I have looked at aliexpress and nobody is selling 1090MHz cavity filters.
very nice build
Iām interested in your filter build. Photos, dimensions, etc. I built 2 a few years ago and enjoyed the process. Thereās a link further up this thread. Thinking of making another one as I still have some copper sheet left.
Interesting information about internal interference
Noise, shielding, cables, and why is that FM signal there?!
Additional noise comes from the switching power supply in the RTL2832U that runs at 1.024MHz. This drops the supplied 3.3v down to the 1.2v needed for internal use.
Laidukasās "Mods and performance of R820T2 based RTL SDR receiver covers replacing all the power rails with external linear regulators, increasing the amount of bypass capacitance on power lines, adding extra chip filtering for the USB 5v line, cutting off the IR receiver part of the PCB, wiring in a TCXO 28.8MHz oscillator, creating a shield with kapton tape and copper foil soldered extensively to the PCB ground, and a new heavy metal case and connectors.
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It appears that Germany like selling these itemsā¦ Jetvision / radaracape, gunter pedals a 7 pole filter. Now in watching this thread, I take it that there are frequencies above and below 1090 that will not cause trouble to ADS B. I
See the design software at
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Thatās the one I used!
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Great! I wrote the original program!
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Ah, yes I see your name as the author of āComputer-Aided Interdigital Bandpass Filter Designā back in 1985.
I have a couple of questions about the construction of these filters. Can I PM you as they may not be relevant to this thread?
As iām sure itāll be interesting reading, open a new thread about Cavity filters! 
Alternatively you can revive this old thread @triggers
Designing cavity filter, may I know more about yours?
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I have built the filters with various materials from PCB to extrusions to machined aluminum. Happy to discuss.
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Well finally some testing. A bit earlier than expected as I found here a RTL-SDR triple 1090 LNA, some of which are otw from China to here. Well image attached sure did fix up my 4G mobile phone tower, 170 metres from here.
Bottom half of the image attached to a Uptronic ceramic LNA, top half, attached to the RTLā¦ Now more testing gear otw to, some 7 captivity filters to experiment with the RTL set up and then the Uptronic running with a cavity filter, but that later when it all arrives. Thoughts?? Oh the test bed, 2 N2ās up in the enclosure, so swapping over between what ever and what ever will be relative simple for testing purposes with one and another set up at the same time. 
Now I am going to leave these 2 set up at the moment as is, one on the RTL and the other on the Uptronic to see what the difference will be stat wise in the next couple of days. Both are set up identically airspy wise, running on 20 mhz on the -e setting of 8 Both antennas identical and at the same height.
For completeness: somewhere around 4-4.5MHz (signalling rate is 2MHz, transmitter spectrum requirements are -3dB attenuation at +/- 1.3MHz around the carrier; the carrier itself can be +/- 1MHz)
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Ok some more filtering options soon to be here for testing locally. Now the options being used, Odroid N2ās. Uptronics ceramic LNA, RTL-SDR triple filter LNA, sysmocom 1090 MHz cavity filter for Mode-S / ADS-B 1090 MHz cavity filter for Mode-S / ADS-B | cf1090-kt30. Now what placements for the cavity filter, in relation to the two LNAās, where does it need to be placed. Now the maximum length of coax we art talking about it 5 metres from the enclosure with N2ās , 240v power etc.
The plan to try each LNA, with the cavity filter, to see which works best to help deal with the 4G mobile phone tower 170 metres from the mast. As seen above, the triple fliter sure did eliminate a lot of the 900 mhz interference and it would appear that the N2 is definitely working better per detectionās and locations
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In theory at least, the order should be:
Antenna - LNA - Filter - feeder - receiver.
Then come the āifāsā and ābutāsā
eg. if your feeder is good quality (eg. including the plugs and sockets) and short, it doesnāt matter much where it goes.
or if you are very close to a noise source (say 170m from a 4G cell), you may find you need the filter before the amp.
but if your range is geographically limited (eg. surrounded by mountains), you may be better off with a filter and no amp.
You may not need an amp, but it shouldnāt reduce the performance.
What is a feeder in this context, do you mean the coax?
5m of LMR400 is not an issue at all, if you use lesser coax you want the amplifier with the antenna.
āthe feederā is the coax plus the connectors (plus the assembly)
Any benefit in using first grade coax is lost if your solder joint is cold or a strand of the shield causes a short.
Good quality coax does not guarantee a good feeder.
True, but bad quality coax pretty much guarantees a poor quality feeder.
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Thanks for your interesting replies. I shall post a coverage graph when I get back home tomorrow morning, showing the detection areas from home, including a brief topographical description of my little part of the world, which will give you an idea relating to blind spots. Still waiting for the cavity filters to arrive but shouldnāt be to far off.