Tagged: low noise amplifier

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.

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Tutorial on Properly Positioning a Preamp (LNA) in a Radio System

Radio blogger Anthony Stirk has made a post on his blog explaining some critical concepts behind understanding why it is important to position a low noise amplifier (LNA) near the radio antenna, rather than near the radio. In the post Anthony explains how the Noise Figure (NF) and linearity (IP3) of a radio system affect reception.

Using the free AppCAD RF design assistant software, Anthony explains how the noise figure of a system increases with longer coax cable runs, and how it can be reduced by placing an LNA right next to the antenna. He also explains why the sensitivity of the radio won’t increase if the LNA is placed close to the radio instead.

In addition to this, he also explains why adding more LNA’s to a system decreases the linearity (IP3) of the system and that if the receiver has a built in LNA that the system linearity can be severely degraded by adding extra LNA’s, causing easy overloading and intermodulation. In conclusion Anthony writes the following:

In summary, a setup with a good antenna system connected to a receiver with a built in LNA:

  • May not benefit from having a preamp at the antenna.
  • The presence of a built in LNA is detrimental to the linearity and may degrade the signals.

So in conclusion:

  • Put the preamp as close to the antenna as possible.
  • Receivers with a built in LNA may not get the most out of an antenna system or preamp.
  • Proper gain distribution guarantees better performance than one-size-fits-all solutions, both in terms of sensitivity and strong signals handling.

Optimal Setup: Antenna -> LNA -> Coax -> Receiver
Optimal Setup: Antenna -> LNA -> Coax -> Receiver
NF and Linearity Calculations
NF and Linearity Calculations in AppCAD

Testing an LNA on receiving a weak signal with the RTL-SDR

Over on YouTube Adam Alicajic the designer of the LNA4ALL low noise amplifier has uploaded a video showing the effect of an LNA on reception of a weak signal. He shows an example of how a very weak signal cannot be received by the RTL-SDR even when the gain is set to maximum unless an LNA is connected.

Adam has posted this video in regards to some statements saying that an LNA will only increase the noise floor and cannot bring signals out of the noise floor. There is a discussion about this on this Reddit thread.

DVB-T dongle + LNA = Myth or Truth

New Inline Low Noise Amplifier Design for the RTL-SDR and RTL-SDR Power Injector Modification

Recently a reader named Fabio wrote in to let us know about his new Low Noise Amplifier (LNA) design for the RTL-SDR. Fabio writes that his design is similar to the LNA4ALL, but is small enough to fit inline with an antenna. An LNA can help improve reception especially if you have long runs of coax cable between the antenna and RTL-SDR.

Fabio’s design requires that the LNA be powered inline with a bias-tee power injector circuit which can be easily built from an inductor and capacitor. But instead of building an external bias-tee he modified the RTL-SDR dongle itself to provide the required 5V output power from the USB bus. He writes that with this modification the RTL-SDR could also be used to power an active antenna.

Fabio has also released his circuit designs on his GitHub page for free.

Inline LNA for the RTL-SDR
Inline LNA for the RTL-SDR
RTL-SDR Bias-T Modification
RTL-SDR Bias-T Modification

Building a Homemade LNA for RTL-SDR

Over on Reddit user soooooil has posted about his work in building an LNA, including etching the PCB. On his imgur page he shows the design and construction process through images, before showing the final result in SDR#. For the LNA he used a ERA-3SM+ MMIC which has 17-23 dB of gain and a NF of 2.6-2.8 dB. While the noise figure is fairly high for an LNA, it is still likely lower than the RTL-SDRs internal amplifier noise figure which is around <4.5 dB.

Homemade LNA
Homemade LNA

LNA4HF Now for Sale Plus Review

Back in November last year we posted about the possibility of an “LNA4HF” low noise amplifier (LNA) for the HF bands being made available for sale. The LNA4HF is now available for purchase.

The LNA4HF is a low noise amplifier with built in low pass filter that runs on a 6-12 V power supply and covers a frequency range of 150khz to 30MHz, with a 18-20 dB gain and 1-2 dB noise figure. It costs 20 Euros. The low pass filter can also be disabled with a small board modification which will allow the amplifier to be useful at up to 2 GHz.

LNA4HF
LNA4HF
LNA4HF Block Diagram
LNA4HF Block Diagram

Akos from the SDR for Mariners blog has reviewed the LNA4HF on his latest post. His results show that the low pass filter significantly reduces broadcast FM interference and that the amplifier also increases signal strength by around 20 dB as advertised.

LNA4HF Comparison
SDR for Mariners LNA4HF Comparison

Radio Astronomy with a 0.2dB Noise Figure LNA

Over on our Facebook page member Александр has let us know about a Russian amateur astronomer, Alex who has been using the RTL-SDR for radio astronomy. Alex uses an Elonics E4000 RTL-SDR combined with a 3.7m mesh parabola dish with 1420 MHz waveguide.

At the center of his system is an LNA with 40dB gain and a very low noise figure of 0.2dB. This LNA appears to be based on G4DDK’s VLNA, but modified to work with the 1420 MHz frequency used for radio astronomy. It seems the LNA can be ordered for 140 USD from the above link.

Note: The above Russian links are machine translated with Google to English.

0.2dB Noise Figure Low Noise Amplifier
0.2dB Noise Figure Low Noise Amplifier
Radio Astronomy Results
Radio Astronomy Results

LNA4HF: Interest Check

Adam, the manufacturer of the LNA4ALL low noise amplifier designed for use with the RTL-SDR, and similar software radios is putting out an interest check for an LNA4HF product. The LNA4HF will be a low noise amplifier with low pass filter designed to work with an existing RTL-SDR upconverter. It will amplify signals between 2 Mhz – 30 Mhz by 16.5dB.

Adam will make a batch once there is interest for at least 50.

lna4hf
LNA4HF
LNA4HF block diagram
LNA4HF block diagram