Stack Filters?

[btlacer]

Dumb question but can you stack band pass filters online before the scanners? Im looking at a airband and a 700 mhz band pass to feed 4 scanners. Is it ok to do or does each scanner need a separate filter?

 

[jtwalker]

If you stack them serially, you will get the AND’ed result of all the filters. Without knowing what the specific filters do, I can’t determine if this would make sense or not.

 

[prcguy]

Some filters yes and some no. You can for example stack a band pass filter for a specific frequency range with a notch filter of another frequency range like a 2m 144-148MHz band pass and an 88-108MHz notch. Or a low pass and a high pass like a DC to 200MHz low pass and a 400MHz high pass which can make a 200MHz to 400MHz notch filter or a 2m/70cm diplexer depending on how they are connected.

You cannot put a 1.8 to 30 MHz band pass in series with a 2m 144-148MH z band pass because the lower frequency band pass will not allow any 2m frequency to pass through, etc. You also have to be aware of the stacked insertion loss of the desired frequencies like two filters that otherwise play nice together might have 1dB insertion loss each of the desired frequencies resulting in 2dB loss which is getting a bit high.

 

[G7RUX]

Dumb question but can you stack band pass filters online before the scanners? Im looking at a airband and a 700 mhz band pass to feed 4 scanners. Is it ok to do or does each scanner need a separate filter?

Sometimes, yes.
This is not a daft question at all and is something that has tripped up very experienced people.

If you stack two bandpass filters then this won't work that well as one will filter out the band for the other to pass. The times it will work is if the second pass band is within the first; say a 100-500 MHz band pass followed by a 150-160 MHz band pass will work fine but a 150-160 MHz followed by 400-450 MHz will not as the first will already have stopped what the second filter passes.

Stacking high pass and low pass filters works but you need to be careful over the order you put them in.
Notch/band stop filters can generally go before other pass filters.

Basically, stop and think about what you are wanting to pass, what you want to stop and what each filter does. If you get stuff wrong then nothing disastrous will happen here, the receiver will, at worst, just not receive anything.

What you can do is to put the individual filters on separate receivers which are fed from a multicoupler/splitter, this works fine.