Characterizing the SWR of an Antenna with a Noise Source and an RTL-SDR

In our last post Adam Alicajic showed us on YouTube how to determine the frequency response of an RF filter using just a wideband noise source an LNA and an RTL-SDR dongle.

In his latest video Adam shows how the SWR of an antenna can be measured using almost the same low cost equipment. One additional piece of hardware required to measure the SWR is a directional coupler which can be bought on Ebay for about $10 USD.

SWR stands for “standing wave ratio” and is a measure that can be used to tune an antenna for a particular frequency. The closer the SWR is to 1:1 at the designed antenna frequency, the better the antenna will receive (and transmit).

In his video Adam shows how he measures the SWR of an ADS-B antenna which he has built and is selling. His results show that the antenna has an SWR of 1:1.02 at 1090 MHz which is quite good.

DIY Characterize the antenna Retrurn Loss / SWR with the DVB-T SDR

Characterizing RF Filters with a Noise Source and RTL-SDR Dongle

Over on YouTube RTL-SDR experimenter Adam Alicajic has uploaded a video showing how it is possible to use the RTL-SDR as a tool to measure the frequency response of an RF filter. To do this he uses a noise source circuit which produces wide band white noise connected to an LNA4ALL, connected to the RF filter and finally connected to the RTL-SDR. Then using the Touchstone spectrum analyzer software he does a 300 MHz bandwidth sweep over a section of the spectrum which shows the response of the filter.

The noise source can be built from a simple diode based circuit as shown in a previous post, or if you have the Ham-it-up upconverter you can buy the parts for the noise generator part of the circuit.

In his video he shows the frequency response of a 145 MHz helix filter, a coax notch filter and a 1090 MHz home brew bandpass filter.

Characterize filters with the DVB-T SDR (part 1)

Characterize filters with the DVB-T SDR dongle (part 2)

More filters and DVB-T dongle

Monitoring House Power Usage using an RTL-SDR

Home automation enthusiast Bruce Winter has recently posted on his blog about how he uses the RTL-SDR as part of his home automation system to automatically monitor his power (and solar power generation) usage. Many home power meters are now wireless which allows meter readers to gather power usage data from afar.

To gather the power usage data he used an RTL-SDR connected to a PC running rtlamr, which is software that can read data from ERT compatible power meters that transmit in the 900 MHz ISM band. He also uses some custom code he wrote that automatically plots the data over time and allows him to integrate it with his home automation system. In addition to his post he also uploaded a video shown below that shows his system in action.

Monitoring house power using a RF RTLSDR USB stick

New Windows AIS Decoder For the RTL-SDR: AISRec

Over on YouTube user Jane feverlay has uploaded a video showing a new AIS decoder called AISRec for Windows that he has developed.

AIS is an acronym for Automatic Identification System and is a system used by ships to broadcast position and vessel information. By monitoring AIS transmissions with the RTL-SDR we can build a boat radar system. We have a tutorial on this here.

The new software is not free, but he offers a trial version that limits the run time to 20 minutes and 5000 max messages. The paid version removes these limits and also decodes both AIS channels simultaneously. The program monitors data from the RTL-SDR and sends decoded data out via UDP. Software such as OpenCPN can then be used to display the AIS data on a map.

We tested the trial version on our machine and found that it worked well at decoding AIS messages. To download the trial go to and enter the code kn44. The download site is in Chinese, but it is obvious where to enter the code. We found the software to be virus free, but remember to always scan unknown software like this yourself. The full price of the software is unknown, but purchasing instructions are given in the trial download readme. The author also writes that his software now supports the Airspy, but not in the trial version.

Note that we discovered that the software doesn’t use a PPM correction setting as expected. Instead it uses a frequency shift setting. To set the shift in the AISRec.ini file, we had to calculate freqshift = 162.025 MHz – frequency of the second AIS channel as shown in SDR# with no PPM correction set.

Another similar software program that was released earlier is PNAis.

Airspy RTL SDR RTL-SDR RTL2832: AIS software receiver for airspy and RTL2832 USB dongles

Using the RTL-SDR in the Pothos Dataflow Processing Platform

Pothos is a dataflow processing platform that can be used for signal processing through a graphical GUI that works in a similar way to GNU Radio. Over on YouTube the developers have released a tutorial video that shows how to use Pothos to interact with an RTL-SDR dongle. In the video they create a design which shows an RF spectrum and waterfall display.

Remote SDR with Pothos

Updates to a Month of RTL-SDR by Keenerd

Back in August of this year RTL-SDR code contributer Keenerd aka Kyle Keen ran an Indiegogo campaign to raise funds to pay for him to work on upgrading RTL-SDR related code for one month. As Kyle only counts the days he manages to put a good effort in towards the upgrades, the coding is still ongoing. He recently put out an update which we quote below.

Programming Report – Week 3

Hey all, sorry about dropping off the radar there.  There were several large unexpected events in recent months, but nothing bad.  I’ve still been hacking on the project.  An anonymous donor gave me a Lyons DSP textbook, which I’ve been working through for more solid foundations.  But studying doesn’t generate any new code for you to play with, so hasn’t been counted towards the project.

I’ve also been working on a very ambitious ADS-B related side project.  It is something no one has ever done before with the RTL-SDR, but mathematically it is feasible and progress is steady.  Not anywhere near for a release (and might yet be impossible in practice), and thus also hasn’t been counted towards the project.

So what has been completed since the last report?

* Automatic audio gain.  Crucial for AM/SSB listening.

* Audio padding (constant rate audio).  If you are streaming rtl_fm, this will prevent stutters when the tuner is busy hopping.

* Heatmap fixes.  The font is downloaded automatically and image slicing is completed.  Run rtl_power 24×7 but render a heatmap of only the last hour, for example.

* Rtl_power fixes.  Including one tricky corner case that caused crashes.

Also a big thank you to Kacper Michajłow who fixed and fleshed out a couple of Windows issues that were giving me trouble.

25 days remain.  As always, if you have comments or suggestions you can reach me by email or in ##rtlsdr.

The latest feature requests and links to the GitHub repositories and Windows builds can be found at


Airspy Second Batch of Preorders Available

The Airspy software defined radio recently completed its first round of 400 pre-orders. Now the second round of pre-orders has become available over on the manufacturers web store. It is priced at $199 USD and they expect the orders to be shipped around the middle of December.

If you were unaware the Airspy is an RX only SDR developed by the author of the SDR# software. It has a tuning range of 24 MHz to 1.7 GHz, up to 10 MHz of instantaneous bandwidth and a 12-bit ADC. The full list of features is shown below.

  • Continuous 24 – 1750 MHz RX range with no gaps
  • 3.5 dB NF between 42 and 1002 MHz
  • Tracking RF filters
  • 35dBm IIP3 RF front end
  • 12bit ADC @ 20 MSPS (80dB Dynamic Range, 64dB SNR, 10.4 ENOB) – Yeah, size does matter.
  • Up to 80 MSPS for custom applications
  • Cortex M4F @ up to 204MHz with Multi Core support (dual M0)
  • 1.5 ppm high precision, low phase noise clock
  • 1 RTC clock (for packet time-stamping)
  • External clock input (10 MHz to 100 MHz via MCX connector) – Ideal for phase coherent radios
  • 10 MHz panoramic spectrum view with 9MHz alias/image free
  • IQ or Real, 16bit fixed or 32bit float output streams
  • No IQ imbalance, DC offset or 1/F noise at the center of the spectrum that plagues all the other SDRs
  • Extension ports: 16 x SGPIO
  • 1 x RF Input (SMA)
  • 1 x RF Output (Loopthrough, U-FL)
  • 2 x High Speed ADC inputs (up to 80 MSPS, U-FL)
  • 4.5v software switched Bias-Tee to power LNA’s and up/down-converters

The Airspy was recently reviewed in two videos by W9RAN, the second video showing some sensitivity measurements. There are also various other videos of the Airspy in action now on YouTube.

Airspy Software Defined Radio
Airspy Software Defined Radio

HackRF Blue: A Lower Cost HackRF

Earlier in the year the HackRF One was released by Micheal Ossmann. It is a transmit and receive capable software defined radio with a 10 MHz to 6 GHz range which currently sells for around $300 USD. Since the HackRF is open source hardware, anyone can make changes to the design and build and sell their own version.

The HackRF Blue is a HackRF clone that aims to sell at a lower cost. By sourcing lower cost parts that still work well in the HackRF circuit, the team behind the HackRF Blue were able to reduce the price of the HackRF down to $200 USD. They claim that the HackRF Blue has the same performance as the HackRF One and is fully compatible with the HackRF software. They are currently seeking funding through an IndieGoGo campaign.

Their main goal through the funding is to help provide underprivileged hackerspaces with a free HackRF.

The HackRF Blue
The HackRF Blue
All About the HackRF Blue

Airspy Sensitivity Measurements and HF Performance

Over on YouTube user ranickel (aka W9RAN) has uploaded a video discussing and reviewing the sensitivity of the Airspy software defined radio. In the video he uses a signal generator to generate a reference signal and shows what the effect of software decimation has on the signal to noise ratio. His results show that the Airspy is a very sensitive and low noise receiver that is comparable to some very expensive hardware.

Airspy Sensitivity Measurement by W9RAN

In another new video W9RAN shows a quick sneak peak at the performance of the Airspy on the 20 meter CW band when using a prototype HF upconverter that he is developing.

Airspy HF Converter Sneak Peek by W9RAN

Sniffing and Analyzing GSM Signals with GR-GSM

Over a year ago we wrote a tutorial on how to analyze GSM cellular phone signals using a RTL-SDR, a Linux computer with GNU Radio, Wireshark and a GSM decoder called Airprobe. With this combination it is possible to easily decode GSM system messages. Setting up Airprobe is can be difficult as it is unmaintained and incompatible with the new version of GNU Radio without patches.

Now a new software package called gr-gsm has been released on GitHub which seems to be a newer and improved version of Airprobe. The gr-gsm software is also much easier to install, uses the newer GNU Radio 3.7 and seems to decode the system data with much less trouble than Airprobe did. We will soon update our tutorial to use gr-gsm, but the instructions on the GitHub are already quite good. The author of gr-gsm also appears to be actively adding new features to the software as well. The video below shows gr-gsm in action.

Sniffing GSM data with gr-gsm and cheap RTL-SDR receivers