Recently the FlightAware Prostick Plus was released. The Prostick is a modified RTL-SDR with a LNA built into the dongle. It is optimized for ADS-B reception and works very well due to the low noise figure of the SKY7150 LNA which is used as the first stage LNA. However, due to the increased gain from the LNA it can easily overload from strong out of band signals, such as broadcast FM, DAB, DVB-T and GSM. To eliminate this problem FlightAware recommend using their 1090 MHz filter in front of the dongle.
The FlightAware Prostick Plus is the same as the Prostick, but the Plus also incorporates a 1090 MHz SAW filter into the dongle itself. The overall cost is about $15.95 USD cheaper than buying the Prostick + Filter combination. See below for a tabulated comparison between the two units.
|FlightAware Prostick + Filter||FlightAware Prostick Plus|
Worldwide: $48.99 + $18 Shipping = $66.99
Worldwide: $29.99 + $12 Shipping = $41.99
|LNA + Filter Arrangement||Filter -> SKY7150 LNA||SKY7150 LNA -> Filter|
|Filter Specs||Type: LC
Passband: 980 – 1150 MHz
Insertion Loss: 1.65 dB
Attenuation: 40 – 50 dB
Passband: 1,075 MHz – 1,105 MHz
Insertion loss: 2.3 dB
Attenuation: 30 dB
|TCXO||Old batches NO.
New batches YES.
|Current Draw||330 mA||300 mA|
The first thing we notice is that the filter arrangement between the two units is reversed. On the Prostick the filter is external and must be placed before the LNA. This has the advantage of excellent rejection of out of band signals, but increases the noise figure (NF) of the system slightly. A higher noise figure means the ADS-B signal will end up being weaker, resulting in less range and reports. However, the FlightAware 1090 MHz filter has low insertion losses and should only increase the NF by 1-2 dB.
The Prostick Plus on the other hand uses a SAW filter positioned after the LNA. SAW filters at 1090 MHz typically have an insertion loss of anywhere between 2-3 dB’s. But since it is placed after the LNA the losses are almost completely eliminated by the gain from the LNA and thus the total NF remains low. The attenuation of the SAW filter is less, but it has a smaller pass band. The small pass band may be useful for people who live near an airport and suffer issues with interference from the 1030 MHz interrogation pulses or from GSM at 950 MHz.
In theory, the Prostick + Filter should operate better in environments with very strong out of band signals (any signal outside of 1090 MHz). And the Prostick Plus should operate better in environments with weaker out of band signals. The theory is that since the LNA is placed first in the signal chain on the Prostick Plus, it is more susceptible to overloading from the strong signals as it has no protection from a filter. The LNA used in both Prosticks is a SKY7150, which has a very high OIP3 rating. High OIP3 means that its performance in the presence of strong signals is excellent, and it will not overload so easily. However, even a very high OIP3 rated LNA cannot withstand the strong broadcast signals in some locations.
The Prostick Plus also has some other enhancements like a TCXO. ADS-B is very tolerant to frequency drift, so a TCXO won’t really improve decoding performance, but the cost of a 28.8 MHz TCXO purchased in bulk is under $1 USD, so they may have decided to add it anyway. They appear to also be using TCXO’s on the new production batches of the Prostick as well. The Plus also only draws 300 mA of current compared to the Prostick which draws 330mA. This may be due to the removal of the LED (Although the new batches of the Prostick might also have the LED removed as they advertise a power draw of 300 mA.) On the image of the PCBs below you can see the difference. The SAW filter is just underneath where the LED used to be.
Again, as we mentioned in our previous review of the Prostick it is a bit odd that the 39 dB OIP3 SKY7150 only appears to be drawing 60 mA, when it should be drawing 100 mA. The lower current usage is probably because they run it from 3.3V instead of 5V. The lower current use probably means that the OIP3 rating is reduced slightly by ~5 dBs.
Real World Testing
Here we test the Prostick and Prostick Plus in a signal environment with lots of strong interfering BCFM, DVB-T and GSM signals around. We’ve seen reports on the FlightAware forums that some users have seen improved performance with the Prostick Plus, whilst others have seen dismal or reduced performance. In these tests and review we are able to show when each stick will perform at its best. We do not test the Prostick without the filter, as without the filter we are unable to receive any ADS-B messages at all due to overloading.
Test 1: Flight Aware ADS-B Antenna
First we set up a test using the FlightAware ADS-B antenna, a 2-way signal splitter and the Prostick Plus and Prostick + Filter. We used Modesdeco2 as the ADS-B software, and ran the test for 45 minutes.
The results show that the Prostick Plus edges ahead of the Prostick + Filter by a small amount. It seems that the 1-2 dB loss in the external filter does not contribute to a huge reduction in ADS-B messaging, but the results do show that the Prostick Plus will give you better results in an environment with favorable reception conditions.
In this test we used the excellent FlightAware ADS-B antenna. This antenna is tuned specifically to 1090 MHz, and performs some rejection of the out of band signals. This rejection is enough to allow the Prostick Plus to work well in our test area without overloading.
In the image slider below we first checked ADS-B reception in SDR#, to see if there was any noticeable visual difference. The reception seemed identical. In the remaining images we checked to see how the reception was on out of band signals with the two units. In these tests we want the out of band signals to be low, so smaller signals are better. The Prostick Plus filters our out band signals significantly less, which can be a reason for increased overload. But the amount of filtering performed by the Plus was sufficient together with this 1090 MHz tuned antenna to not cause any overload at max gain.
Test 2: Discone Antenna
In test 2 we show what can happen if the out of band signals going into the Prosticks are really strong. This could especially happen if you are using a wideband antenna that is not specifically tuned to 1090 MHz, or if the out of band signals in your area are exceptionally strong (living near a transmission tower for example). In this test we used the same setup as in test 1, but used a wideband discone as the antenna instead. This means that the natural out of band signal filtering from the FlightAware antenna is not present anymore, and thus out of band signals come into the dongle much stronger.
Here we found that the Prostick Plus produced dismal results. The out of band signals were too strong for the LNA to handle, thus causing overload and significant desensitization of the ADS-B signals. The messages received by the Prostick + Filter was significantly higher.
In the SDR# screenshots below we can clearly see that the Prostick Plus has very poor ADS-B reception at 1090 MHz with this antenna. The noise floor is much higher due to desensitization and overload from broadcast FM and DVB-T signals. Reducing the gain on the RTL-SDR does not help a lot, since most of the overload occurs in the first stage SKY7150 LNA. This can also be seen in the amount of signal overload that is present when tuned to the broadcast FM and other bands in SDR#.
The Prostick and Prostick Plus dongles are both excellent low cost ADS-B receivers. If you want to set up a permanent ADS-B monitoring station they are highly recommended.
So what are the lessons learned from these tests?
- If you live in an environment with extremely strong out of band signals you’ll need to place the filter first. So in this case use the Prostick + external filter combination (or Prostick Plus + external Filter).
- Otherwise use the Prostick Plus for slightly better performance and lower cost.
- To reduce the possibility of overload with the Prostick Plus use an antenna tuned to 1090 MHz.
The table below summarizes the recommendations again.
Antenna -> LNA -> Filter
|Antenna -> Filter -> LNA
(Prostick + FA Filter)
Noise figure (NF) is dominated by the LNA, thus this method gives minimum NF.
Losses in filter overcome by LNA gain.
LNA will not be susceptible to overloading from out of band signals.
The LNA can overload from out of band signals since it is not protected by a filter.
The insertion loss (IL) of the filter directly adds to the noise figure (NF). For example a 2 dB IL filter will add 2 dB to the system NF. This may result in a few dB’s lower SNR.
|When to use||Use this method if you do not have strong out of band signals in your area and/or if you have an LNA with a high OIP3 rating, like with the SKY7150 LNA which is used on the Prostick’s.||Use this method if you have very strong out of band signals in your area.|
For most people the Prostick Plus should work fine and be the better choice. Also rest assured that if you purchase a Prostick Plus and find that it overloads in your environment, you still always have the option of placing an external filter in front of it. Then you’ll practically have the same performance as with the standard Prostick + Filter combination. A Prostick Plus + External Filter combination may even be more beneficial for users in very very strong signal environments.
Also remember that the Prostick’s are designed to be placed as close to the antenna as possible, without the use of coax cable. You can use USB extension cables, or run the Prostick on a remote Raspberry Pi computing unit to achieve this. If you want to run coax between the antenna and Prostick, you will see heavily reduced performance due to the losses in the coax cable. In this situation you should instead place an LNA like the LNA4ALL or Uputronics ADS-B LNA by the antenna, and use a bias tee to power it.