filter questions?

[paulmohr]

Ok, I know next to nothing about RF filters so these might seem like pretty basic questions, because they are lol. I have a basic idea of how filters work because I tinker with building my own speaker systems. So I know what a bandpass, low pass and notch filter is. I just don't know how they work in an RF system vs an audio signal.

So, this is what I am trying to do. For starters I have a weather signal very close to me that is overpowering other channels and bleeding through, 162.450. To be honest I could do away with the whole marine band and it would be ok with me. So that would be a notch filter correct?

Then I would like to filter out anything under 150.00 MHz or so, mainly broadcast FM stations. I have a local station really close to me that is really strong as well, 103.9. I also don't need anything above 860 MHz or so. Although there doesn't seem to be anything up that high that is interfering with anything. So that would mean I need either a high pass filter, or a bandpass filter correct? Or possibly two notch filters?

So, the real questions are:
1) Can you use a notch filter AND a low pass/bandpass filter together, or multiple notch filters? Will they isolate frequencies like that, or if I place them in line with each other will they just add all the components together and make a big mess and do nothing close to what I want them to do?

2) Where would I get such devices, at a reasonable cost? I don't want to pay 100 to 300 dollars for high end filters just for my little scanner and discone antenna. I tried a quick google and ebay search but really didn't come up with anything. What I did find was expensive and not the frequencies I am looking for. Chances are I just don't know what I am looking for or the proper name to search for it.

What are my options, aside from blowing up the local antenna? That was a JOKE, don't call the cops! I have a scanner so I will know they are coming lol. I can live with the problem if I have to, but I would like to solve it if I can do it for a decently low price.

I could probably build them if I knew exactly what components I needed, but the math of designing them is beyond me.

Thanks in advance for any suggestions.

 

[majoco]

Googling for "VHF FM Band Stop filters" will get you a lot of answers to your second question - I don't know why you wanted to block out everything below 150MHz, you'll lose some public service stuff, the 2m ham band and all the good aircraft band too.

As for your 162MHz signal, you may have to dig a bit deeper. There are articles on making your own tunable filters but they get a bit technical and quite tricky to set up at the higher frequencies - they need to be in a screened box but you need to get near them to tune but your hand detunes the circuit and so on.... ! Yes, you can put them in series and hopefully one doesn't affect the other.... If you feel like experimenting buy two VHF FM blockers and modify one to get the higher frequency

 

[paulmohr]

- I don't know why you wanted to block out everything below 150MHz, you'll lose some public service stuff, the 2m ham band and all the good aircraft band too.

Basically because I don't listen to anything under 155 Mhz, I think their might be one channel that is 152. something and that would be the lowest. Same with anything over 860 MHz or so, there is simply nothing there I listen to. As well as the marine band. I don't really have a "need" to filter them out, I just figured a bandpass filter from 150 MHz to 850 MHz with a notch filter for the 162 MHz frequency would be simple.

I guess two notch filters for the FM broadcast band and marine band would work too.

I was worried in order to make it work I would either need multiple antennas or scanners. Because with an audio crossover you combine multiple filters for different frequencies, but each one feeds a different driver. Not all of it going to one driver. Although you can incorporate a notch filter into one, so I guess that would be the same thing. I have never built a speaker system that needed a notch filter so I never really messed with them.

I have an RCA pre amp that has separate VHF and UHF inputs and an FM trap. I did some testing with my SDR and the VHF/UHF separation seems to work ok, but the FM trap filter doesn't seem to do anything. Which is actually more than I expected for a 40 dollar TV pre amp from Lowes lol. And as it turns out I don't even need the amp, it just makes my problems worse.

Thanks for the input.

 

[majoco]

Yes, the amp when you have strong signal around is a curse rather than an aid because the amplifier is overloaded itself and probably overloads the front of your receiver. When anything like that gets overloaded it produces all sorts of spurious signal all over the frequency spectrum so initially you think WOW this is good but in fact it's absolute garbage!

Audio filters, as you say, tend to be in parallel and feed individual speakers which have limited responce curves. If you wanted to feed two receivers from one antenna that's what you would do here but we don't want that, we want to block two bands of frequencies so we put two 'blockers' in series - hopefully they will pass through everything we do want to hear. I have some doubts about the interaction between the two filters though - one test will see!

You can think a filter and a blocker is the same thing - a filter passes a select band of frequencies through to the radio and passes everything else to ground. If you turn it around the blocker passes the selected frequencies to ground and leaves the rest to go to the radio.

 

[Ubbe]

I would suggest to try a stub filter. It is a piece of open end coax connected to the antenna coax with a T connector and works as a notch filter. You can have several of them and you could first try them connected at the scanner and then also before the amplifier depending of what works best.

The electrical lenght of the open end coax should be 1/4 wave of the frequency to be notched.
Standard RG58 coax have a delay of 0.66 so that is the value to multiply with the wavelenght.

Radiowaves travel 300 000 000 meters per second and divide that with the frequency in Hertz.
When using MHz it's 300 divided by 162 to get the full wavelenght for your weather channel and divide by 4 to get the quarter wavelenght which is 46centimeters. A RG58 coax will multiply with 0.66 and the result is 30cm or 12 inches for a 162MHz stubfilter.

/Ubbe

Picture from Arcticpeak The Quarter-wave stub

 

[paulmohr]

Man I am glad you brought this up, I just looked at that thread last night and tried it. It had ZERO effect.

I used a cable tv splitter with one side running to the scanner with 3 feet of RG6 between the splitter and the scanner. On the other side of the splitter I used the Stub, 1.44 feet of RG6Q. I started with about 2 feet and trimmed it to see if it had any effect, nothing made a difference. I was testing it with my RTL SDR and SDR#.

I am assuming I did something really wrong here lol. Either a splitter does not work the same as a T fitting. Or it needs to be attached right at the scanner feed port. Or a combination of both?

I was very excited because this seems like a very cheap way to address the problem I am having. I can live with a broad range of the spectrum dropping some because I have pretty strong signals in my area, and various amps I could use to compensate for it. However it simply didn't work lol. It had no effect on any signal.

I have chosen specific signals in my area, saved them as favorites and charted the peak, noise floor and SNR with various antenna combos. Like the bare antenna, with the pre amp, with the fm notch filter engaged and testing the vhf/uhf separator built into my pre amp.

I played with a filter design program and that is out of the question too. The values for some of the components are so low they don't make them. I guess you would need some kind of active filter with an op amp or something and that is WAY above my skill level.

For the stub, could I just solder a leg into the coax feed line? Or would that mes up how the coax line works? And does the Stub need any kind of termination to it, or do you just cut it flush and leave it go?

Thanks for the help so far guys.

Paul

 

[jonwienke]

You need to ust a T, not a splitter. A splitter has a transformer to prevent signal input from one side from bleeding over to the other. So any filter effect from the stub would be blocked using a splitter.

 

[paulmohr]

You need to ust a T, not a splitter. A splitter has a transformer to prevent signal input from one side from bleeding over to the other. So any filter effect from the stub would be blocked using a splitter.

Thanks for the quick response Jon. That is my issue then. I kind of figured it was something like that when I tried it and it had no effect what so ever.

I will have to track down a T fitting and then try it I guess.

 

[Ubbe]

If you use SDR to check, you cut away the RG6 in 1/2 inch pieces while looking at the signal level and when it begins to reduce the level you start to cut smaller bits. If you make the coax too short and use screw on F connectors you just unscrew the coax a turn or two to fine adjust. Start with a coax that are a bit longer than needed.

Most RG6 coaxes have a delay of 0.85

/Ubbe

 

[paulmohr]

Ok, I finally got a BNC T fitting today and played with trimming a stub. It works, if I set the gain on my SDR so 162.450 is at -10db the lowest I could drop it to was around -30db, so a 20db attenuation. It doesn't kill the signal, but it lowers it enough that I can amplify the other frequencies and that channel does't bleed over or anything.

I ended up getting a T fitting at a local communications company that installs and services the public safety system in my county. I asked him about how to go about dealing with that signal and the overloading my scanner issue. He didn't have a clue what I was talking about and said he had no idea what I should do.

 

[gmclam]

Notch filter

I am using this filter between my antenna and active splitter. It reduces the AM & FM broadcast bands. This one won't do anything to 162 MHz.

 

[JamesO]

These guys make a really good/high dB FM broadcast notch filter: http://stores.ebay.com/GPIO-Labs/Notch-Filters-/_i.html?_fsub=28020159018

You might contact them and see if they would be willing to tweak the FM Notch filter and narrow it up for Weather Band for a reasonable price as well:

http://stores.ebay.com/GPIO-Labs/Notch-Filters-/_i.html?_fsub=28020159018

I see the T stub filter seems to be helping, but you may want something with a bit more rejection, just keep in mind it may impact some of the spectrum and services either side of the band you are trying to address.

 

[paulmohr]

These guys make a really good/high dB FM broadcast notch filter: http://stores.ebay.com/GPIO-Labs/Notch-Filters-/_i.html?_fsub=28020159018

You might contact them and see if they would be willing to tweak the FM Notch filter and narrow it up for Weather Band for a reasonable price as well:

http://stores.ebay.com/GPIO-Labs/Notch-Filters-/_i.html?_fsub=28020159018

I see the T stub filter seems to be helping, but you may want something with a bit more rejection, just keep in mind it may impact some of the spectrum and services either side of the band you are trying to address.

Thanks for the links James, I will contact them and see what they say. Prices are for sure good enough, I would pay at least twice that if they would tweak one or custom build one for me.

And ya, I knew about the broader range of the T stub going in. The channels I need around that area are pretty strong so it shouldn't be a big issue. What this basically did was bring down the weather signal to where it is about equal to the other signals around it. It may still be a tad higher, but not enough that is bleeding over. When I started the weather signal was 20db higher than most of the other stuff! And if I turned the gain up enough I could pick it up bleeding through all the way down to around 40 MHz. And the spike would float too, it was really strange. I will take a video of it later if I get a chance.

The T stub was a very cheap and simple solution to my problem though. If I can get a notch filter for 40 or 50 bucks I will go that route later on down the line, but 3 or 4 hundred for a filter just wasn't in my budget lol.

Thanks for the help guys, I am getting to close to nailing this thing finally. Even if I don't, what I have now will work.

 

[JamesO]

Hopefully the GPIO group would consider doing a "custom" filter based on their FM Broadcast notch filter. They effectively have the circuit board and hopefully could just change the component values to get it close, maybe not 85 DB, but even 20-30 dB might be the answer.

I also tell people this is often a trial and error process. Some people may get excited that the filter might affect some adjacent service by 3 dB or so and think the filter might impact a service you are interested in.

Sometimes when the strong signal is attenuated the adjacent signal, even attenuated will actually perform better. You never know until you try it.

I hope they will be able to help out at a reasonable price.

 

[JamesO]

A bit expensive and may not have enough rejection, looks to be around 15-16 dB. Looks like basically a higher quality/shielded stub filter.

VHF Tunable Notch Filter Cavity Ham 146-175 MHz for Extra Rcvr Isolation | eBay

 

[JamesO]

You will have to email for pricing - VHF SYM 162HT Specifications and Plot | PAR Electronics

 

[paulmohr]

If GPIO can't help me looks like I will giving Dale at Par Electronics a ring. looks like he could easily do exactly what I want for under a hundred bucks from what I can tell. A bit more than I want to spend, but I know it would work. I believe he visits these forums too if I am not mistaken.

Thanks for all your help James. My searches were not turning up much, your links definitely have me pointed in the right direction. I did take a look at Par once before, but the prices I was seeing were much higher.

 

[Ubbe]

.... played with trimming a stub. It works, if I set the gain on my SDR so 162.450 is at -10db the lowest I could drop it to was around -30db, so a 20db attenuation.....

If you trim it exactly you will easily get 40-50dB attenuation. But it is very sensitive where that actual notch is.
It is only 1-2mm tolerance and in reality you must have some sort of fine tuning. No crimp F connectors but twist on to be able to adjust. Or unscrew the BNC connector and lift it 1-2 mm when the coax stub is a bit too short. Outside of the precise notch you will have 20dB and at 1MHz offset it is some 10dB.

If you manage to use it between amplifier and scanner it will interfere the least to other frequencies. Only use between antenna and amplifier as the least resort.

/Ubbe

 

[cmdrwill]

If you manage to use it between amplifier and scanner it will interfere the least to other frequencies. Only use between antenna and amplifier as the least resort.
/Ubbe

I would suggest the 'filter' stub be before the preamp. That would keep the interfering signal out of the preamp where it can cause intermod and distortion to your wanted frequencies.