@k6rtm
Bob, your comprehensive comments are result of your sharp eye and vast experience. Thanks
The tuning / trimming by expanding/collapsing the coil turns is a time proven technique.
In the days of analog radios, the FM band’s Antenna and Local Oscillator were trimmed by spreading the turns of respective coils. See photo below.
However, in FM radio, the frequency is of the order of 100 MHz, while in the ADS-B filter frequency is in the range of 1000 MHz, i.e. 10 times. Trimming by expanding/collapsing turns requires a very skilled and steady hand.
Not news, but it may come in handy - the Sysmocom company (Berlin, Germany) has in its lineup bandpass filters, including a cavity filter for the operating frequency 1090 MHz: 7-pole cavity filter for 1090 MHz Mode-S / ADS-B .
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However, I note that the band-pass filter at the input is justified when there is (powerful) interference outside the band - here is 1080-1100 MHz.
In any case, it will not improve the existing S / N ratio - but it is capable of “removing” (weakening) out-of-band interference causing overloading of the receiver input (or LNA). Especially one that cuts, for example, the signals of cellular stations (921-960 MHz GSM 900) by 80 dB.
I doubt this is needed anywhere when using the rtl-sdr LNA, it seems very resistent against overloading.
But i suppose if you are less than 100 ft from a mobile tower or something at exactly the same altitude, even the rlt-sdr LNA may need this to quiet the 900 MHz band.
My no name LNA is very resident to overloading. I suppose the others are at lest the same.
I actually ordered a triple filter from them, just for curiosity, although I don’t think I need more filtering.
Most if the places don’t need that level of filtering to demand a cavity filter.
good cavity (or 3 pole) filters with very low attenuation (like the one from jetvision) in the passband are not only about possible overloading of the following amplifier probes but also about very weak signals from a great distance. here a little more range is achieved and especially more regular reception of the signals.
The use of a highly selective filter, such as a mechanical cavity filter, may have benefits more subtle than the statement “it blocks strong signals” implies.
Intermodulation distortion of multiple out of band signals has the effect of a noise-floor like increase with resulting loss of pre-detection SNR.
My “proof” of this claim is highly subjective: My urban FA site is very noisy from LF into GHz. The use of a cavity filter compared to the FA filter plus FA Pro Stick Plus (which has internal bandpass filter) shows clear advantage for the cavity filter. The with-cavity results are several % higher aircraft count averaged over a month.
Was comparing to the rtl-sdr LNA, which is just a superior design compared to that combination of filtering and one LNA.
Yeah maybe even a good two stage LNA with input filter can benefit from a cavity filter.
My terrain is quite limiting, but the 20 Euro filter from Sysmocom is tempting nonetheless!
I’ll give it a try just for the fun of it.
See if i can get some more reflections of other aircraft or however some of those signals from behind a mountain get to me (multiple positions, not bogus, not MLAT).
Edit: Darn it, they don’t offer the 20 Euro filter anymore, probably because i linked it in this forum and they got orders for it from somewhere on the globe.
Yeah 45 Euros is too much for this experiment.
This is a part of the spectrum below 1090MHz without and with a Sysmocom cavity filter for the place where I live with a cell tower about 150m from my antenna (Jetvision A3).
