculating coils!

Regarding Q-values I am not convinced that you do get more than 150 for inductors at this frequency range, and one will get a lot less than 100 if using slugtuned and screened. As for the capacitors, Q>1000 would mean porcelean caps (microwave stuff)?? Expensive, using NP0 would be more realistic and I guess a Q of 200 or so.

Finally a very hand notch can be made with a narrow helix bandpass filter, terminaded into 50 ohm, connected to a power splitter (sum port) in the other end. When outside of the bandpass the filter shows high impedance and gives almost full reflection and the power goes from port 1 to 2. When inside the pass-band you get full absorption in the 50 ohm resistor an no reflection, no power transfered from port 1 to 2 all is to the sum-port. The major drawback is the 6 dB loss you get but as there is plenty of signal strength this should be no trouble.

Good Luck

build tv notch filter

Hello webdog :lol:

Can you be more specific on helix bandpass filter. I never heard of it. Do you have some kind of schematics for it, design steps (simulating with Serenade maybe?), tuning the filter?


notch filters tv

Helix filter is just a way of making the resonator acheive very high Q.
You enclose the inductor and introduce stray capacitance and get a resonance. Putting two (or more) of these inductors close and have an adjustable aperture you can change the coupling between the resonators and get a bandpass response. If you look in any RF handbook they should have numerous references to helix resonators. The trick of using bandpass filters and a hybrid power divider as a notch is **NOT** my invention, I read about this in RF Desing Magazine.

I recomend the book "Introduction to RF Design" by Wes Hayward. It is sold by ARRL and does cover filters and resonators.

Good Luck

filte* helica* resonat*

Hi Rostfrei,

A more precise term is helical resonator and you may find several urls if you do a search with the words "helical resonator notch filter, for instance you may find h**p:// or h**p://

So you can get a feel on how a helical resonator looks like. It can have a much higher figure of merit, unloaded Q, then a normal coil has.
It is possible to make a single coil helical resonator put into a rectangular-shaped printed circuit board enclosure. The size of the enclosure and the coil diameter and its number of turns are of interest of course. With Serenade you cannot simulate it but the other Ansoft product the HFSS can nicely do it.
Unfortunately I do not have HFSS but someone else on this Forum may have an installed and running one....

rgds, unkarc

PS to webdog: with proper air wound coil diameter/lenght ratio, one can get a Q of 300-400; just experiment with coil.exe, you can trust in it! Regarding capacitors's Q, you are right, the NPO ceramic smd caps are the better ones and I think they must have Qs of towards several hundreds. Porcelain caps are not needed for this design but small air-trimmer caps are.
Of course, getting any of them in hand may be a challange for Rostfrei.

one pass helical resonator

Hi unkarc,
always a pleasure discuss RF with others, one always pick up new stuff! :p

I think that Rostfrei must get some "local" help on the way. Starting from scratch with no measuring equipment is very difficult and time consuming, but of course it could be done.

Looking at Q-values of inductors, I have always assumed that at frequencies around 100 MHz it would be almost impossible to get higer than say 200.... I do not question the coil design program but have to take a look in my textbooks and try to recapture the theory!


homemade tier trap filter

Hello webdog adn unkarc :p

First of all thank you both for your helpful information and directions on RF construction. That will be great start for me. I'll try and do my best building that filter and I'll report results to you in a week. I simulated filter from the link I provided in the Serenade and I like the results. Is the simulation in SPICE possible for such big frequencies (I mean are they acurate)?

Does anybody of you two know where to get working Genesys (not demo...cracked) program? I would like to try it out since everybody on the board are saying great things about it.
And I would relly appreciate if you could direct me to some eBooks (PDF, chm) on RF introduction and construction.

For the last...I never used TOKO coils. Where can I get some information on using Toko coils in a designs. I have been on Toko web page, but there are no application notes available there.


homemade fm trap filter

Hi Rostfrei,

I think you could download Genesys directly from their website ( Probably you will have to log in with your name/e-mail but that is all. Look for 811setup.exe this is the latest and full version with size of 53 187 648 bytes (about 55MB) Crack is in FM 1 under Bleaz directory but search the Forum for further info on that crack!

Do not worry on either Serenade or Genesys accuracy, they are considered professional simulators up to several GHz!

Regarding Rf Design by Hayward or other good books on Rf topics, I suggest you to turn to a local library because there exists a so-called "interlibrary service" and your local library searches for requested books at other libraries either inside your country or abroad and will obtain them for you in 2 or 3 weeks' time. Try it, it works!!! Such books are not easily available on the web in e-book form and many of them do not exist in e-book.

Do not consider TOKO coils as magic, they are nice-looking coils, that is all. If you meet a circuit design in which the designer used a specific TOKO coil, you can always substitute it with a home-made coil, the difference will be that your coil will not be so nice-looking...or small... Of course TOKO has a big advantage in that they can easily obtain components to manufacture coils + have instrumentation and a home constructor cannot obtain easily that in most of the cases.

rgds, unkarc


homemade notch filters

Hello Rostfrei :p

There was a thread on helical filters on the Other Design Forum around this February 24, see **broken link removed** which may be of further help to you. Have you read it?

I found on the internet a nice utility program on designing helical resonators and I uploaded "" into my unkarc directory in FM 1. Hope will be of some use for you.

regards, unkarc

build notch filter

Hello unkarc :roll:

Thank you for helical filters design guides. I read the forum topics you suplied and I PM-ed the guy who made a notch filter with helical filter design. I like the program you uploaded ( but i don't fully understand how the represented calculated helical filter should look like when built. Where is input and where is output? Can you give me some guides on entered parameters:

- resonator impedace...why should that bother me ?
- impedance at tap...I guess this is 75 Ohms for cable TV
- number of resonators ?!?
- pass band riple ?

About SPICE I meant if it is possible (acurate) to simulate filters (not helical but ordinary filters) with it? I know how to use SPICE from college as for Serenade or Genesys I would have to learn how to use them.

Waiting for your reply. :lol:


tv notch filter build

You can use SPICE but the SPICE models for inductors and capacitors do not include Q, so you need to add a resistor in series or parallel with all of your components to model their Q. Much better to use SerenadeSV as suggested and it's free!

notch filter 43 tv

Dear Rostfrei,

Sorry but the includes "only" a program for calculating band pass filters with helical resonators and your need is notch or band stop filter.

-resonator impedance = means the transmission line impedance what is formed by the coil and the enclosure together (and this ranges from some hundred up to around thousand ohms and this is what is to be matched to 50 or in your case to 75 ohms)

-impedance tap = indeed the input and the output imp, 75 ohms in your case

-number of resonators = in the above program the number of helical filters coupled to each other with the aperture gap, this program was designed for a minimum of 2 resonators (i.e. a double tuned circuit)

-passband ripple = means how "wavy" or bumpy the band pass filter in its useful pass band inside the 3 (or 6dB) points

Regarding your question of where is the input/output:

Well, it does not turn out for your notch filter from the program.

Imagine a rectangular fully metal box with very rigid walls, say width=5cm depth=5cm and hight=7cm (You can make one from double-sided PCB and solder the edges carefully)

Let's put it on the table vertically (onto on its 5cmx5cm base) and imagine two antenna connectors (one female and one male) on its any of the opposite sides from ,say, 1cm down from its top. Connect the two connectors inner pin together with 0.8-1.5mm bare copper wire (short circuit). The helical winding comes up from the bottom (solder the lower end to the bottom) in the geometric centre as high as approximately nears the bare wire (but does not touches). This way you couple this high-Q resonator to your 75 ohm cable coax line and you can adjust the coupling by very small pF capacitors (or fraction of pFs), hence you may control the deepness of the notch. Of course this detunes a little the original resonator frequency what can be corrected by a fine threaded screw of 1-2cm long (diam=3-4mm) which comes down from the top center (in fact offset a little from the center) and goes down as far as the inner area of the coil's upper end. How deeply comes down it so changes the resonant frequency. Of course you can stretch out or push closer the turns of the coil to tune it roughly.

More or less this is what you may build, the sizes and the number of turns of the coil what the above program can approach to you.

Hopefully you get help from the person you pm-ed also.

rgds, unkarc

amateur radio pager notch filter

Hello unkarc 8)

Thank you for your VERY detailed instructions. Today I'm going to make filter with first design I provided. I'm gonna let you know how succesful I was. I think that helix design is too tuff for a newcomer like me. If I'll succeed with that "easyer" design, I'll give it a try with helix design too.

Coils.exe is great program and helped me a lot. I calculated coils for my needs and I simulated design in Genesys and it works great in theory. I hope it will work so great in practice too.

You'll hear from me soon. :wink:


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