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: http://forums.solidsignal.com/showthread.php/5843-Solid-Signal-s-WHITE-PAPER-The-NEW-DIRECTV-Residential-Experience)
Taps vs. Splitter Example (Source: http://forums.solidsignal.com/showthread.php/5843-Solid-Signal-s-WHITE-PAPER-The-NEW-DIRECTV-Residential-Experience)

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.

LimeSDR and SDRAngel Running on a LattePanda

Thanks to Marty Wittrock for sharing with us his latest news about his experiences with the LimeSDR. Over on YouTube he’s uploaded a video showing that the LimeSDR can run perfectly on a ‘LattePanda‘ which is a full Win10 64-bit PC in a Raspberry Pi sized package. The one Marty uses costs about $209 with a fully activated Windows 10 licence and has 64G eMMC memory, a 1.44 GHz CPU and 4GB of RAM. The cheaper version with 2GB of RAM and 32GB eMMC memory only costs $119 USD. Of interest is that the LattePanda also comes with an Arduino co-processor for various GPIO projects, which could be useful for switching in and out various radio blocks like filters and LNAs.

In his video Marty shows that the SDRAngel software run the LimeSDR smoothly on the LattePanda, and he demonstrates receiving the 30M band. He writes:

Ladies and Gentlemen may I present to you: The LimeSDR operating on receive on the 30m Shortwave Band using SDRAngel running on a Windows 10, 64-bit, 4G (RAM) 64G (FLASH) operating at 1.8 GHz on a LattePanda – – THE SMALLEST WINDOWS 10 PC ON EARTH..! No kidding, this makes my MSI ‘hockey puck’ PC look monstrous…The LattePanda is about the same size as my LimeSDR and a VERY POWERFUL little PC, too…I’m INCREDIBLY impressed with this tiny PC…This Single Board Computer also has an integrated Arduino processor such that all the bandswitching and other functions required to make a Software Defined Transceiver will be very trivial to add-in…The ‘proof of concept’ is complete – – this shows that it CAN BE DONE to make the LimeSDR into a compact, wideband Software Defined Transceiver that can be run on a car battery if needed…Watch the video and see for yourself…CHEERS..!

In a second newer video he demonstrates the system running on a 7″ LCD touchscreen. He writes:

Here’s a follow-up YouTube video I did when I laced-up the LattePanda to a 7″ LCD and the companion Capacitive Touchscreen for the LimeSDR. This thing is awesome and very compact – it’s amazing. I’m planning to take those same components and mount them into a walnut case that I’m getting made from furniture-grade wood from Amana. The case will be sloped and will have enough room to put the LattePanda, LimeSDR, USB 3.0 hub, and *maybe* the PA – but I have to think that the PA will be outboard along with the Bandpass Filter Card assembly, too. I am planning to make a preselector/receive preamplifier for the LimeSDR that will reside in the case, too. The LattePanda not only has the Intel CherryTrail processor there for Windows, but it also has an integrated Arduino processor on the board along with the Arduino development software and the GPIO on the LattePanda to drive the BPF, PA, T/R switching, and the receive preamplifier/preselector/filter. No kidding – when this is all done this thing is going to be unstoppable. SDRAngel is open source so adding the communications for band switching will not be hard to do between Edouiard’s source in Win32 and to the Arduino through DLL calls. I’m even giving some serious thought about how LimeSDR-Mini will be included as an alternative with an even smaller footprint for this.

Looks like the LimeSDR is slowly starting to all come together as a fully usable system for ham radio thanks to the efforts of people like Marty. Remember that Lime are currently crowdfunding for their latest LimeSDR Mini product, which is a cheaper $139 version of their LimeSDR. Currently almost $85k of the required $100k has already been raised, with still 40 days to go.

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 Radioworld.com 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 titusradio.com.

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.

 

GNURadioCon17 CyberSpectrum Special Meetup Now Live

Live right now is CyberSpectrum #22, currently being held at the GNU Radio Convention in San Diego. Cyberspectrum is an often monthly meetup where SDR enthusiasts come from around the world to share their work. The video will be available offline once the stream is over too. But if watched live you can use the #cyberspectrum hashtag on Twitter, or join the #cyberspectrum on Freenode IRC to discuss the presentations live.

Speakers include:

• The Phil Karn (@ka9q

Low-cost general coverage/HAM receiver for Raspberry Pi & FUNcube dongle, or other cheap SDR.

• Clayton Smith (@argilo

By day, Clayton is a security researcher at ecommerce company Shopify, and by night a GNU Radio enthusiast and amateur radio operator (VE3IRR). He’s worked on projects such as gr-dsd (digital voice), gr-qam (digital television), gr-elster (utility metering), gr-rds (radio data) and sdr-examples. Tonight he’ll tell you about his recent work on HD Radio. 

• Josh Blum (@pothosware

Interesting features in Pothos framework, how we pull in GNU Radio project, and proposed additions to the GNU Radio project. 

• Alexander Chemeris (@chemeris

The first public demo of XTRX – a tiny high-performance miniPCIe SDR for the real world. Up to 120MSPS for simultaneous 2×2 MIMO receive/transmit. 

LimeSDR Mini Crowdfunding: $99 Early Bird RX/TX, 12-Bit, 10 MHz – 3.5 GHz SDR

Back in June 2016 the first LimeSDR crowdfunding campaign completed raising over a million dollars in pre-orders at a cost of $249 – $299 per LimeSDR unit. THe LimeSDR is a RX and TX capable SDR with a frequency range of 100 kHz – 3.8 GHz, bandwidth of up to 61.44 MHz, 12-bit ADC and 2×2 RX/TX channels.

Recently LimeSDR have begun crowd funding for their latest product called the ‘LimeSDR Mini’. This is a smaller and cheaper unit with slightly reduced specifications. The main changes are the slightly restricted frequency range of 10 MHz – 3.5 GHz, and half the maximum bandwidth at 30.72 MHz. The mini also only has 1×1 TX/RX channels.

LimeSDR Comparison

Currently the LimeSDR Mini is being sold on the crowdfunding site CrowdSupply for $139, but the first 500 early bird backer can get the lower price of $99. Accessories such as an acrylic enclosure and set of whip antennas are also available for $40. Crowdfunding is due to end on October 30 and the units are expected to ship on Dec 31, 2017. Note that in the last few minutes that it took to write this article the number of pledges went up by 5 (started at 41), so we’d suggest being quick to claim the early bird if you are interested.

The LimeSDR Mini looks like it could compete favorably with the PlutoSDR, which is another recently released $99 SDR with TX capabilities. Both the PlutoSDR and LimeSDR Mini are 12-bit devices, but the LimeSDR Mini has the larger 30 MHz bandwidth available, and can tune lower. In contrast the PlutoSDR only has a stable bandwidth of about 4 MHz, although it can be pushed higher with dropped samples. The PlutoSDR also has a tuning range (with hack) of 70 MHz – 6 GHz, vs the 10 MHz – 3.5 GHz of the LimeSDR Mini. Another plus of the LimeSDR products is that they are fully open source.

These are exciting times for SDR enthusiasts with cheap TX capable radios now starting to proliferate on the market!

LimeSDR Mini Renderings
LimeSDR Mini Renderings