Review: FlightAware ADS-B RTL-SDR + LNA Positioning

Recently FlightAware released a new RTL-SDR dongle sold at zero profit at $16.95 USD. It’s main feature is that it comes with an ADS-B optimized low noise amplifier (LNA) built directly into the dongle. FlightAware.com is a flight tracking service that aims to track aircraft via many volunteer ADS-B contributors around the world who use low cost receivers such as the RTL-SDR. In this post we will review their new dongle and hopefully at the same time provide some basic insights to LNA positioning theory to show in what situations this dongle will work well.

FlightAware Dongle Outside
FlightAware Dongle Outside

A good LNA has a low noise figure and a high IIP3 value. Here is what these things mean.

Noise Figure

An LNA is generally used in a RX radio system to reduce the overall noise figure (NF). In simple terms, the NF is a metric that measures how much noise (in dB) components such as amplifiers, coax cables and filters are contributing to the system. The overall noise figure is dominated by the first amplifier in the system, and so by adding a low NF amplifier (LNA) right by the antenna, the NF of the entire radio system can be significantly reduced. For example if the total NF of your radio system was 10 dB, and you added a 1 dB LNA by the antenna, then the overall noise figure would be close to 1 dB.

By placing the LNA by the receiver (as what FlightAware have done with their dongle), the benefits of the LNA are limited as the noise figure of components before the LNA like the coax and filters can not be overcome. However, there is still one significant benefit. The noise figure of the R820T/2 chip is advertised at 3.5 dB at maximum gain, but in reality it is more like 6 dB. So by placing a 0.4 dB NF LNA before the R820T2 tuner the overall system noise figure can still be reduced by about 5 dB’s. From noise figure calculations which we don’t specify here, this means that you will get (approximately) an 7-8 dB stronger signal at 1090 MHz, simply by using the 0.4 dB NF LNA. 

However, although the NF is reduced, there is still the problem of there now being potentially too much gain in the system, which may cause the RTL-SDR to overload on out of band signals. Reducing the RTL-SDR gain would increase the NF of the R820T chip, bringing the overall noise figure back up again and reducing the SNR gained. Instead of reducing the gain the overloading problem can be eliminated by using a filter, and this is indeed suggested by FlightAware. The filter will remove strong out of band signals, and since ADS-B signals are not very strong there is no chance of in band overloading.

IIP3/OIP3

The IIP3 (third order input intercept point) and OIP3 (third order output intercept point) are measurements for amplifiers that explain how linear they are. The IIP3/OIP3 linearity measurements explain how strong of a signal can be input (or output) before the LNA will saturate and overload, thus causing very poor reception. The OIP3 is simply the IIP3 + the gain of the amplifier. Higher values for the IIP3/OIP3 are better as it means that stronger signals can be received without overloading.

The IIP3/OIP3 of the entire system is reduced as more LNA’s are added. So adding too many LNA’s can cause trouble with overloading. The problem of overloading for ADS-B can easily be overcome by adding in a sharp filter, such as the FA ADS-B filter to block out other very strong signals such as broadcast FM, pagers and GSM. As long as the in band ADS-B signals are not overly strong (which they are not), the system will be fine even with a lower IIP3/OIP3.

If the LNA has a high IIP3/OIP3 value, and is not overloading, then it is optimal to place the LNA first, before any filters, so that the filter insertion loss can be overcome. If the LNA IIP3/OIP3 is low and it is overloading, then place the filter before the LNA to reduce strong out of band signals, or use an attenuator.

The FlightAware dongle advertises an OIP3 of 39 dBm for its built in LNA which is at the high end and so is good. For comparison the popularly used LNA4ALL has a OIP3 of about 33.5 dBm.

Placing the LNA by the Antenna Pro/Cons

The pros and cons of placing an LNA by the antenna are shown below.

ProsCons
  • Can achieve a very low noise figure by overcoming the NF increase caused by the:
    • R820T tuner
    • Coax cable
    • Filters
    • Switches/Splitters etc.
  • Is the optimal arrangement for best reception.
  • Need to place the LNA by the antenna (may be difficult to access).
  • Needs DC power from a bias tee or other source.
  • May need weather proofing if placed outdoors.

Placing the LNA by the Receiver Pro/Cons

The pros and cons of placing an LNA by the receiver are shown below.

ProsCons
  • Can overcome the high NF of the R820T tuner.
  • Easy to implement and cheaper.
  • Smaller improvement compared to placing the LNA by the antenna.
  • IIP3/OIP3 will be reduced if another external LNA needs to be used to overcome a long coax cable.
  • Without filtering the increased gain can cause overloading, requiring that the R820T gain be reduced, increasing it’s NF and thus causing the overall NF to rise again to levels seen without the extra LNA.

Product Review

The dongle itself comes in a good looking and tough orange plastic enclosure. It is flat on the bottom, with a curved topside. There are several ventilation holes on the bottom. It uses an SMA connector for the RF input. It uses an R820T2 chip, but does not come with a TCXO. (A TCXO is not needed for wideband signals like ADS-B). The dongle is made by Newsky who make the majority of RTL-SDR dongles at the moment.

On the inside we can see that most of the dongle remains the same, except for the LNA circuit which can be seen to the top right, near the RF input. The LNA is labelled SKY 7150, which appears to correspond to the Skyworks SKY67150-396LF chip. The specs are 0.3-2.2 GHz operation, with a noise figure (NF) of 0.23 @ 849 MHz and a high OIP3 figure of up to 39 dBm. FlightAware themselves advertise a NF of 0.4 and OIP3 of 39dBm which may be the figures for 1090 MHz. The chip costs about $2.70 USD for volumes of 500 or more.

FlightAware Dongle Circuit
FlightAware Dongle Circuit

The total current use of the dongle sits at about 330 mA. A regular dongle consumes about 270 mA, so it appears that the LNA is using about 60 mA. This current usage is suspiciously low for a 39 dBm LNA, which should be drawing about 100 mA.

The low current usage can be explained because they are powering the LNA from the 3.3V line and not the 5V line. In the PGA-103+, a similar LNA, using 3V instead of 5V causes a 40mA drop in current usage and about a 8 dBm drop in OIP3 according to the datasheet. The SKY67150 also has adjustable current draw and they may have adjusted the circuit to make it draw less current so that mini PC’s like the Raspberry Pi would not be too strained by the increased load. Less current draw means that the LNA will not be performing with an OIP3 of 39dBm, and is more likely to be performing at around 33 dBm or less which is still pretty good.

Test and Theory Results

In these tests we used a roof mounted FlightAware ADS-B antenna connected to 15m of RG6 coax which has low loss at 1090 MHz. Using a VNA we measured the cable loss including adapters to be about 6dB. To test against an external LNA we used the LNA4ALL which is a PSA4-5043 based LNA. We also used a FlightAware ADS-B filter and set the RTL-SDR gain to maximum to get the best noise figure. Each receiver was run simultaneously on the same PC via an isolating coax splitter box. The software we used was dump1090 and Virtual Radar Server. Each test lasted about 10 minutes.

For theoretical measurements we used AppCAD, which is software that can allow you to determine a radio systems overall noise figure and IIP3. The post at http://ava.upuaut.net/?p=836 also shows AppCAD in action and explains how to properly position an LNA.

Note that we don’t know the R820T’s internal LNA gain for sure, but we guess 35 dB for these results.

Standard RTL-SDR vs FlightAware RTL-SDR Dongle

  • Antenna -> 15m Coax -> FA Filter -> Standard RTL-SDR
  • Antenna -> 15m Coax -> FA Filter -> FA RTL-SDR

In this first test we simply test the dongles together, without any external LNA’s used.

Noise Calculation Theory
Noise Calculation Theory

The theory shows that the standard RTL-SDR system has a noise figure of 13.60 dB, whilst the FlightAware dongle system has a NF of 8.15 dB, giving the FlightAware dongle about a 5 dB better signal. However the IIP3 of the FlightAware dongle is reduced to -21.4dBm vs -2.4dBm for the standard dongle, meaning that the FlightAware dongle is more likely to overload from strong signals (again not a problem with a filter though).

The results show that the FlightAware dongle with its 5dB lower noise figure was able to receive about double the number of Mode-S messages.

RTL-SDR.com Receiver vs FlightAware Dongle (No external LNA's used)
RTL-SDR.com Receiver vs FlightAware Dongle (No external LNA’s used)

Standard RTL-SDR + LNA4ALL at receiver side vs FlightAware RTL-SDR Dongle

  • Antenna -> 15m Coax -> FA Filter -> LNA4ALL -> Standard RTL-SDR
  • Antenna -> 15m Coax -> FA Filter -> FA RTL-SDR

In the next test we added an LNA4ALL directly by the standard RTL-SDR, after the coax and filter, so that it was set up equivalently to the FlightAware dongle. The LNA4ALL has similar specs to the LNA used in the FlightAware LNA, but has a NF of about 1 dB at 1090 MHz – about 0.6 dB worse.

The theory results show that the NF and IIP3 of the system is nearly identical with the FA dongle system having about a 0.6dB better NF. The NF of the standard dongle is 8.76dB and the NF of the FA dongle system is 8.15dB.

Noise Calculation Theory
Noise Calculation Theory

The real test results show that both setups received an almost identical number of messages, with the FA dongle receiving slightly more due to its 0.6 dB lower NF.

Standard RTL-SDR + LNA4ALL at receiver side vs FlightAware RTL-SDR Dongle
Standard RTL-SDR + LNA4ALL at receiver side vs FlightAware RTL-SDR Dongle

Standard RTL-SDR + LNA4ALL at antenna side vs FlightAware RTL-SDR Dongle + LNA4ALL at antenna side

  • Antenna -> LNA4ALL -> 15m Coax -> FA Filter -> Standard RTL-SDR
  • Antenna -> LNA4ALL -> 15m Coax -> FA Filter -> FA RTL-SDR

In the next test we placed the LNA4ALL by the antenna. The coax, filter and splitter were the same as before.

In these theory results we can see that the NF is now much smaller than in the previous tests thanks to the LNA4ALL placed by the antenna (about 7dB better as the loss of the coax and filter has been eliminated). For the standard dongle the NF is 2.06dB, and for the FA dongle it is 1.29dB. You can see that in this situation the second LNA on the FA dongle does not make a huge difference. In terms of linearity, the IIP3 is reduced down to -39.4dBm on the FA dongle due to the use of two LNAs, whereas it is 20dBm higher at -20.4dBm on the standard dongle.

Noise Calculation Theory
Noise Calculation Theory

In this test both dongles again received a nearly identical number of messages, with the FlightAware dongle receiving a bit more due to the 1 dB lower NF.

LNA4ALL placed on the antenna side.
LNA4ALL placed on the antenna side.

Standard RTL-SDR + LNA4ALL at antenna side vs FlightAware RTL-SDR Dongle

  • Antenna -> LNA4ALL -> 15m Coax -> FA Filter -> Standard RTL-SDR
  • Antenna -> 15m Coax -> FA Filter -> FA RTL-SDR

In the final test we ran the standard RTL-SDR with the LNA4ALL at the antenna, and the FlightAware dongle without the LNA. This time we had to test each dongle separately, and did so 10 minutes after the other.

The theory results show that the standard dongle with LNA4ALL by the antenna has a NF of 2.06dB and an IIP3 of -20.4dBm. The FlightAware dongle with no antenna side LNA has a noise figure of 8.15dB and an IIP3 of -21.40dBm. In this case we would expect the standard dongle with LNA4ALL at the antenna to perform much better. If the coax cable length was increased or a more lossy cable was used the benefit of the antenna side LNA would increase further.

Noise Calculation Theory
Noise Calculation Theory

In these real results the standard dongle with the LNA4ALL performed significantly better as expected.

LNA4ALL Antenna side. FlightAware dongle by itself.
LNA4ALL Antenna side. FlightAware dongle by itself.

Conclusions

It appears that the goal of the FlightAware dongle is to not necessarily go for the optimal high performance setup, but rather go for the easiest to use and cheapest performance setup. This is fine as they are trying to get as many people to participate in their program as possible, especially those on a budget or those without the technical knowledge to set up a proper antenna system.

In terms of performance, an LNA at the receiver won’t be able to overcome coax cable and filter losses, but it will still reduce the RTL-SDR noise figure from 6 dB to 1 dB, giving a 7-8 dB better signal. The extra gain in the system may cause overloading, but with a sharp ADS-B filter such as the FA Filter, the overloading problem can be eliminated.

We also briefly tested the FA dongles operation on frequencies other than 1090 MHz. However, overloading at max gain was common and we usually had to reduce the RTL-SDR gain down 20 to 40 dB’s, and even then there was still sometimes overloading from strong broadcast FM, DVB-T and GSM signals. So using an LNA by the receiver will only really work well if you can do the following:

  1. Filter out of band strong signals. (Use the FA ADS-B Filter)
  2. Ensure your in band signals are not too strong. (ADS-B is not too strong)
  3. Use maximum gain on the RTL-SDR to get the lowest NF for the R820T chip

Overall for the purposes of improving ADS-B reception the FlightAware dongle + filter combination works very well and you will receive significantly more ADS-B messages with it if you do not already have an LNA. An external LNA + bias tee module would have been a higher performance product, but we do note that it would be more difficult to set up, and also would probably be a more expensive solution.

Recommendations

A FlightAware dongle will work better than a standard dongle if the dongle is placed near the antenna, without long lossy coax runs and if used with a sharp filter, with the RTL-SDR gain set to maximum.

An external antenna side LNA + standard dongle will work better than the FA dongle if a few meters of coax cable is used to connect the antenna to the dongle or if there are other lossy components in the RF path.

The FlightAware dongle costs $16.95 + shipping on Amazon for US buyers, and $24.95 + shipping on eBay for international buyers. Amazon is currently out of stock, but they should be back in a few weeks.

If you are interested we also previously reviewed their ADS-B filter and antenna solutions.

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