ADS-B Filter

Stealth

New Member
It's unlikely that I'd be in anybody's log! I started running a BBS in 1996 and I've hardly touched a microphone since! The BBS grew to 1x70cm, 2x2m, 1x20m, 1x30m and 1x40m ports. I was on 25 acres so room for antennas was readily available. When Packet started to die I moved to an APRS Igate and satgate. Then a gap in activities until I started to get the satgate running at this new location. That, ADS-B and compiling Linux programs and kernels keep the mind active. When I think EME I think of the late VK3ATN who achieved 2m moonbounce in the 60s despite being told it would never work. Ray had a huge, steerable stacked Rhombic antenna. I've heard a few FM signals trough the LEOs but voice doesn't interest me these days.
The call does ring a bell now. I ran FBB and NOS with some IP and HF forwarding for a while. Then Pactor and a bit of Winlink.
 

Lee Armstrong

Administrator
Staff member
As an aside if anyone stumbles upon this with a Plane Finder radar no filters are needed with that.

We use 3 filters and 2 LNAs in the RF front end on that.
 

Stealth

New Member
As an aside if anyone stumbles upon this with a Plane Finder radar no filters are needed with that.

We use 3 filters and 2 LNAs in the RF front end on that.
Perhaps you ought to send me one over Lee, love to do a comparison with the Mode-S Beast :)
 

freddoair

New Member
The call does ring a bell now. I ran FBB and NOS with some IP and HF forwarding for a while. Then Pactor and a bit of Winlink.
I started with NOS under DOS, then a two port packet switch that booted and ran Linux from a floppy. I started the BBS with FBB/BPQ under DOS and then Win95 before moving all radio gear to Linux in about 1996. I "think" I used the 2.0.x kernel back then, upgrading to 2.0.36 before moving to kernel 2.2.x, back when Linux could boast it wasn't bloated, like Windows. My how things can change! I moved 99% of my desktop needs to Linux near ten years ago.
 

freddoair

New Member
Some of the linux isos are rather porky these days. Just imagine at a 300 baud donload speed!
I started with Debian and apart from one fruitless attempt at Redhat a long time ago, I've stayed with Debian. I once downloaded 14 CD images over 256kb/s ADSL and that took 6-8 hours for each disk, but these days I always download a net install iso and then get only the packages I need. I started with Debian when the package manager was dselect, prior to apt, et al being available. dselect still survives as the underlying layer for apt. Using dselect makes Chinese water torture a pleasant experience!

One unfortunate thing that's going on now is that some kernel development is not being tested against kernel ax25, and ax25 is breaking. I've just set up an old Pentium 4 to run some old (late 80s) packet hardware. To avoid some recent kernel ax25 issues I've gone back to Debian Jessie with a 3.16 kernel and it's working fine. I wouldn't do that for my desktop system - Debian Buster (Testing but just on due for release) - but for an aprs-only machine the old version and kernel is fine. I'll likely leave the ADS-B system on the old RPi with battery back up, but the rest of the radio stuff will go to the P4.

Speaking of the RPi (and dongle), I've been playing with agc/gain settings to assess performance changes. Previously I had agc selected in the dump1090 configs but it's now on 48dB. I initially knocked it down to 30 and promptly saw about half the planes disappear so I stepped up to 45 and now 48dB, and for the last two settings I "think" I'm seeing the same range/plane count as with agc set. It's quite difficult to make really meaningful tests when the playing field keeps changing. What I have noticed is the range of RSSI has changed from about 20dB with agc, to around 30dB with fixed gain. Not knowing how the internals of the dongle work means I don't know how I should interpret those numbers.
 

wiedehopf

New Member
-20dB/-30dB

0dB (FS for full scale) is just the maximum value the ADC can handle.
(The software then averages the signal over the entire messages and it's only a very rough estimation)

In practice there are rarely signals above -1 dBFS detected by dump1090.
The rtl-sdr radio front end tends to get overloaded at that point.

I started a little thread about gain over at FA if you want to have a look (https://discussions.flightaware.com/t/thoughts-on-optimizing-gain/44482)
The second post is more specific in regards to dump1090.
 

Stealth

New Member
Gain settings seem very fickle and certainly not a 'one size fits all'. With TV masts, an SSR radar head and sundry cell towers within a 3km radius, I am in an RF rich environment and overload was an issue with the SDR I was using. An additional SAW filter helped a lot, but I switched to a Mode-S Beast and got more consistent and better results.
 

freddoair

New Member
-20dB/-30dB

0dB (FS for full scale) is just the maximum value the ADC can handle.
(The software then averages the signal over the entire messages and it's only a very rough estimation)

In practice there are rarely signals above -1 dBFS detected by dump1090.
The rtl-sdr radio front end tends to get overloaded at that point.

I started a little thread about gain over at FA if you want to have a look (https://discussions.flightaware.com/t/thoughts-on-optimizing-gain/44482)
The second post is more specific in regards to dump1090.
I'm familiar with 0dBFS in the digital world. Until yesterday I had a regular supply of local aircraft flying over and around near here to provide a wide variety of signal levels, but all those pilots - from an aviation college - graduated yesterday so they'll now head off home. I "think" setting the gain at around 45-48dB is optimum for my radio environment but time will tell.

Thanks for the links. In the fullness of time I'll gain a better understanding of what happens within the dongle!
 

freddoair

New Member
Gain settings seem very fickle and certainly not a 'one size fits all'. With TV masts, an SSR radar head and sundry cell towers within a 3km radius, I am in an RF rich environment and overload was an issue with the SDR I was using. An additional SAW filter helped a lot, but I switched to a Mode-S Beast and got more consistent and better results.
I don't have to deal with your "headaches", just a single cell site 285m away, and I think the Minicircuits filter is mostly dealing with that. However, I have considered a second unit to go after the LNA as a belt and braces approach for the RF environment seen by the dongle.
 
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