FM Notch Filters / Tunable Traps

[Andrew K]

Does anyone in this forum have knowledge regarding FM notch filters or tunable traps? I found a company that custom makes these filters to attenuate nearby powerful FM frequencies. They can notch many specific frequencies at once depending on whatever you desire, but the cost is outrageously expensive... over $100 per frequency. I found the Winegard FT-7600 for $39 which looks like it can do the same thing. It will attenuate a single frequency up to 26 dB or two frequencies at 10 dB each. I'm supposing I could purchase 2 or 3 of these for attenuating the frequencies I want to eliminate and connecting them together. Good idea or not?? Or is there another product that's better for my situation?

 

[rbrucecarter5]

Does anyone in this forum have knowledge regarding FM notch filters or tunable traps? I found a company that custom makes these filters to attenuate nearby powerful FM frequencies. They can notch many specific frequencies at once depending on whatever you desire, but the cost is outrageously expensive... over $100 per frequency. I found the Winegard FT-7600 for $39 which looks like it can do the same thing. It will attenuate a single frequency up to 26 dB or two frequencies at 10 dB each. I'm supposing I could purchase 2 or 3 of these for attenuating the frequencies I want to eliminate and connecting them together. Good idea or not?? Or is there another product that's better for my situation?

Very bad idea! For that price - build it yourself would be a better - much less expensive - option. I'd take a splitter and gut it. You can buy fixed resistors to do the job very well. Calculations for a "pi" attenuator are all over the web, but I can give you resistor values and connections if you can't do it yourself.

 

[Big 121]

Winegard does not specify the "width" of the attenuated signal. The notch width and symmetry may affect 1-3 FM channels on each side of the tuned freq., and it's effectivness may vary across the band.Also the insertion loss is not specified. It will be higher in the max mode.Just this alone may wipe out a very weak signal elsewhere on the band you're trying to receive. A plus is that this unit appears to be metal/shielded, and is probably a decent impedance match.I would say $40 for this is quite reasonable, given custom,fixed, lumped-element filters can run several hundred $ each.

 

[Andrew K]

Does anyone in this forum have knowledge regarding FM notch filters or tunable traps? I found a company that custom makes these filters to attenuate nearby powerful FM frequencies. They can notch many specific frequencies at once depending on whatever you desire, but the cost is outrageously expensive... over $100 per frequency. I found the Winegard FT-7600 for $39 which looks like it can do the same thing. It will attenuate a single frequency up to 26 dB or two frequencies at 10 dB each. I'm supposing I could purchase 2 or 3 of these for attenuating the frequencies I want to eliminate and connecting them together. Good idea or not?? Or is there another product that's better for my situation?

Very bad idea! For that price - build it yourself would be a better - much less expensive - option. I'd take a splitter and gut it. You can buy fixed resistors to do the job very well. Calculations for a "pi" attenuator are all over the web, but I can give you resistor values and connections if you can't do it yourself.

I'm not technical enough to build my own unless I had very well written instructions with specific descriptions of the parts needed. Ultimately I want to attenuate 88.1, 89.7, 91.3, 94.9 and 97.5 each about a minimum 25 dB without affecting nearby frequencies. I hope this is realistic.

 

[frankberry]

I've used these Winegard FM traps. They perform well for the money. Go for it.

 

[Andrew K]

I've used these Winegard FM traps. They perform well for the money. Go for it.

Thanks for everyone's input. I really appreciate it!! I'll have to wait until I get the Winegard in the mail, and I'll post my results. If anyone else has suggestions on how to fix my overloading problem, I'd appreciate anything. I'm literally 1 mile from two towers that are blasting FM stations at me and blocking the weak ones.

 

[ddsparxx]

What do you do in a situation when you can't have an outdoor FM antenna? I used to be in a townhouse near 2 FM transmitters and didn't even thought about the FM traps at that time. I used a "rabbit ears" dipole at that time and used it to null out the strong signals as much as I can at that time...I used an FM tuner that handled strong signals well.

 

[kenglish]

Are you just looking for a single set of filters for one location, or do you need to buy a bunch for Blanketing Interference problems?

Microwave Filter Company makes "bullet traps" for Cable TV-type applications, and can tune them to whatever frequencies you want. Last I checked, they were $6 to $10 dollars each.
If it's for your home system, the Winegard is good, or you can look on e-bay for something like the old Blonder-Tongue dual notch VHF filters. They have two slugs, as well as an attenuator.

 

[rbrucecarter5]

Does anyone in this forum have knowledge regarding FM notch filters or tunable traps? I found a company that custom makes these filters to attenuate nearby powerful FM frequencies. They can notch many specific frequencies at once depending on whatever you desire, but the cost is outrageously expensive... over $100 per frequency. I found the Winegard FT-7600 for $39 which looks like it can do the same thing. It will attenuate a single frequency up to 26 dB or two frequencies at 10 dB each. I'm supposing I could purchase 2 or 3 of these for attenuating the frequencies I want to eliminate and connecting them together. Good idea or not?? Or is there another product that's better for my situation?

Very bad idea! For that price - build it yourself would be a better - much less expensive - option. I'd take a splitter and gut it. You can buy fixed resistors to do the job very well. Calculations for a "pi" attenuator are all over the web, but I can give you resistor values and connections if you can't do it yourself.

I'm not technical enough to build my own unless I had very well written instructions with specific descriptions of the parts needed. Ultimately I want to attenuate 88.1, 89.7, 91.3, 94.9 and 97.5 each about a minimum 25 dB without affecting nearby frequencies. I hope this is realistic.

Assuming you are using matching transformers and have 75 Ohm lead in (which is a good idea in strong signal environments) - here are the directions:

(1) Take apart a splitter, getting the back off will be the hardest part.

(2) Take out all the internal parts. The splitter has three 75 Ohm connectors left. You will only be connecting to two of them.

(3) Solder an 82 Ohm Resistor from center conductor to the splitter case on the two connectors you plan on using for your attenuator.

(4) Solder a 680 Ohm resistor from the center conductor of the first connector you are using to the center conductor of the second you are using.

(5) Put the back on the splitter and it is now a 25 dB attenuator!

Those resistor values will get you very close. If you have access to 1% resistors, the values should be 84.5 Ohm and 665 Ohm. But the difference in performance will be negligible. The attenuator is bi-directional, you don't have to worry which is "in" and "out".

You just saved a bunch of money for an attenuator which probably has those same resistor values inside!

Here is one calculator I found on the web:

http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm

 

[frankberry]

He needs a frequency-selective attenuator. A notch filter ... not just a simple broadband attenuator.

 

[Andrew K]

To clarify exactly what my problem is, I would like to do some FM DXing, but this is made impossible by nearby FM towers that are overloading the FM band. I’ve tried several radios in my collection, and the Sangean ATS-909X and Kaito KA1103 are the best performing when it comes to selectivity and sensitivity. I still have issues though. When I point my AntennaCraft FM6 east to receive the desired Youngstown stations, the antenna is right in line with the nearby FM tower that overloads the band. I want to attenuate the frequency of 97.5 that broadcasts from that tower. And hopefully that will eliminate the interference that shows up on other frequencies throughout the dial and blocks out the Youngstown stations I want to receive in Akron. I will hear distorted audio interference from 97.5 on other frequencies like 93.3, 98.9, 101.1, 105.1 and so on. I’m not sure what determines the frequencies, but it’s there. And I hope that by attenuating 97.5, that it will eliminate the associated interference that it causes to other stations.

 

[rfry]

Andrew - Rejecting one FM station with little effect on other stations close to that frequency is difficult, and maybe more expensive than some can justify.

One notch filter that performs well is the Eagle TNF412-4BF. It has a reject bandwidth of about 0.7%, or about 700 kHz when tuned to reject 97.5 MHz. This bandwidth would not pass FM stations from 97.1 to 97.9 without losses, though.

I checked with the Eagle factory, and the price with BNC female connectors is $315, and typical availability is 2 weeks or less. It does need to be used in a 50 ohm antenna system in order to produce the best performance.

Also note that the insertion loss for frequencies outside of the reject bandwidth can be as much as 7 dB, which will reduce the effective gain of your FM6 receiving antenna.

There are cheaper filters including some that can be home made using 1/4-wave stubs, but their max rejection is smaller and their reject bandwidth is wider than the Eagle filter.

Here is a link to the Eagle filter lines..

http://www.eagle-1st.com/eagle1st.nsf/e4fa682eb52a27e10725763c007961ea/16929a0821e52be207257674005fecd6!OpenDocument

 

[Andrew K]

Andrew - Rejecting one FM station with little effect on other stations close to that frequency is difficult, and maybe more expensive than some can justify.

One notch filter that performs well is the Eagle TNF412-4BF. It has a reject bandwidth of about 0.7%, or about 700 kHz when tuned to reject 97.5 MHz. This bandwidth would not pass FM stations from 97.1 to 97.9 without losses, though.

I checked with the Eagle factory, and the price with BNC female connectors is $315, and typical availability is 2 weeks or less. It does need to be used in a 50 ohm antenna system in order to produce the best performance.

Also note that the insertion loss for frequencies outside of the reject bandwidth can be as much as 7 dB, which will reduce the effective gain of your FM6 receiving antenna.

There are cheaper filters including some that can be home made using 1/4-wave stubs, but their max rejection is smaller and their reject bandwidth is wider than the Eagle filter.

Here is a link to the Eagle filter lines..

http://www.eagle-1st.com/eagle1st.nsf/e4fa682eb52a27e10725763c007961ea/16929a0821e52be207257674005fecd6!OpenDocument

Actually, I'm not expecting to receive any adjacent FM stations like 97.1 or 97.3. I would be happy to just attenuate 97.5 (or whatever the notch width may be) and eliminate any associated interference it causes throughout the FM dial... the soft static/hissing or distorted audio overload I hear from 97.5 that's way up or down the dial from that frequency. One of my observations is that 97.5 will mix with 94.9, and I will hear the audio from both of those frequencies combined onto another random frequency like 93.3 for example.

 

[rbrucecarter5]

Actually, I'm not expecting to receive any adjacent FM stations like 97.1 or 97.3. I would be happy to just attenuate 97.5 (or whatever the notch width may be) and eliminate any associated interference it causes throughout the FM dial... the soft static/hissing or distorted audio overload I hear from 97.5 that's way up or down the dial from that frequency. One of my observations is that 97.5 will mix with 94.9, and I will hear the audio from both of those frequencies combined onto another random frequency like 93.3 for example.

The best tunable filter you can buy are the ones that are supplied in high quality vintage FM tuners with many gangs in the front end. Although the primary selectivity is achieved in the IF section, multiple RF gangs will give you better rejection of strong nearby signals. Those multi-gang tuners aren't cheap - they show up on eBay occasionally and usually go for a few hundred dollars. Your present tuner sounds like a mess - you need a better one. Anything less than a multi-gang front end would be like applying bandaids to the Titanic - it won't address the fundamental problem you are facing - which is overload due to poor RF front end design and circuitry in your tuner.

 

[Andrew K]

Actually, I'm not expecting to receive any adjacent FM stations like 97.1 or 97.3. I would be happy to just attenuate 97.5 (or whatever the notch width may be) and eliminate any associated interference it causes throughout the FM dial... the soft static/hissing or distorted audio overload I hear from 97.5 that's way up or down the dial from that frequency. One of my observations is that 97.5 will mix with 94.9, and I will hear the audio from both of those frequencies combined onto another random frequency like 93.3 for example.

The best tunable filter you can buy are the ones that are supplied in high quality vintage FM tuners with many gangs in the front end. Although the primary selectivity is achieved in the IF section, multiple RF gangs will give you better rejection of strong nearby signals. Those multi-gang tuners aren't cheap - they show up on eBay occasionally and usually go for a few hundred dollars. Your present tuner sounds like a mess - you need a better one. Anything less than a multi-gang front end would be like applying bandaids to the Titanic - it won't address the fundamental problem you are facing - which is overload due to poor RF front end design and circuitry in your tuner.

I've been told that a receiver with good IP3 characteristics will perform well. I researched a recommended unit... the AOR AR5000 which costs $1,500!!! I don't have that kind of money to throw toward a radio, although I'm sure a unit like that performs well. For now, I'm dealing with what I have... the Sangean ATS-909X and Kaito KA1103, which have decent reviews for the price.

Are there any specific vintage radios to recommend??

 

[ddsparxx]

The FMTunerInfo.com is the best website for component FM tuners that I know of. I have read that the Yamaha T-85 tuner is very good at handling strong signals. I've never owned one but I have an Onkyo T-9090 II for many years that did pretty well when I was 2 or 3 miles from an FM tower.

 

[BobOnTheJob]

I have a Winegard TAC2-FM that's over 30 years old. I have an FM yagi about 100' below a 4.9KW ERP FM and it notches that freq enough that I have zero overload and I can still hear a class A 1 channel removed that's nearly 80 miles way. Question is...where can you buy one of these today?

 

[Tincap]

Actually, I'm not expecting to receive any adjacent FM stations like 97.1 or 97.3. I would be happy to just attenuate 97.5 (or whatever the notch width may be) and eliminate any associated interference it causes throughout the FM dial... the soft static/hissing or distorted audio overload I hear from 97.5 that's way up or down the dial from that frequency. One of my observations is that 97.5 will mix with 94.9, and I will hear the audio from both of those frequencies combined onto another random frequency like 93.3 for example.

The best tunable filter you can buy are the ones that are supplied in high quality vintage FM tuners with many gangs in the front end. Although the primary selectivity is achieved in the IF section, multiple RF gangs will give you better rejection of strong nearby signals. Those multi-gang tuners aren't cheap - they show up on eBay occasionally and usually go for a few hundred dollars. Your present tuner sounds like a mess - you need a better one. Anything less than a multi-gang front end would be like applying bandaids to the Titanic - it won't address the fundamental problem you are facing - which is overload due to poor RF front end design and circuitry in your tuner.

I've been told that a receiver with good IP3 characteristics will perform well. I researched a recommended unit... the AOR AR5000 which costs $1,500!!! I don't have that kind of money to throw toward a radio, although I'm sure a unit like that performs well. For now, I'm dealing with what I have... the Sangean ATS-909X and Kaito KA1103, which have decent reviews for the price.

Are there any specific vintage radios to recommend??

Try a Grundig G8. I kid you not. Slip the bare end of a piece of RG6 under the base of the antenna, hook it up to your FM6 and see what you get. You may be pleasantly surprised. Both mimo and I can attest to its abilities, in filtering out strong signals, while not affecting weaker signals (I was so impressed that I bought a 2nd G8, which is now a dedicated tuner, hooked up to my FM6 and patched through my Luxman). And even if that still doesn't work for you, you'll have a brilliant little radio for $50. 8)

~BG

 

[Andrew K]

Actually, I'm not expecting to receive any adjacent FM stations like 97.1 or 97.3. I would be happy to just attenuate 97.5 (or whatever the notch width may be) and eliminate any associated interference it causes throughout the FM dial... the soft static/hissing or distorted audio overload I hear from 97.5 that's way up or down the dial from that frequency. One of my observations is that 97.5 will mix with 94.9, and I will hear the audio from both of those frequencies combined onto another random frequency like 93.3 for example.

The best tunable filter you can buy are the ones that are supplied in high quality vintage FM tuners with many gangs in the front end. Although the primary selectivity is achieved in the IF section, multiple RF gangs will give you better rejection of strong nearby signals. Those multi-gang tuners aren't cheap - they show up on eBay occasionally and usually go for a few hundred dollars. Your present tuner sounds like a mess - you need a better one. Anything less than a multi-gang front end would be like applying bandaids to the Titanic - it won't address the fundamental problem you are facing - which is overload due to poor RF front end design and circuitry in your tuner.

I've been told that a receiver with good IP3 characteristics will perform well. I researched a recommended unit... the AOR AR5000 which costs $1,500!!! I don't have that kind of money to throw toward a radio, although I'm sure a unit like that performs well. For now, I'm dealing with what I have... the Sangean ATS-909X and Kaito KA1103, which have decent reviews for the price.

Are there any specific vintage radios to recommend??

Try a Grundig G8. I kid you not. Slip the bare end of a piece of RG6 under the base of the antenna, hook it up to your FM6 and see what you get. You may be pleasantly surprised. Both mimo and I can attest to its abilities, in filtering out strong signals, while not affecting weaker signals (I was so impressed that I bought a 2nd G8, which is now a dedicated tuner, hooked up to my FM6 and patched through my Luxman). And even if that still doesn't work for you, you'll have a brilliant little radio for $50. 8)

~BG

The Grundig G8 is a nice little unit with some cool features, but as far as sensitivity goes, it just doesn't match up to the Kaito KA1103 I have, which is both sensitive and really selective... even more so than the Sangean ATS-909X, which costs $250. The Kaito is able to receive WRDL 88.9 well, and the Sangean can BARELY pick up listenable audio with the same antenna. The G8 gets NOTHING on 88.9.

I didn't have much luck with the attenuators last night. I couldn't properly tune the Winegard notch filter, so I'll have to try another time. The variable attenuator just attenuated everything with undesired results.

It looks like a really nice unit will be the way to go, but people's opinions can be mixed. I need to step up from a portable to a good tabletop unit designed for DXing!!

 

[rbrucecarter5]

It looks like a really nice unit will be the way to go, but people's opinions can be mixed. I need to step up from a portable to a good tabletop unit designed for DXing!!

The people here: http://fmtunerinfo.com/ are really knowledgeable and will be happy to help you. There is a companion yahoo group where you might want to ask for help: http://tech.groups.yahoo.com/group/FMtuners