#1381 Microstrip RF Bandpass Filter

Episode 1381
I bought this just for fun
marked 'ADS-B'

[markusberg]

In the United States, ADS-B-equipped aircraft and vehicles exchange information on one of two frequencies: 978 MHz or 1090 MHz.

[MatthiasWelwarsky]

These are made to improve the dynamic range of SDR ADS-B receivers, eg for "Flightradar24" ground stations. Direct conversion SDRs typically suffer from interference from strong signals nearby, because the cheap ones usually downmix to zero IF and then directly digitize the signal, without any intermediate stages for filtering.

[joeteejoetee]

Transponders transmit and receive 1030 Mhz, and 1090 (Respectively), and all avionics use 60 MHz IF for that reason. Rec IF is SAW filtered @60, feeding 4 I/Q receivers, and then 10 bit digitized ->DSP from 4 antennas for direction finding, and the transmission is electronically phase-shifted for 360 degree coverage, using over 1KW pulses with added BPSK data.
You are absolutely correct about the 45 degree angle reflecting the sheet of electrons cleanly.
Bonus: There is no 90 degree corner "point" that would have a very high field intensity.
(Retired from Collins Aerospace)

[byronwatkins]

Those "capacitors" are directional couplers. There are quarter-wave stubs at input and output. Each of the 'J loops' is a resonator; I believe the inner two are slightly lower frequency (I think their 'J' is slightly deeper). The trace width, board dielectric constant, and board thickness set the impedance. Wider traces also increases the resonator bandwidth. The couplers have two different lengths making them couple two different frequency bands. The pass band is very flat and the insertion loss is quite low at 50 Ohm. Very well designed and manufactured.

[kobusswart]

it is designed for 1090Mhz to receive aircraft transponder telemetry messages.

[bobkozlarekwasqq]

ADS-B is used for aircraft location data. Easy to decode using an SDR receiver.

[johnwest]

I used to avoid microwave circuits. I just called them 'plumbing'. But once I got a NanoVNA and could finally see what was going on, the devices became just some interesting RF circuits that were slightly different mechanically, and were quite fun to work with. The newer Nano's and the TinySA Ultra have really opened up microwaves to the hobbyist.

[portblock]

would love to see you rig one up with aluminum tape and just see various changes on the spectrum analyzer as you cut pieces away, like square corners, then cut into 45 live as we watch it

[huseyinurigulmez]

I am doing my Phd on microwave filters and i can easily say that it is a conventional hairpin filter. It consists of coupled half wavelength transmission line resonators. Unlike the half wavelength resonators in combline and interdigital filters, those resonators are folded to make it more compact and do not require precisely manufactured grounding via. Each folded section directly couples energy to adjacent one and no cross coupling is observed. Hence, it has moderate stop band performance compared to cross coupled filters.

[Randrew]

Nice. I got a couple of PCB BPFs from HobbyPCB. One for 2m and the other for 70cm. I've tested them with my SSA3015X Plus - The 2m one is very good, but the 70cm BPF's high end roll-off is quite a bit slower. Those two BPFs are even smaller than your 1090 MHz one here and incorporate some tiny SMD capacitors. I've had some copper "stained glass foiling tape" for a while now with an eye to experimenting with this kind of thing... I need to get on that :)

The passband of your ADS-B filter looks like it's designed to cover both the 976 MHz ("UAT" for under 18, 000 ft aircraft) as well as 1090 MHz.

[vincei]

ADSB is some aircraft frequency I believe. Just checked, it's for aircraft tracking.

[rogergreenwood]

Interesting to see the matching stubs, or "magic wands" as I think of them.... :-)

[georgesampson]

It is interesting that although light and RF are part of a continuous spectrum we still differentiate them based on our biological sensor's ability to see one and not the other.

I recently read an article, in QEX I think it was, where Amateur Radio people were using modified RF techniques to push closer and closer to what we call light frequencies. If I remember right they are getting pretty close and maybe into far infrared territory. So I guess use of the full electromagnetic spectrum just boils down to appropriate combinations of magic smoke and mirrors as you pointed out in this filter.

[Peter-W]

These types are called "Hairpin filter"