There is a good community out there on github and discord that have flight tracking as a hobby. In addition to ADS-B there is also voice radio transmissions from aircraft (airband) and text messages (ACARS) etc. If you get hooked there is lots of different things to tinker with.
Better also add a SAW bandpass and a properly designed amplification (LNA) stage to this.
The RTL-SDRs are great - but heavily limited in their dynamics (8 Bit ADC). They get deaf quickly if you are located next to strong transmitters like FM/DAB/DVB-T/etc.
Aliexpress has cheap antennas that include all of this.
For optimal ADS-B reception, pair that RTL-SDR with a 1090MHz bandpass filter and place your antenna outdoors with clear sky view, as these signals are line-of-sight and easily attenuated by buildings.
It is perhaps telling the author lives in the Netherlands! My findings were also that it's really super important to have them up high... at which point you need decent cable, termination and general radio knowledge otherwise you are exposing yourself to other issues. Basically, if there's complex topography, give up on this ever giving excellent results unless you have a radio tower or tree.
The most common design for wire antennas at 1090 MHz is the quarter-wavelength monopole (such as a ground-plane or whip). For 1090 MHz, the quarter-wavelength in free space is approximately 68.8mm, but practical build recommendations round this to 65-66mm to account for the velocity factor of the conductor, especially if the antenna element is made from solid wire rather than thin tubing. For a dipole (the total length being half the wavelength), the length is approximately 137mm (ie. 2x68.5mm). Commercial antennas sold for 1090 MHz may appear longer because they are collinear or have internal elements designed for added gain or bandwidth.
> My findings were also that it's really super important to have them up high... at which point you need decent cable, termination and general radio knowledge otherwise you are exposing yourself to other issues.
Well, 1090 Mhz is a crap frequency to run through coax cable for long distances. The attenuation is pretty high at that frequency already. So I wouldn't just mount the antenna on the roof and then coax the RF down a few floors. You will reduce your range.
I would simply mount the SDR itself right up on top of the mast, along with a raspberry pi to decode everything. Then add a PoE HAT and all you need to bring down is Ethernet for which long distances are no issue. The PoE uses 48V so it also bypasses the problem of voltage drop on 5V with high current. Put it all in a waterproof box of course. Ideally metal for heat dissipation because waterproof means no airflow from outside to inside.
Make sure you use real copper Ethernet cable and not that CCA crap (copper clad aluminium) because that has much higher resistance and does have big problems with PoE. All the cheap cables are CCA these days so beware.
I've had good experience with the AirNav Radar FlightStick - it comes with a built-in 1090 MHz filter. It's a smaller package with the same SDR hardware.
