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....
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