1.6 LNA
A LNA, short for Low Noise Amplifier, is a device that can be installed in the coax line from the antenna to amplify the signal.
There is a lot to consider before buying and installing an LNA. Please read carefully before making a decision, because installing an LNA may decrease the performance of your setup under many circumstances. Nevertheless, there are setups where a LNA can increase performance.
An LNA usually has an RF input, an RF output and a power input. The signal coming from the antenna will enter the LNA at the RF input connector and will exit at the RF output connector going towards the SDR. You need to apply power for the LNA to work, depending on your LNA, via USB, solder points or Bias-T. The LNA will increase the signal level by a given amount e.g. 20db, including all the noise around. Hopefully it will not add a lot of own noise, that is what the LN -> LowNoise stands for in LNA.
The RTL SDRs usually have a signal strength working range of roughly about -1db to -30db.
Stronger signals will not be usable and may even stop the SDR to receive other weaker Signals. Weaker signals will not be usable either, but will not harm the receivers performance.
The signal strength is subject to a number of factors:
- The transmitter power of the airplane
- The distance of the plane and the receiver
- The line of sight
- The weather
- The type of the station antenna
- Signal loss on the receiver end which is a combination of
- The length and quality of the antenna cable used
- The number and quality of connectors and adapters used in the signal path
- The number and quality of filters used
- It is better not to loose signal strength than to amplify, therefore we try not to loose signal strength by
- Shortening the coax cable run
- Using good matching connectors to omit the need for adapters
- It is better to amplify a strong signal early in the signal path, than a weak signal later in the signal path, therefore we install an LNA as close to the antenna as feasible
With a lower general signal strength we will have no problem to receive planes close by and strong signals. If we want to increase the reception of planes further away and weaker signals by increasing the overall signal level with an LNA we have to keep in mind not to overdo it and over amplify the strong signals of planes nearby.
LNAs are available with different amplification levels e.g 0.6db or 20db
The strongest signals in a setup without LNA + the value of the LNA should always result in a signal lower than -1db
For example the 20db LNA can only be used if the strongest signals in the setup without the LNA are -21db or lower.
In a usual setup of a SDR with integrated Filter and a PCB antenna on a short run of quality coax there is no need for an LNA, it may even impact your performance negatively. If you see weak values nevertheless there is most likely something wrong with your setup.
The same thing applies when using an SDR without integrated filter no matter if combined with an external one or not.
If we combine a SDR with internal filter and an external filter and have a longer run of coax e.g. 5m+ of RG-58 then you will encounter relatively low signals and a LNA can help improving the performance by a good bit.
While you can place an LNA between antenna and cable and feed it power via Bias-T it is much more convenient to place it inside between coax cable end the filter.
In a test setup a station with the green Radarbox Stick combined with a FlightAware filter and about 8m of RG58 to the PCB antenna received 181 planes. By adding an 20db LNA between coax and filter we were able to increase the number of received planes to 266.
As always: your mileage may vary
As with ADS-B just use a good antenna and short cable, there is no need for an LNA. We have not encountered a simple setup with an antenna connected to one SDR where an LNA was able to improve reception.
In a more complex setup with antenna splitter and filter involved, an LNA can be useful.
As with AIS just use a good antenna and short cable, no LNA. We have not encountered a simple setup with an antenna connected to one SDR where a LNA was able to improve reception.
In a more complex setup with antenna splitter and filter involved a LNA can be useful.
High quality but expensive LNA with three different options of powering.
- 5V pin header
- Micro USB in
- Bias-T in
Inexpensive LNA available with different parameters.
The 5 - 6000MHz +20db Variant works well with ADS-B
