VHF air band interference suggestions

[kmacka]

Heres what I have.

Scanntenna fed into a 8 port electroline splitter. 3 BC898T's, 3 2052's, and 2 BC780XLT's.

With those radios I am only monitoring VHF/UHF local civil/artcc and military air.

The 3 BC898T's and 3 2052's are all programmed with specific frequencies and the BC780's are used for searching.

When I scan my frequencies I have no problems with the UHF freqs, no intereference at all. But at times I have problems with the various VHF freqs. Sometimes the scanner will stop on one of the vhf frequencies with a type of interference. The intereference is what sounds like a tone out (ems/fire) of some sorts and then I can hear my local (lack co.) dispatch. I will make a recording of this and post it, but it sounds like it only happens when they tone/page out a fire or ems unit and you can hear the various tones and them talking and then it stops.

I was wondering if this product would remove this type interference: http://www.scannermaster.com/Jim_ABF_0604_Air_Band_Filter_p/36-531261.htm

Now you guys may say this is intermod but I don't know. I'm not sure what the difference is between paging signals and tone out (fire/ems) signals. When people talk about paging signals, are they talking about pagers or paging out ems/fire depts. I hear that these paging signals also cause unwanted interference. What's the difference between the two or are they the same thing?

I want to try and get rid of this problem because I have to lock out these frequencies and then I forget about them. I'm afraid that I will miss a legitimate communication on these frequencies. I hope someone can give me some info.

 

[W6KRU]

I've never had to use one of these but it looks good on paper. You only lose 2db in the pass band and it knocks down the junk by 20db. Looks viable to me. Don't order it yet though, someone on here may have tried one.

 

[jpryor]

I have a handful of the Scanner Master filters and those types of filters have indeed helped with eliminating various sources of interference.

You essentially have two different methods to attack that kind of an interference problem.

One method is to use a band pass style filter (like the one you have linked to) to only pass the signals from the band you want to hear, and attenuate all other signals. I have this one specifically (http://www.scannermaster.com/BPF_AIR_Air_Band_Band_Pass_Filter_p/24-531022.htm) and it has done a great job of cleaning up my VHF air band reception.

The second method is to use a notch style filter to eliminate and attenuate the specific frequency or frequencies that are causing your problems (inter mod, mixing, overloading, etc). Their are various notches that eliminate FM and VHF Paging that can help for those specific frequencies.

If you know that the fire paging you are hearing is the only problem, and you know that specific frequency, you can notch that out to minimize the impact. You can also use a combination of notches and band pass style filters to attack these kinds of problems from both ends. Essentially minimize the signals you don't want to hear and pass everything you do want to hear.

 

[kmacka]

I have a handful of the Scanner Master filters and those types of filters have indeed helped with eliminating various sources of interference.

You essentially have two different methods to attack that kind of an interference problem.

One method is to use a band pass style filter (like the one you have linked to) to only pass the signals from the band you want to hear, and attenuate all other signals. I have this one specifically (http://www.scannermaster.com/BPF_AIR_Air_Band_Band_Pass_Filter_p/24-531022.htm) and it has done a great job of cleaning up my VHF air band reception.

The second method is to use a notch style filter to eliminate and attenuate the specific frequency or frequencies that are causing your problems (inter mod, mixing, overloading, etc). Their are various notches that eliminate FM and VHF Paging that can help for those specific frequencies.

If you know that the fire paging you are hearing is the only problem, and you know that specific frequency, you can notch that out to minimize the impact. You can also use a combination of notches and band pass style filters to attack these kinds of problems from both ends. Essentially minimize the signals you don't want to hear and pass everything you do want to hear.

Would the type of filter you posted a link to, would that do harm to my uhf listening? Degrage the signal at all?

 

[W6KRU]

Would the type of filter you posted a link to, would that do harm to my uhf listening? Degrage the signal at all?

Yes the filter he posted and the one you posted will hurt UHF significantly. If you want to listen to UHF with one antenna you will have to go to notch filters to kill the harmful frequencies. If you could segregate the scanners somehow and filter the airband scanners but not the UHF scanners, that could work as well. Then you could buy another filter for the UHF bank. :lol:

 

[jpryor]

Both the http://www.scannermaster.com/Jim_ABF_0604_Air_Band_Filter_p/36-531261.htm and http://www.scannermaster.com/BPF_AIR_Air_Band_Band_Pass_Filter_p/24-531022.htm VHF air band pass filters will only pass signals between the 118 MHz and ~138 MHz range. Both of these will harm any reception outside of the VHF air band, including the UHF military band. You'd only really want to use a VHF air band pass filter if you can dedicate one radio to only the VHF air band frequencies, the leave the filter off the radios for the UHF bands. If you don't want to go the band pass filter route, you would probably be better to find a notch filter to knock down just the one problem frequency.

 

[W6KRU]

Both the http://www.scannermaster.com/Jim_ABF_0604_Air_Band_Filter_p/36-531261.htm and http://www.scannermaster.com/BPF_AIR_Air_Band_Band_Pass_Filter_p/24-531022.htm VHF air band pass filters will only pass signals between the 118 MHz and ~138 MHz range. Both of these will harm any reception outside of the VHF air band, including the UHF military band. You'd only really want to use a VHF air band pass filter if you can dedicate one radio to only the VHF air band frequencies, the leave the filter off the radios for the UHF bands. If you don't want to go the band pass filter route, you would probably be better to find a notch filter to knock down just the one problem frequency.

Beatcha! :roll: Yeah, we're thinking the same way. I would think about a splitter coming off of the main distribution and filtering just that but then you have the signal loss due to the split. An active splitter in that role could work if there was too much loss. This could be done several ways. But how much is too much?

 

[kmacka]

Ok so in summation. I basically need to get another antenna in service. Then I can separate my scanners. I can then place one of those filters on one set of scanners to block the unwanted paging signals and the other antenna could be used for UHF, which hasen't been a problem thus far.

 

[jpryor]

There is no such thing as too much, ha, usually.

I have a multi coupler. Between the antenna and the coupler, I have the FM Notch filter (so this knocks down the FM broadcast interference to all radios). Then I can connect four radios. So between the coupler and the radios, one radio has the VHF air band filter for all air band activity, one radio has a VHF band filter for VHF AM aviation (137-150 MHz), and then one radio for the UHF military band. So as long as you can strategically separate your radios and the frequencies you want to monitor, that can help maximize the use of different filters.

 

[W6KRU]

Ok so in summation. I basically need to get another antenna in service. Then I can separate my scanners. I can then place one of those filters on one set of scanners to block the unwanted paging signals and the other antenna could be used for UHF, which hasen't been a problem thus far.

It depends on your situation in my opinion. You can also use one antenna and then cascade amps and filters to fit the need. This would be so much easier with a whiteboard but here goes. The antenna goes to a FM broadcast filter that attaches to amp1 with 2 outputs. One of the outputs has a airband pass filter on it and that connects to amp2 that feeds a splitter that feeds the VHF scanners. The other output of amp1 goes to amp3 that feeds a splitter that feeds the UHF bank of scanners.

Of course you might not need amp2 or amp3. You need to experiment to find out what you really need. I'm just giving you ideas. For all I know your second antenna might be the best option if it's easy to do.

 

[zz0468]

Before you go hog wild adding filters and amps, you should take a good hard look at what you're doing now. Is that 8 port Electroline splitter and active splitter? What you're describing COULD be intermod, and if you're running an active splitter, there's a good possibility that the intermod is being created right in your own equipment. There are relatively easy tests to determine that, if one has the equipment, but I'm guessing you don't.

So... here's something you CAN do. Take a single scanner, preferably one with a built in attenuator, and connect to the antenna directly - no splitter. Listen to all your troubled frequencies, and see if perhaps the trouble goes away. If it doesn't, try the attenuator and see what happens. If the interference goes away with this configuration, the active splitter could well be the problem, and the wrong kind of filtering in the wrong place may well not fix the problem.

The thing with filters, and amps, and splitters is, if you're not applying the proper cure at the proper place in the system, you're going to spend a lot of money for no results. Adding amps to make up for filter loss is insane. So is cascading amplifiers. If a filter is required, just deal with the minimal insertion loss - I promise, you'll never notice it. Get rid of the 8 port active splitters, use several antennas and several 3 way passive splitters (less loss) , and only use a (low noise!) preamp if you have a specific need to hear a system that's otherwise marginal.

It's amazing how much intermod goes away when you get rid of the excessive gain.

 

[W6KRU]

Before you go hog wild adding filters and amps, you should take a good hard look at what you're doing now. Is that 8 port Electroline splitter and active splitter? What you're describing COULD be intermod, and if you're running an active splitter, there's a good possibility that the intermod is being created right in your own equipment. There are relatively easy tests to determine that, if one has the equipment, but I'm guessing you don't.

So... here's something you CAN do. Take a single scanner, preferably one with a built in attenuator, and connect to the antenna directly - no splitter. Listen to all your troubled frequencies, and see if perhaps the trouble goes away.

The thing with filters, and amps, and splitters is, if you're not applying the proper cure at the proper place in the system, you're going to spend a lot of money for no results. Adding amps to make up for filter loss is insane. So is cascading amplifiers. If a filter is required, just deal with the minimal insertion loss - I promise, you'll never notice it. Get rid of the 8 port active splitters, use several antennas and several 3 way passive splitters (less loss) , and only use a (low noise!) preamp if you have a specific need to hear a system that's otherwise marginal.

It's amazing how much intermod goes away when you get rid of the excessive gain.

Thanks for adding some sanity to my post. I wanted to say some of that but it was already getting long winded. I've went down the wrong fork in the road before. I actually had an attenuator between an amp and the scanner once. Duh.

 

[zz0468]

I actually had an attenuator between an amp and the scanner once. Duh.

Don't laugh. That's frequently the best way to get optimal noise figure and gain out of cascaded amps. The first amp sets the noise figure, the attenuator prevents the first amp from over driving the second, and the second amp provides the required power gain for the application.