Thursday, 7 April 2016

ADS-B Bias-T Filter and External amplification

The FlightAware (FA) dongle is great for beginners to get into ADS-B reception.

However, if you are not a beginner... or want the best performance... for the same money...

ADS-B filter with bias-T (link), plus
LNA4ALL with bias-T enabled (link).

Total cost: $50 dollars shipped to the USA.
FA dongle plus filter costs the same.
Update 1: apparently FA dongle+filter combo can be bought for less. Check local availability and shipping costs, and read end of post if outside the US.

The LNA4ALL has been reviewed earlier on this blog (link), and the bias-T option with's dongle as well (link).
The setup described here is plug-and-play, and works with any SDR receiver, not just RTL-SDR dongles.

LNA placement

Just to recap on basics:
A preamp / LNA should be placed at the antenna, because electrical noise pickup by coax cable is horrendous: Broadcast FM, pager, baby monitors, cars starting and so on will influence your reception.
In the image below, bias-T enabled LNA4ALL used in conjunction with's bias-T enabled dongle, 20 ft / 7 m double-shielded good quality coax.
Signal pickup is by coax cable only, less signal in the image (smaller peak and less color) is better. LNA placement is the only difference.

This noise scenario applies to anyone living in a city or suburban environment and using coax cable to connect to the antenna.
A setup with direct antenna - receiver connection, or someone living in the Australian Outback where you visit your neighbors by plane are two options where this recommendation does not apply.
If you say "Hi" to Johnny / Jane on a daily basis, the following configuration will pull in more messages per aircraft.

Bias-T setup with Raspberry Pi

Check FlightAware's feeder statistics - most users use a Raspberry Pi. If you don't know what's a Raspberry, save on initial setup cost and get my book (link), then plug in cables and start feeding ADS-B data.
The Raspberry Pi already has 5V (top right corner pin with USB ports facing you) and Ground (second down outer pin) ready at the GPIO pins, so why not use them to power a bias-T enabled LNA at the antenna? Right where an LNA should be?
To do that, use the ADS-B filter from Adam, which is a small piece of electronic board with two SMA connectors for signal In and Out, and power connectors.
1090 MHz filtering will work without any power, which is only required for bias-T operation.
Bias T for the LNA works with any supply, e.g. if your LNA uses 13.8 V simply connect the voltage via the holes and the bias-T filter will send 13.8 V down the coax.
Note that standard jumper cables coming with Pi "Ultimate" or "Hacker" kits, as in the image, will fit the power holes.
The LNA4ALL used here came factory built for 5V supply, which is also the voltage used by the RPi.
Place the filter right after the SDR receiver, provide 5V from GPIO pins, or the 5V rail if using a breadboard  (or DC downconverter for PoE).
Place a bias-T enabled LNA4ALL at the antenna feedpoint, lay back and revel in the knowledge that you got the best ADS-B amplified antenna system.

That's all.

Further thoughts

FlightAware's RTL-SDR dongle with its onboard Low-Noise Amplifier has been, by all accounts, a runaway success - it sold out on many online marketplaces due to cost vs performance.
Err, probably on cost, as it's less than 20 dollars.
Khmm, less than $20 in the USA, but I get to that point in a minute.
A preamp is good for improving reception, but it should be placed right at the antenna, which is impossible with the FA dongle unless an extremely short cable run is used, or the antenna is directly connected.
In a strong signal environment, e.g. urban or suburban setting an LNA will often need
- either the gain turned down, so FlightAware published instructions how to do so, or
- requires unwanted signal reduction in the shape of a filter, which is recommended, but costs $20 dollars.
If you read this from the States, please know that your brethens in Europe would have to pay significantly more; for instance, a Flightaware dongle with filter costs a whopping $66.99 shipped to Ireland from Ebay.

To sum it up:

- FA dongle and filter costs a lot in Europe,
- for the same cost, you get an LNA and a filter than can power the LNA using the coax cable,
- LNA can be placed at the antenna, where it should be.

This setup is the best known alternative for enthusiasts for the same price as the new FA dongle and filter IF you run a RasPi AND have coax to the ADS-B antenna.

Want to know even more? Please order my RTL-SDR
Guide Book on Amazon Kindle.
Close to 200 pages of knowledge, information and diagrams for all levels of expertise.

Kindle is available for computers (Windows and Mac), and for smartphones and tablets (Android and Iphone, iPad and iPod touch).

If you're interested in the Raspberry Pi, my book details the ins and outs of setting up, using and enjoying the Pi 3 microcomputer.

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