I'm trying to receive weather satellite images (NOAA and Meteor), transmitted at 137-138MHz.
Problem is, periodically there is a powerful 144.8MHz ARPS transmitter nearby, which completely overwhelms the weak satellite signal.
To get good reception from the NOAAs i need about 60db of gain, while the ARPS signal is already very strong without any gain at all.
So, i need some sort of a notch/bandstop filter to exclude 144.8MHz without interfering much with 137 MHz. That is, with less than 12 MHz of bandwidth.
The question is - what sort of filter can do that, and what to look for?
I looked at passive filters, but most calculators give me impossibly small part values, i.e. http://www.wa4dsy.net/cgi-bin/lc_filter3?FilterResponse=Bandstop&poles=6&CF=144.8&cfunits=MHZ&cutoff=13&funits=MHZ&Z=50
60dB of gain is a massive figure.
Where is this gain?
At the antenna?
At the input to the receiver?
What kind of receiver?
Is it tuned in any way?
If you are using a scanner, they have virtually no input selectivity.
Because of this, they usually have a high first IF to stop "image" interference.
High,unfortunately goes with wide bandwidth,so suppression of a strong signal 7MHz is pretty much minimal.
The second IF is where all the selectivity is,but by the time you get there, you may already have intermodulation products within the IF passband.
Simple receivers,like "Direct Conversion" types are even worse,as they rely heavily on input selectivity, along with phase cancellation techniques---these can be easily "trumped" by a very strong signal.
I don't know a lot about the simple "dongle" type SDRs, but it seems likely that they have similar problems.
I would suggest a tuned converter at the antenna, with an IF at HF going down the coax to a receiver for an appropriate band.