OpenWebRX - SDR

Edit, 11th August 2024: This system is currently offline.

1. Introduction

Software Defined Radio (SDR) is a way of turning a computer, phone or tablet into a radio receiver, covering a wide range of bands, modes and typically, a frequency range of 0 - 2GHz.

For those not involved in the hobby, most of that probably means very little. This post will explain a little about what this means then how to access an online SDR that is operating from my home.

1.2 SDR

SDR is a catch-all term describing a means of accessing radio signals through a device plugged into a computer or other device.

Once signals are received, they are "decoded" by the computer and produce sound or data depending on what is being received.

Advantages of SDR are:

• Wide frequency range - covering 0 - 2GHz (others cover different ranges but these will be useful for most users).
• Cheap - I use two of these and find them excellent receivers - these kits come with a basic antenna setup which is often "good enough" for general reception. Compared with the hardware alternatives for what these can do, they represent excellent value for money, users might need to spend thousands of pounds on dedicated equipment to get a similar result.
• Versatile - In the simplest guise, using these to receive broadcast FM radio just requires installation of the dongle, the antenna attached and a free SDR application installed on the computer. For those interested, I use the excellent SDR Console. Exploring further, analogue voice signals operating on various modes can be received very simply by recognising what you are turned to and selecting the appropriate mode. This can also easily be used to decode many data signals - from ADSB (aircraft tracking) to Digital Voice signals such as DMR, "true" data modes such as RTTY, faxes, FT4, FT8 and just about anything else you can think of. Even the expensive hardware radios need to be connected to a computer to make use of some of these. 

In short, SDR is a receiver for a wide range of signal types that is cheap, easy to work with and accessible to anyone with a computer, phone or tablet. 

1.3 Bands

The radio spectrum is huge and contains users of all kinds - from amateur operators to the military, commercial use, civilian and military airband, satellites and many more applications and users.

To manage this, the spectrum is divided into bands. Perhaps the most familiar "band" is the FM broadcast band - this is around 87.6MHz to 107.9MHz but this is a very narrow portion of the spectrum - covering just 1% of what is able to be received by an SDR device.

Bands tend to be allocated to user types and modes - for instance, amateurs and the military won't transmit on the FM Broadcast band and Broadcasters won't encroach into military bands or those designated for other use.

There are a huge amount of "bands" allocated but the basic principles are:

A frequency range designated to a particular type of end user. Within these bands, they can be further sub-divided into different modes of use (data, voice and so on).

More information on the UK band allocations can be found here.

Amateur users operate on a number of different bands which are broadly grouped into the following:

HF - Generally long distance - with the right conditions, can easily reach around the world. Signals are "bounced" off the ionosphere, ranging in 200km-400km above the earth.

VHF - Less range - manly "line of sight" between transmitter and receiver.

UHF - Similar to VHF.

Often, bands are spoken of in terms of metres, for example, the 40m amateur band covers 7.0Mhz to 7.2MHz - a tiny fraction of the overall spectrum but, the radio wavelength in this band is roughly 40m from peak to peak.

1.4 Modes

Continuing with the example of the 40m band, we can operate different "modes" within the range of 7.0 - 7.2MHz. The example below is taken from the 40m band plan showing what amateur users can do on each frequency within the band.

We can see that, for ease, we are now talking in terms of KHz rather than MHz. For example, 7.1Mhz is now 7100KHz - its the same thing just using different unit to make smaller values easier to work with.

So what do these all mean?

CW - Continuous Wave (Morse code).

Data - Typically FT8, RTTY or other data modes used to send and receive by amateurs. More on this shortly.

SSB - The bands allocated for amateur use are often quite narrow with a lot of users - particularly when considering the worldwide capabilities of HF. SSB is a way of making analogue voice transmissions more efficient, allowing more users to operate in the frequencies allocated. 

For those who are interested - SSB is essentially half the bandwidth (space occupied in the band) of an AM transmission. 

1.5 Summary

Becoming familiar with the radio spectrum, allocations, propagation characteristics of "bands" and what the various modes are, how to work with them, transmit and receive, are massive subjects in their own right and learning about and experimenting with them is part of the broader hobby. 

It is sufficient, for the purposes of working with SDR to understand:

• The radio spectrum is a continuous set of frequencies allocated to many users for different purposes.
• Whilst we are talking of 0 - 2GHz, the spectrum (and allocations) continues way beyond this - up to 300GHz.
• Bands are usually subdivided for different uses within the core allocation.
• Modes are different ways of operating on radio frequencies and will vary depending on user type but broadly consist of voice or data transmissions which need to be decoded to make sense of them.

2. Web SDR Concepts

Having discussed some basic principles, we can now look further at Web SDR and give some examples as to how to explore the bands and modes. Once you've tried some of these, it should enable you to explore further and see what you can find.

2.1 Access

The Web SDR can be accessed on this link and you should see something like this when you open the site:

This is the main screen for accessing the web SDR and it is important to become familiar with the three concepts highlighted.

2.2 Waterfall - This is a view of what the system is receiving over its antenna in the frequency range selected (in this case, 13MHz - 15MHz). Each line represents a different transmission, many of which are packed very close together.

Points to note: 

• This is a view over time, with the most recent signals showing at the top and the oldest at the bottom, for each frequency.
• It is recommended that you zoom in to the waterfall using the scroll wheel on your mouse. This is done by moving your mouse to the area you're interested on the waterfall and zooming in. 

In the example below, I have zoomed into an AM signal (We know this is AM due to the strong signal in the centre with the variation in signal on either side - note that both sides mirror each other).

Becoming familiar with how different modes appear on the waterfall takes time but also makes life easier when deciding how to decode the signal. More on this when we discuss the control panel.

2.3 Frequency Range and Bookmarks - Much like tuning in any radio, it has to be set to a frequency, the bar highlighted for this shows where we are in the spectrum. Bookmarks are pre-loaded frequencies with the relevant mode pre-set. 

In short, if you see activity under these bookmarks, you can click them and it will set the relevant mode, decoding etc and should be good to go.

Note: Transmissions can come and go so it is possible that the signal will not always be what the bookmark states, this is unavoidable for various reasons but in short - if nothing seems to come through when you've selected a book-mark, probably best to explore another.

2.4 Control Panel - This is the main means of control for receiving signals and decoding them and is worth exploring in some detail.

From top to bottom.

The current frequency the SDR is tuned to.

The drop-down menu with pre-configured bands. In this example, currently Device 0 (there are two dongles attached to my setup, the other is RTL1), is tuned to the 20M band. 

RTL0 is set to cover HF (long distance) frequencies.

RTL1 is set to cover VHF and UHF (line of sight frequencies such as broadcast FM and Airband).

In this section we define the mode being used. 

FM - Typically used on VHF bands for amateur use such as the 2m band.

WFM - Wide-FM - mainly used for FM Broadcast radio.

AM - More often found on the HF bands and mostly used by long distance broadcast radio as with the example above (see waterfall section). Is also used for airband transmissions.

LSB/USB - Lower side-band and Upper side-band - This is an AM signal, cut in half as previously discussed. As a general rule, voice transmissions below 10MHz are on LSB, above 10MHz uses USB.

CW - Continuous Wave, used for Morse Code.

SAM - Never used this so can't comment!

D-Star, NXDN, YSF, M17, FreeDV - These are Digital Voice modes - I will provide an example to listen to digital voice in the examples section.

When we find a digital mode that we want to decode, selecting the appropriate decoder from this list will set the SDR up to receive that signal.

At the time of drafting this, I have it set to fax and tuned into 13.8550MHz which is bookmarked as DDK6 - Hamburg. This is currently receiving a weather fax chart from Germany.

When used with bookmarks, this will begin itself but can be manually set if you recognise what is being transmitted and set the mode manually. There are many in the list and knowledge of these comes with time and experimentation.

There are three main functions in the controls section.

Top left is the volume control - this is currently set to mute as I'm decoding a digital signal and don't need the audio through my speakers. Click the speaker icon and use the slider to set volume that suits your setup.

Below that is an "SQ" button. This is used to eliminate white noise in-between transmissions. As a general rule, when you tune into a signal, just hit the SQ button and it will take care of this itself. The slider allows for fine tuning if you're getting broken (or no) audio.

To the right of this is the "A" button. As a rule, when you select a new frequency on the waterfall, just hit this button. It will ensure the waterfall is as clear as possible (adjusts colours relative to signal strength).

2.5 Summary

Becoming familiar with the basic concepts in this section will enable users to begin to explore the RF spectrum and start hearing audio or decoding data.

Given my configuration and activity on the bands, you are most likely to find more to look at in the HF bands (RTL0 frequencies from the drop down menu).

It is important to have some understanding of the layout and navigation and control options within the system but broadly speaking, just browse around and find different signals, become familiar with how they operate and can be decoded.

3. Examples to get started

Having explored the basic concepts behind the radio spectrum and the software itself, it is probably worth providing some examples as to how this may be used in a practical sense.

3.1 Tracking Aircraft (ADSB)

Most civilian and military aircraft in the world now carry ADSB transceivers. If you have ever used Flightradar24 or any of the other flight tracking websites, broadly speaking - this is how they receive the signals.

Sites such as FR24 work on the basis of many receivers around the world feeding into their system as range for each is limited to "line of sight" - that is, relatively short range up to around 100miles depending on factors such as topography and antenna.

This link will load straight into the settings needed to receive ADSB, you should see the following on screen (to the left of the waterfall).

This is a live view of the signals being received from aircraft. In the example above, there is a mixture of data showing location, altitude, speed and bearing.

Clicking on the map button at the top right of the screen will load a visual showing these aircraft plotted as signals are received, on a map.

Alternatively, you can simply load the map from this link.

3.2 Listening to Airband

Airband is the voice element of plane-ground communications. I'm about 40 miles from Newcastle Airport so we do not receive the ground comms but we do hear aircraft on approach or departing the airport.

Newcastle Approach can be received on this link.

It is recommended to hit the "A" button (see 2.4 control panel) immediately to adjust the waterfall. Whilst it can be quiet depending on how much activity is passing through the airport, it generally receives quite well.

If you are seeing transmissions on the waterfall for this frequency but not hearing audio, reduce the slider for "SQ" - you want it just higher than the "white noise" background then, if necessary, check you don't have the audio muted in the SDR or on your computer.

It is also recommended to zoom into the waterfall on the Newcastle Approach bookmark using your mouse wheel so you can clearly see when signals are being received, such as this one below.

When you become familiar with the waterfall concept, you will inevitably spot other transmissions from ATC and aircraft. Feel free to select these by clicking on them in the waterfall.

Note: Civilian Airband is to be found between 108MHz and 137MHz but as a general rule, there isn't much audio below 118MHz as below this is typically used for various beacons. 

Note 2: Airband is divided into 2MHz chunks in the drop down menu in the control panel.

3.3 Amateur Radio - Digital Voice (DMR)

Alongside many pieces of equipment I use for radio, I have a DMR Hotspot which allows me to transmit and receive on digital radio networks operated for amateur users.

The hotspot transmits and receives on 438.8MHz (UHF) and can be received by OpenWebRX.

Click on this link to receive audio from the hotspot.

As a general rule, when not in use, this will be tuned to a global talkgroup (think of it as a channel) which means you may receive transmissions from people anywhere in the world.

As this is a digital mode, you will also see the callsign and name of the person transmitting on Timeslot 2. Timeslot one is not used on this hotspot so will never show any activity.

The waterfall will look significantly different from analogue voice (airband, for example) transmissions.

3.4 FT8 - Signal reports from around the world

FT8 is a digital mode (data) and is used by amateurs to send out signals to other operators around the world and, hold a "qso" (contact) with them. As a general rule, FT8 is a simple exchange of callsigns and signal reports.

It can be quite engaging seeing how far your signal has got and trying to "log" (another subject for another day) a contact in every country in the world which is only limited by one or two countries not allowing amateur radio operators to do so from their territory (North Korea, for example).

This link will load FT8 reception and decoding on one of the HF (long distance) bands (20m in this case) and will display a table to the left of the waterfall.

Some background on what the messages mean can be found here but if we open the map (top right hand side of the waterfall) we will see, in real time, where these signals have come from and, clicking on one of the blue squares will provide more details about the operator and their location.

3.5 Summary

I have included a mixture of different bands and modes in these examples to give users an idea of the kind of things which can be done with this type of setup.

As I think these show, this is a very wide ranging subject and we have only skimmed the surface so far. 

4. Web SDR Setup

For anyone interested in the setup here:

I have a Raspberry Pi 4:

Installed in a metal case:

With two SDR dongles attached:

This is connected to an end-fed antenna for HF (RTL0) and the stock antenna from the RTL kit for VHF and UHF (RTL1). Whilst the latter isn't ideal, it does receive pretty well most of the time.

Software is OpenWebRX+ running on an SD card in the Pi.