Category: Applications

Using a TV Antenna Tap as a Directional Coupler for Antenna Measurements with an RTL-SDR

Over on his blog Tomi Engdahl has been exploring his options for measuring the VSWR of antennas with an RTL-SDR. As discussed in one of our previous tutorials, by using an RTL-SDR, noise source and directional coupler it is possible to roughly estimate the resonant frequency of an antenna.

However, being without a directional coupler Tomi looked for other options and realized that cheap TV antenna network taps are also directional couplers. Taps are commonly used with Cable and Satellite TV installations to split a signal from an antenna over multiple TVs. They are designed as directional couplers to ensure that unwanted signals do not feed back into the antenna system and so that there is a pass through port to continue the strong signal down a long cable. 

Note that there is a difference between a tap and a splitter. Taps are used when multiple devices need a signal over a long run of cabling. A splitter divides the signal strength by the number of out ports and can feedback unwanted signals into the system.

Taps vs. Splitter Example (Source:
Taps vs. Splitter Example (Source:

In his tests Tomi found that TV taps worked acceptably well to determine the resonance frequency of an antenna that he was testing. Taps can be found for as cheap as $2 on sites like eBay, although for some listings it is unclear over what frequency range they work well at as sellers assume that they will be used for TV frequencies.

Tomi also tested to see if he could use a signal splitter instead of a directional coupler tap. His results showed that the splitter still worked, and he was able to see the resonant points, but the results where not as good as with the directional coupler.

Measuring the resonant point of a antenna with a noise source, tap, and RTL-SDR.
Measuring the resonant point of a antenna with a noise source, tap, and RTL-SDR.

Transmitting DVB-S with a PlutoSDR and Receiving it with an RTL-SDR

Over on YouTube Christopher Bridges has uploaded a video showing him using a PlutoSDR and a GNU Radio program to transmit a DVB-S signal, which is then received with an RTL-SDR. DVB-S is a digital video broadcasting standard designed for satellite transmissions and digital amateur television video (DATV) also uses DVB-S in the 1.2 GHz amateur band. In this example the PlutoSDR transmits at 1.28 GHz.

Chris uses the rtl_sdr command line software to receive the raw IQ data at 1 MSPS, and then uses the leandvb software to decode the raw IQ file directly into a video file.

If you’re interested in TXing DVB-S/DATV but don’t have a transmit capable SDR, then we note that even a Raspberry Pi just by itself can be used to transmit it with rpidatv.

iqToSharp: Convert rtl_sdr IQ files to SDR# Format

Thanks to Marcin Jakubowski for submitting news about his new software tool called iqToSharp which is a simple tool that allows you to convert rtl_sdr IQ files into the SDR# IQ format. The rtl_sdr command line tool records raw IQ files but by default they are not compatible with the format used by SDR# so a conversion is required.

This is useful as for example you could set a command line script to record an entire band for a few hours on a portable Linux device like a Raspberry Pi, and then use the converter to listen to the file on SDRSharp at a later time. Recording the raw IQ file allows you to record all signals within the entire bandwidth at full quality.

Note that IQ files can become very large so for archiving compressing them with FLAC can be useful. You might also be interested in the SDR# FilePlayer plugin which allows you to easily skip back and forth in time through a recorded IQ file.

LocalRadio: A new RTL-SDR App for MacOS

Thanks to Doug Ward (@dsward) for letting us know about his new RTL-SDR compatible MacOS based app called LocalRadio. LocalRadio is an open source web browser based app that connects to a MacOS server running an RTL-SDR. The software allows you to listen in on any frequency supported by the RTL-SDR in AM or FM modes, and audio is capable of being streamed to multiple devices via a built the LAME MP3 encoder, EZStream and Icecast server. It does not provide an FFT or waterfall display however.

The software introduction reads:

LocalRadio is an experimental, GPL-2 licensed open-source application for listening to “software defined radio” on your Mac and mobile devices. With an inexpensive RTL-SDR device plugged into the Mac’s USB port, LocalRadio provides a casual listening experience for your favorite local FM broadcasts, free music, news, sports, weather, public safety and aviation scanner monitoring, and other radio sources.

LocalRadio’s easy-to-use web interface allows the radio to be shared from a Mac to iPhones, iPads, Android devices, and other PCs on your home network. No additional software or hardware is required for sharing with mobile devices, simply use the built-in mobile web browser to connect to LocalRadio and tune to your favorite stations. You can also listen to LocalRadio audio on your Apple TV and other AirPlay-compatible devices.

LocalRadio does not provide features like FFT waterfalls, panadapters, or signal recording that are found on other SDR software. For those features, GQRX for Mac is highly recommended. GQRX is a good way to discover radio frequencies that can be used with LocalRadio.

LocalRadio is intended for use as in-home entertainment, using a local area network with a private IP address. It has not been tested with a public IP address, particularly for security testing, therefore it is not recommended for that purpose. For simply listening to LocalRadio on the Mac with the RTL-SDR device plugged in, no network is required at all.

LocalRadio Interface in the Safari Web Browser
LocalRadio Interface in the Safari Web Browser

PantronX Titus II Ready for Production

Since September 2016 we’ve been slowly hearing news about the PantronX Titus II portable SDR system, but as of yet nothing seems to have eventuated. The Titus II is essentially an Android touch screen tablet running their custom software, a set of speakers, an antenna and an SDR chip with 100 kHz to 2 GHz tuning range all in one portable system that has been estimated by them to retail for less than $100 USD. The main goal with the system is to provide low cost receivers for digital broadcast standards like DRM, DAB and DAB+ to try and boost their popularity.

Titus II receiver features include:

  • DRM in the AM bands (MW, SW, LW) and VHF bands (FM-band, VHF band-I, VHF band-III) with latest xHE-AAC audio codec.
  • DAB Classic/DAB+ (VHF band-III).
  • FM stereo with RDS (Service Signaling).
  • AM with AMSS (AM Signaling Service).
  • Integrated service list management and service selection.
  • DRM/DAB Data Apps: Text Messages, Dynamic Label/DL+, Journaline, (Categorized) Slideshow, EPG, Transparent File Transmission (e.g., for educational services), etc.
  • Remote Radio Hotspot: Built-in WiFi hotspot feature, which allows any mobile device with an HTML5 web browser to connect to the Titus II via Wi-Fi, select radio services, listening to aud (HTML5 audio streaming) and accessing all the DRM/DAB data apps.
  • Recording feature and Archiving interface to select existing recordings for playback.

Recently there has been some new news over on the magazine about radio broadcasting stating that the Titus II is now ready for production. They write:

Titus SDR, a division of PantronX, says the Titus II multi-standard digital radio receiver is ready for production.

The consumer software-defined radio digital receiver platform, which is the result of collaboration between Titus SDR/Patron X, Jasmin-Infotech, TWR, and Fraunhofer IIS, supports multi-standard radio reception, including DRM, DAB and DAB+ and core data applications. The system is based on a custom Android tablet platform, featuring multipoint touch, WiFi/Bluetooth and stereo sound.

Titus II units will be available as a stand-alone product from Titus SDR as well as from selected OEMs. Titus SDR explains that as a module, Titus II can serve as a full-featured basis for third-party product development, adding that PantronX provided the platform and RF expertise, while Fraunhofer IIS enabled the digital and analog radio features.

With latest xHE-AAC audio codec, Titus II supports DRM in the AM and VHF bands; DAB/DAB+; FM stereo with RDS; AM with AMSS; integrated service list management and service selection; DRM/DAB data apps; text messages and Journaline.

No news yet on exact release dates, but if you are interested you can sign up to their pre-order notification list at

The Titus II
The Titus II

From YouTube we’ve also found a short video of them demonstrating the Titus II from DBS2017 back in March. Another video showing the interface up close can be seen here.


Decoding Amateur Radio Digital Voice with an RTL-SDR and the QRadioLink Android App

Thank you to Adrian for submitting his video about using the Android App called QRadioLink and an RTL-SDR to decode digital amateur radio voice transmissions. Adrian writes that in the video the RTL-SDR connects to the Android phone with a USB OTG cable and uses a sample rate of 1 MSPS. He also writes the following about QRadioLink:

QRadioLink is a building platform which allows experimenting with VHF-UHF SDR transceivers using different modulation schemes for digital data transmissions. So far digital voice and text transmission is supported, using either a narrow band modem and Codec2 or a high bandwidth modem and Opus. Supported hardware includes the RTL-SDR, Ettus USRP, HackRF, BladeRF and in general all devices supported by libgnuradio-osmosdr.

A Solar Powered Raspberry Pi + RTL-SDR NOAA Weather Satellite Receiver

Over on YouTube user Fuzz has uploaded a video showing his solar powered NOAA weather satellite receiver.

The system is based on a Raspberry Pi connected to an dongle. The front-end input of the RTL-SDR dongle consists of an LNA and FM reject filter, and this is all connected up to a QFH antenna in his front yard. The electronics are completely solar powered, with the solar system consisting of solar panel, solar controller and four 12v batteries used for energy storage. A 12V to 5V step down converter is used to power the Raspberry Pi, with the 12V LNA being powered directly by the batteries. The system is able to be accessed remotely via the Raspberry Pi’s WiFi connection.

Over on his Facebook page Fuzz has uploaded some additional photos, and some of the images he’s receiving.

Fuzz's solar powered NOAA weather satellite receiver.
Fuzz’s solar powered NOAA weather satellite receiver.

SDR on an Android Wristwatch with Airspy HF+ and SpyServer

Over on Twitter @lambdaprog and @mm6dos, developers of SDR# and Airspy SDR products have tweeted videos showing off an Android watch being used as an SDR interface. They use a prototype of their upcoming Airspy HF+ SDR, their SpyServer streaming software and an Android watch. The Android watch receives the streaming FFT and audio data from a server running the SpyServer and Airspy HF+.

They write that this new SpyServer client is mainly for phones and tablets and is efficient enough to run on a watch. It appears that this lightweight version of the SpyServer sends compressed FFT and audio instead of a slice of the IQ data like the current SpyServer, making it very light on the client side CPU and network usage.

If you’re interested in the Airspy HF+ we have an initial review available here.