After a while, however, you start the get the hang of things.
Last year I had been really feeling like there was something missing in my life, and it was likely RF. After watching lots of friends on Facebook build impressive ADS-B monitoring systems, I decided it was simple enough, cheap enough, to get my toes wet in the hobby waters.
So, a Raspberry Pi, Flightaware Pro stick and filter were purchased, and I was off.
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| assembled on the test bench |
Next, an external antenna was procured, but where to mount it? For the time being, it would be in the second floor of my barn, for various reasons why I won't go into here. Having the resonant antenna with a small amount of gain helped me see quite a few more aircraft. This was getting exciting!
Before long, I told my BFF, Jay Tarantino, AB1II, about this setup and I had him hooked after about 2 second of talking to him on Facebook Chat.
Jay got the same system and put his antenna in his attic. Jay, unlike me, is in a FANTASTIC location to hear aircraft close into KBOS. In fact, he's close enough to hear the ground aircraft. We were both hooked!
Within a few months, I realized that my RF hobby was starting to come back. Jay and I were chatting regularly about radio topics. We were both finding oddball aircraft and flight paths, we were both having a lot of fun.
But then it started to happen. How can I make my system better? How come Jay can see more stuff than ME? What can I be doing to improve my system?.....down the rabbit hole I went.
The first thing was to get the antenna out of the attic and into the air. I slowly started to think about how to mount it and started digging through an old pile of antenna masts, mounts and accessories to see what I could put together. I had forgotten about an H-frame I had made years ago that was originally built to mount a pair of M2 eggbeater antennas. I decided that If I was going to do the work to mount one antenna, I may as well mount a few antennas! The first batch of antennas included:
- a Daimond X50 (144/444Mhz)
- a DPD dual air-band antenna (VHF/Mil-Air)
- an old Marine GPS antenna of unknown origin
- a Flightaware 5dbi ADS-B vertical
- an M2 435MHz eggbeater
Next, find a piece of mast to mount it all on and get it vertical. I had some 2" Aluminum mast from another project, so I was able to cut a piece of and get the whole thing mounted over the course of a weekend.
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| The initial RX antenna array |
That did the trick! I was able to see quite a bit more ADS-B traffic and as a bonus was able to start listening to ATC using the VHF air-band antenna with a RTL-SDR receiver into a second Raspberry Pi (using rtl_tcp and GQRX).
There was a problem, however, quite frequently Jay would be seeing traffic down close to me that I wasn't seeing. That was driving me mad, so next I started reading more and started thinking about looking for greater efficiencies. The first step was about understanding the SDR receiver and it's gain settings. By default, the Flightaware Pro stick and PiAware are configured to use hardware automatic gain control (AGC). This is probably fine for most, but if you have any noise or out of band signals nearby, it will cause your receiver to miss a lot. I started working with the gain settings, but wasn't finding much in the way of consistent results.
My next detour was to look at filtering and pre-amplification. The Pro stick claims to have a LNA built into the device, but how decent of an LNA? Low noise, to me, should be something in the vicinity of 0.5db nf. I wasn't sure what the spec was on the Pro stick, so I started looking online. There were many to choose from, I ended up trying one I found on Ebay from a Canadian company called GPIO labs. a GPIO labs bandpass filter was also procured. The issue I noticed with these devices was that they came as pre-assembled circuit boards not in enclosures. When I received the boards in the mail I noticed they were very fragile and as I would soon discover, not particularly well soldered (at least the SMA connectors).
Meanwhile, I had also come across a mini-circuits bandpass filter that was appropriate for the 1090MHz signals I was monitoring, and was physically much studier than the GPIO labs filter. I also happened to have a small aluminum box with holes just the right size for SMA connectors with enough room for the filter and pre-amp. A few bits of UT-141 coax with SMA connectors from the junk box and voila!
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| GPIO labs ADS-B 5v pre-amp, Mini-Circuits VBFZ-1065-S+ bandpass filter |
At the same time, I decided to try swapping out a different receiver. I went with a Nooelec NESDR SMArt. I was concerned that having an external LNA into the oboard LNA of the Pro stick might just end up being more gain that would be useful, and besides, we're talking $25 per receiver! The receiver ends up being one of the cheaper components of the system. That, in fact, is a new paradigm that I can get behind.
The new receiver, pr e-amp and band-pass filter all show improvements, each time getting me a little more signal to decode, but we're not quite done yet. It was time to replace the cheap Flightaware 5dbi antenna with something a bit more substantial and a little more gain. I ended up with the DPD Productions 9dbi vertical for ADS-B. That.....is what I needed.
Now, finally, I'm able to track aircraft much further out on the edges and I'm suddenly seeing a ton more close-in aircraft as well. I'm now finding consistent results when adjusting the receiver gain under dump1090, with my gain currently set at 27.
This was a really fun project that took me from "mildly interested in getting back into an rf project" to "I now can't stop thinking about all things RF".
There are MANY more projects now in various states of planning. Next up, GNSS monitoring on a global scale and a side benefit, a stratum 1 time source.
73
de N1JFU
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| The RX array with the new DPD 9dbi ADS-B antenna far right |
