ADS-B Wisdom

Following advice is based on practical experience testing equipment, hands-on knowledge gained from being involved in ADS-B for years, building and testing numerous antennas, and enjoying 1090 MHz aircraft signal reception in multiple countries on three continents.
Also been running side-by side testing since February, for more information, please read the relevant testing notes.
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Equipment doesn't matter for casual use


Check out the difference between a $8 generic dongle with an off-measurements homemade Coketenna versus $90 worth of gear on a $45 antenna:


Surprised? I'm not: even a generic RTL-SDR dongle is a capable powerhouse. In the example above, I had twice as much position reports from the semi-pro setup, a wonderful feeling in itself demonstrating quality of components. At ten times the price: after a certain performance level and cash outlay, aiming for a small improvement will exponentially increase costs.

The range myth


Antennas don't have a range. RTL-SDR dongles do not have a range. Equipment doesn't come out of the factory with a label of "300 mile maximum" or "20,000 position reports", because they're just pieces of metal, silicone chips and are fundamentally inanimate objects: performance matters on what you do with said equipment.
Using the best gear helped a lot in the previous example to get as much coverage as possible, however, I will not be able to reliably receive a signal 200 nm away, no matter what I put out. Physics rules the universe.
If you can not improve your location, think about upgrading equipment.

LNAs and filters


Let's illustrate amping and filtering in a visually pleasing way:


Back to radio: coverage difference between a stock v.3, a Pro Stick (amped) and a Pro Stick Plus (amped and filtered) on three identical antennas, 10" / 25cm difference in horizontal positioning:


Amplification, in any shape or form, makes a world of difference, read the FlightAware Pro Stick review for more plots if you're looking for a built-in LNA in a dongle, or get an LNA4ALL, Janilab, novakx5 preamp, or your choice, either will bring a smile to your face.
Filtering is also beneficial, up the ante and compare two semi-pro LNAs, Uputronics Wideband vs 1090 MHz Filtered:



A filter almost always adds more performance by blocking unwanted signals reaching the receiver; a single-purpose version is called a 1090 MHz bandpass filter, which only lets ADS-B signals through.
For dedicated ADS-B use, a combined solution (Pro Stick Plus or Uputronics 1090 Mhz filtered preamp) is the least hassle with the best performance due to less insertion losses; for general radio use, a separate wideband preamp is a better purchase - you can add a filter later if you want to, but removing a built-in bandpass filter is naught impossible this side of being a soldering Yoda.

Antenna location is everything


If you have elbow room:



Antennas should be as high as possible, with as much unobstructed line of sight as feasible. Please feel free to fork out your inheritance on expensive equipment, then place the antenna on the ground floor balcony - your neighbor with a $21 Pro Stick Plus and a homemade antenna taped to the chimney will enjoy better performance.
Once more, antenna elevation and unobstructed line of sight matters the most, much, much more than equipment used. You don't see antenna towers in valleys, do you?

Antenna positioning matters


Even a small difference in antenna position will have an affect, look at four mounts on my windowsill, 10" / 25 cm horizontal difference, black bags for weatherproofing:



Despite four factory-fresh SMArt bundles used, huge differences show in totals:


Closer scrutiny:


All four mounts had the same maximum range, yet coverage vastly differs. Experiment by shifting antennas, who knows, you might improve system performance.

Account for day-to-day changes


Common and shriek-inducing posts on Facebook and on various online pits of infinite wisdom detail a setup, then close with "I had 50,000 reports and 250 mile range after making a change: reading aloud the relevant section from ARRL Antenna Handbook to my cables whilst gently caressing the preamp with a feather". Often, I break down crying.
Weather and traffic density plays a huge part, let's look at a random user's FlightAware control panel:


Performance graph is never a straight line; ADS-B signals are emitted by aircraft, less aircraft = less signal. Reading sentence above, I feel like stating the obvious, however, there are enthusiasts out there who fail to grasp the importance of variance.
Same setup, different day at my location:



Again, vastly different results. Had I altered the setup in any way, I could have reached a flawed conclusion - wow, nearly 10% improvement! No, it was simply raining.
The only way to reliably assess the effect of any change is by changing only one variable then doing a comparison: either with two identical systems and an antenna splitter on the same day, or by looking at local feeders' data.

Monitor peers


FlightAware's control panel has a useful feature: "View ADS-B coverage map" on top shows other local feeders in your area, detailed statistics at the bottom are also available.



Highlight local feeder's data and copy-paste into a spreadsheet software of your choice, then establish performance ratios - these will remain more or less the same, as they also experience the same weather and flight density.
If you make any alteration to your setup, such as replacing the receiver, repositioning the antenna, or even swapping or adding a connector, you'll see in a few days whether the change resulted in any improvement.
Data doesn't lie. Without absolute cross-referencing from as many sources as possible, the mental conculsion will start with "I think, maybe, possibly", instead of "I know for sure".

3 comments:

  1. What about a directional antenna? (https://en.wikipedia.org/wiki/Cantenna or https://en.wikipedia.org/wiki/Yagi%E2%80%93Uda_antenna ) I sometimes have the feeling that I get so many messages that they must somehow collide. If I listen to one direction, I'll probably get weaker messages undisturbed. No?

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  2. No. Antenna gain will be much higher, therefore you'll run into front-end saturation problems real quick. Waveguides / Yagi-Uda or helicals require a high level of precision too.
    Try playing with gain first with an omnidirectional antenna and find the best setting, then try adding amplification - Satcoms e.g. Outernet uses double amping and filters and a directional antena to get signals from satellites, but the covered area is just a tiny portion of the sky.

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  3. "Even a small difference in antenna position will have an affect, look at four mounts on my windowsill, 10" / 25 cm horizontal difference" - Can you please expand on this. Are these all the same distance away from the building? I found the sentence and photograph a bit difficult to make sense of.

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