Is a RF notch filter for a VERY narrow slice of bandwidth feasible? - Page 1
Forum section: electronics / beginners

author: Mpegger
author_rank: Contributor
timestamp: October 20, 2023, 04:38:40 am
content: I've been playing around with some SDR usb sticks and one of the things I like to do is try to pickup distant FM radio stations. Unfortunetly I've found that there is a radio station tower less then 2 miles away from me pushing 2kW of power which overpowers alot of the FM radio band and makes it impossible to catch distant stations. I've been playing around with a open stub filter and it helps some, reducing the signal strength of that 1 station enough that I can actually use the LNA to try and pickup the low power, and distant FM stations, but I'm finding it's still not enough.I know there are other filters one could make, but most guides on DIY filters that I have found on the net, seem to be for large spectrums of an RF band, and not for a very narrow, specific tiny slice of spectrum (200kHz). So I'm wondering, is there ANY filter that could do something about such a small slice of spectrum? If there is, would it even be feasible to DIY one? What about combinations of different filters?

author: fourfathom
author_rank: Super Contributor
timestamp: October 20, 2023, 06:40:33 am
content: Coax stubs don't provide particularly sharp notches. Look at "duplexers", often used in ham radio repeater installations. These are essentially very high-Q cavity resonators. You probably don't need the performance of the multi-section ones, but the principles might be useful. Here's a link to a "built it yourself" article that's about 40 years old, but the basics haven't changed:https://www.repeater-builder.com/projects/2m-duplexer.htmlOr look up "helical resonators".

author: Terry Bites
author_rank: Super Contributor
timestamp: October 20, 2023, 10:30:38 am
content: A helical resonator may fix your your problem. Seewww.pa0nhc.nl/IMDfilter/EN/Notch/Single/HelicalNotchfilters2mEN.htmGet yourself a copy of "Handbook Of Filter Synthesis" A. I. Zverevsee page 499 for how design them.https://archive.org/details/HandbookOfFilterSynthesis/page/n509/mode/2uporhttps://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjO9PPNrYSCAxVdhf0HHXQeAmgQFnoECAkQAQ&url=https%3A%2F%2Fwww.wiley.com%2Fen-gb%2FHandbook%2Bof%2BFilter%2BSynthesis-p-9780471749424&usg=AOvVaw3DE5dpnYZoJDqhnffpJ-aX&opi=89978449Every lab should have a copy of the real thing. About $150 new. I nabbed a pre-loved one off ebay for $20.

author: mawyatt
author_rank: Super Contributor
timestamp: October 20, 2023, 01:33:34 pm
content: Agree, Zverev is a "Classic". We had two copies, one hard, one soft. Loaned the hardcopy and it was never returned!!Best,

author: Mpegger
author_rank: Contributor
timestamp: October 20, 2023, 08:17:35 pm
content: Quote from: fourfathom on October 20, 2023, 06:40:33 amI never came across this term in my previous searches for various RF filters. First search I did with "helical resonators" gave me this link:http://www.hubbatech.com.au/AMATEUR%20RADIO/RF%20Filters/Rf%20Filters.htmThey actually made a filter out of a "coffee can" and various others parts they had laying around.If I can find a "calculator" that works with this design I have enough spare parts laying around too that I should be able to make a filter like that as well.The pdf's are a good read, but the math is way over my head.

author: TimFox
author_rank: Super Contributor
timestamp: October 20, 2023, 09:32:15 pm
content: There are on-line calculators for helical band-pass filters based on Zerev.https://www.rfcafe.com/references/electrical/helical-resonator.htmhttps://coil32.net/online-calculators/helical-resonator-bandpass-filter.htmlA notch-filter design:https://www.satsignal.eu/wxsat/filters/HelicalNotch.htm

author: KE5FX
author_rank: Super Contributor
timestamp: October 20, 2023, 09:41:53 pm
content: What are you using for an antenna? That's the #1 thing to look at before anything else. Given enough antenna gain in the desired direction, a simple attenuator may be enough to get rid of the interference, if anything is still needed at all.

author: Mpegger
author_rank: Contributor
timestamp: October 21, 2023, 12:00:17 am
content: Given how close and how much power that station is putting out, even a simple 1/4 wave dipole inside my home, which because of the building material and location relative to many of the transmitters gets horrible reception, my reciever still gets overwhelmed by that station. That one station will easily hover around 15dB or higher (according to the SDR++ software I use, 100dB being no signal, 0dB being max) while the major radio stations that are less then 20 miles away, and put out 10-20kW of power, can barely sustain 40-35dB, the digital portions of thier signal not much higher then the noise floor. Any amount of Gain I try to dial in may bring in those stations abit better, but at some point, that transmitter in my backyard will just come through throughout the entire FM band, making low powered/far statations dead. I even have a LNA in line, but all I can do is simple turn it on, and place the Gain on the SDR to its minimum, or else once again, that 1 station will just bleed over the entire band.My usual setup is a old Radio Shack turnstyle type FM antenna (yes, I've been DX'ing FM radio stations since the 90's), which for me is perfect since the radio stations transmitters are either West, North, or East of me. The major stations (and the occasional pirate stations that popup) are almost all to the West of me, with the minor/far stations (low powered or 30+ miles away) are to the North and East. Unfortunetly, this 2kW station is NE of me which hurts my reception of those further/weaker stations (and of course the major stations). I have a 8 foot Yagi FM antenna, though it's just sitting in the garage gathering dust (can't put it to use right now for various reasons), but I doubt it would help much cause of how close that transmitter is. It's practically directly in the path both North and East of those stations I could use the Yagi for.The open stub cable helps attuenate the signal enough that I can actually tell there is a signal of some of those weak/far stations on a frequency (though I can't hear what's playing), and actually make use of the LNA to improve the dB and SNR overall throughout the entire FM band (the digital portion of those stations that transmit a HD Radio signal are actually much higher then the noise floor now), but at a certain point of upping the Gain, that close station once again will start coming through. It even comes through on the Air Band when I listen to the local airports if I raise the gain too high, so aircraft that should be well within listening range are inaudiable to me cause I can't use any Gain.

author: David Hess
author_rank: Super Contributor
timestamp: October 21, 2023, 02:34:57 am
content: Quote from: Terry Bites on October 20, 2023, 10:30:38 amA helical resonator is the way to go around the FM broadcast band because of its convenient size. I have used common double sided copper printed circuit board material and 14 awg tin plated wire for the coil. For tuning, pistons trimmers work great, but plastic trimmers may be good enough.

author: EPAIII
author_rank: Super Contributor
timestamp: October 21, 2023, 03:51:26 am
content: I faced this problem when the TV station that I worked for wanted to receive a distant station so they could record the programs for re-broadcast at a later time. This was to allow the recording of two different network programs at the same time when my station had only one network feed line via ATnT. With all parties consenting, of course. But there was a local station on the adjacent channel. I will try to describe the process I went through, but it has been decades since then so I can't remember the details, like the exact numbers of dbs at each step.The first thing was a good, highly directional antenna. Due to the distance involved it had to be mounted at about the 450 foot level on our transmitter tower in order to see the distant station around the curve of the earth. VHF does not work well past the horizon.I purchased a professional, single channel receiver which was intended for the cable TV industry where the passing of off air programming was common. But that, with the antenna was no way near enough.I purchased an additional, single channel filter, again a professional grade product also used by cable TV. Still not enough for a clean picture.I replaced the single, directional antenna with a matched pair of professional grade antennae, again made for the cable TV industry. They were mounted side by side on a boom which placed them at the same distance (so zero phase difference) from the distant station and with a spacing that had them 180 degrees out of phase for the local station. A professional level RF coupler was used to combine their signals. I do recall that this gave me a final 20 db of separation between the two signals (desired and not desired). This finally gave me an almost acceptable picture and sound.However, there was still a trace of interference which I tracked down to the FM audio carrier of the local station, which was just below the video carrier of the desired, distant station. A final purchase of a notch filter at the frequency of that offending FM carrier finally removed that last problem and we were able to record good quality audio and video from that distant TV station.This illustrates that you can approach the problem from both ends: yes you need band pass filters for the desired signal, but you can also eliminate the interfering signal with a band stop or notch filter.The guiding numbers that I used throughout that process was the separation of the two signals in db. I first calculated the signal strengths for the two stations. As I said, I can't remember the numbers, but it was at least 80 or 90 db in favor of the local station. And I needed at least 40 db in the other direction, so around 120 or 130 db or more was needed. None of the items that I used was able, on it's own, to produce that much separation of the adjacent signals. It took the combination of ALL of them to finally get acceptable reception.I am sure that things have changed in the decades since then. So I won't even try to suggest any sources. You need to do that research yourself or perhaps others here can suggest some. I would start with a good antenna and receiver and some calculations or measurements, if you can, of signal strength and proceed from there. It is all about the separation of the signals in dbs.

author: radiolistener
author_rank: Super Contributor
timestamp: October 21, 2023, 04:08:10 am
content: Receiver with good dynamic range can receive weak signal even in presence if near very strong signal, so you're needs receiver with good dynamic range. But DX FM stations are jammed with noise from digital electronics, switching mode power supplies and LED lamps in modern city. So the better way is to get away from the city and buildings to receive weak signals...By the way FM modulation is not good for long distance communication, it's better to use SSB or CW...

author: Terry Bites
author_rank: Super Contributor
timestamp: October 23, 2023, 09:41:17 am
content: here's handy calculator for conventional LC filtersThe tighter the notch the more unwanted responses- not so easy!https://markimicrowave.com/technical-resources/tools/lc-filter-design-tool/