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WebSDR

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WebSDR
What it isA receiver anyone can tune through a web browser
Based onSoftware-defined radio receivers connected to the internet
Best-known siteUniversity of Twente WebSDR, Enschede, Netherlands
WebSDR softwareWritten by Pieter-Tjerk de Boer, PA3FWM
Related systems
KiwiSDRNetworked receiver covering about 10 kHz–30 MHz, a few users per unit
OthersOpenWebRX and similar browser receivers
Cost to listenFree; no license needed
Common usesChecking your own signal, propagation, shortwave listening

A WebSDR is a radio receiver connected to the internet that anyone can tune from a web browser. The receiver is a software-defined radio (SDR), which digitizes a wide slice of spectrum and lets software do the tuning and demodulation. The listener sees a scrolling waterfall display of signals, clicks on one, chooses the mode, such as SSB, AM, FM or CW, and hears the audio a few moments later. Hundreds of these receivers are online around the world, and many hams use them every week. Strictly, "WebSDR" is the name of one software package, but the word is often used for any web-based receiver.

The Twente WebSDR

The WebSDR software was written by Pieter-Tjerk de Boer, PA3FWM, and the best-known installation is the receiver at the University of Twente in Enschede, the Netherlands, run with the university's amateur radio club. It covers the whole range from near 0 to about 30 MHz, so every HF amateur band is available at once. Because the hardware digitizes the whole range at once and the server's software produces a separate audio stream for each listener, a single receiver can serve hundreds of people simultaneously, each tuned to a different frequency. A directory of WebSDR sites, many of them run by clubs and individual hams, is kept at websdr.org, and some sites cover VHF and UHF bands or satellite downlinks as well as HF.

KiwiSDR network

The KiwiSDR is a receiver board designed by John Seamons, ZL/KF6VO, that runs on a small single-board computer and covers roughly 10 kHz to 30 MHz. Each unit serves only a few listeners at a time, but because the hardware is affordable, many owners have made theirs public, and together they form a worldwide network of receivers in remote, quiet locations. A public map and list show which receivers are online and how busy they are. The board is a "cape" that plugs onto a BeagleBone single-board computer and pairs a 14-bit analog-to-digital converter rated for 65 MHz with a Xilinx Artix-7 field-programmable gate array (FPGA); the original version served up to four listeners at once. A second-generation KiwiSDR 2, built on a BeagleBone Green in an aluminum case, also supports four users and comes with a GPS antenna. KiwiSDRs include a GPS receiver for accurate timing, which makes possible a direction-finding feature that compares the arrival times of a signal at several receivers to estimate where it came from. Other browser-based receiver software, such as OpenWebRX, works in a similar way.

Finding a receiver

Several directories help listeners pick a receiver. Besides websdr.org and the KiwiSDR public list, Receiverbook maps WebSDR, KiwiSDR and OpenWebRX receivers together and can filter them by band and type; owners can submit new receivers through a form.

The antenna matters as much as the location. The Northern Utah WebSDR, run by Clint Turner, KA7OEI, at a former HF research site near Corinne, Utah, about 60 miles north of Salt Lake City, is a good example. It runs several servers on different antennas: some use an omnidirectional log-periodic, one uses a log-periodic beam fixed toward the eastern United States, Canada and the Caribbean, and another points toward the Pacific and Asia, so a listener can choose the server aimed at the region of interest. Its receivers are a mix of small SDRs, and its S-meters are calibrated so that S9 equals -73 dBm. A separate site covers the 2-meter and 70-centimeter bands, and the station's KiwiSDRs share the same antennas.

Listening to your own signal

One of the most practical uses of a WebSDR is hearing yourself as others do. A ham transmits, then listens on a distant receiver to check whether the signal is getting out, whether the audio sounds natural, and whether there is distortion, splatter or hum. It's a quick way to compare two antennas, see whether a band is open to a particular region, or check an SSB transmitter's microphone gain and speech processing settings.

A few points make this work better:

  • Expect a delay. Audio usually arrives a second or more behind real time, because it travels through the internet and the browser buffers it.
  • Use headphones. Playing the WebSDR audio through a speaker near the microphone creates feedback, and transmitting a distant receiver's audio back onto the band is poor practice.
  • Choose a receiver at a useful distance. A receiver too close may hear the signal by ground wave; one a few hundred to a few thousand miles away shows how sky-wave propagation is treating the signal.
  • Remember others are listening. Tune carefully, keep test transmissions short, and follow normal identification rules.
To check whether your signal is getting out on CW or digital modes, combine a WebSDR with the Reverse Beacon Network or PSK Reporter. The automated spots show where you are heard, and the WebSDR lets you hear how you sound.

Other uses and limits

WebSDRs let newcomers explore the bands before buying equipment, and they are a staple of shortwave listening: broadcasters, aviation and maritime stations, time signals and amateur nets can all be heard. Hams also use them to monitor a net they can't hear at home, or to check reception from another continent before calling a DX station. Since receive-only listening needs no license, anyone can use them.

Contacts made by hearing the other station only on someone else's receiver are generally not accepted by contest and award programs, so check the rules before relying on one. Popular receivers fill up at busy times, and public receivers sometimes go offline without notice.

See also

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