How To Use A WebSDR

A WebSDR may seem complicated at first. There are so many things to cover that whole books were written on the subject. This tutorial will not cover a whole lot of it but will get you started with the basics so you can at least hunt down, fine tune and listen to different signals across the radio frequency spectrum.

WebSDR interfaces do vary from one site to another. Only the most common and basic functions will be covered here. Two sites that will be used in the tutorial are the one at the University of Twente in Enchede, The Nethrlands and a local one via SDR.hu (these use OpenWebRX).

The Waterfall

When you first go to a WebSDR page (you may need to scroll down) you will see what is commonly called a waterfall similar to the image below:



These are the frequencies and signals. The lighter or more prominent the color, the stronger the signal. Some waterfalls may differ in color but the concept (and navigation) is basically the same.

To navigate, you click and drag left or right inside the waterfall or (above it where the tabs/tags are in OpenWebRX based radios). Note that you may need to zoom out and in to get this navigation to work the first time each time you visit the page. To zoom, hover the mouse over the waterfall part and scroll with the mouse wheel (or equivelent). This is useful for getting to a certain frequency and narrowing in on a signal.

Frequencies: Tags and Labels

At the bottom (UofTwente) or top (OpenWebRX based radios) there are labels (or tags). These tell you what band the frequeny is in and shows you what frequencies are being viewed.

UofTwente:



OpenWebRX (SDR.hu):



Note that the text pointing to frequencies, bands and labels do not appear on the site, but were added for illustration.

In the top example (UofTwente), you can see the frequencies and bands at the bottom of the waterfall. If you zoomed in on the waterfall, then you would also start to see the labels between the frequency and band notations.

In the OpenWebRX example, these items are all above the waterfall.

The labels can be quite useful. Looking at the OpenWebRX example above, you can see on the far right a label for CB19 which is the frequency for the Citizens Band Radio (CB) Channel 19 that Truckers normally use. If you were to zoom into that area on the waterfall, you would see the signals on that frequency become more prominent.

Bands

As illustrated above, you can see different bands. As you navigate through the waterfall left or right, you will see the bands change. Such as 60m (for 60 meter), 49m (for 40 meter), and so forth. These give clues into what type of radios are used to receive these bands. 10m, in the OpenWebRX example above is the Amateur Radio band, and includes Citizen's Band radio. Certain frequencies only fall into certain bands.

One thing that may be confusing at first is that there is another type of band you also will need to be aware of. These refer to things like Upper Side Band (USB) or Lower Side Band (LSB) or CW (Morse Code). These are band modes as discussed below.

NOTE: There are arrows in the controls on the OpenWebRX radios which let you slide the controls in or out to free up room and get a better view of the waterfall.

Band Modes

Band modes let you further fine tune a signal. Sometimes a signal might not be audible or may be distorted until you switch to a different mode.

The LSB, USB, CW and other band modes can be switched under the waterfall in the UofTwente site:



In the OpenWebRX interfaces, these are in the side control panel:



You probably already are familiar with AM and FM bands.

AM is primarily best for use with voice, such as talk shows but there are also some music stations as well.

FM has more support for music, including stereophonic music.

CW (Continuous Wave) is what Morse Code is usually found on, however it also could be found on LSB bands as well.

LSB stands for Lower Side Band. This is what a lot of data and even Morse Code (as mentioned previously) is found on.

USB stands for Upper Side Band. This is generally used for voice transmissions but other types of transmissions might be heard on this band.

How to Fine-Tune a Signal

The waterfall usually shows a lot of signals going on and many times they seem all clumped together. On your speaker (or headphones) you hear a lot of static. You may hear what might sound like digital modems (beeps, buzzes, squeaks and high-pitched ringing or static), or beeps (as is the case with Morse code) or even garbled or robotic sounding voices. You can use the cursor to tune into different parts of the waterfall.

To use the cursor, click on the center line and drag it around. The size or span of the cursor may be larger or smaller depending on what band (ie. LSB, USB, CW, etc.) you selected. You can adjust the size of the cursor by clicking on the left or right slope and draging it away from or towards the center line. In some cases the slopes are limited in how far in or out they can go. What this does is lets you hear the signal that is within the slopes of that ursor. This is great for picking out, say, a morse code signal out of other signals that are on frequencies or bands close to it.

The cursor is usually found around the frequency markers (labels). Either at the bottom (UofTwente) or top (OpenWebRX).

Bleadingis when a signal bleeds through (ie. can be heard on) another band. For example, suppose a strong signal is being broadcast on the CW band. You may even hear the beeps on the USB or LSB band as well. When tuning in, you may need to switch bands until you get the best quality signal.

Using what was covered thus far, you can now find and narrow down any signal on the waterfall. See Identifying Signal Types for information on the more common types of signals you may find.

Identifying the WebSDR Radio

Most of the time, the WebSDR radio information is on the top of the site or on the main page of the site. Let's take a look at two of them.

UofTwente (The Netherlands)

They have information at the top of the page but it doesn't give too much information other than where they are located. They do not give their call sign, that I could see.

OpenWebRX (SDR.hu)

When you got to SDR.hu they generally have the receiver's location in the listing. If you click on a receiver to monitor, you'll see a contacts box on the left inside the waterfall (or the lower arrow on the left side of the waterfall). This gives you information on the receiver. If you wait a few moments after the page loads, some of the top images slides up. Then you'll see more information. Clicking onT the Map link will pull up a Google Maps with a pin at approximately where the receiver is located. The call sign is after the text Software-defined receiver at but it does not link to the receiver's info. You can look up the call sign for the receiver at QRZCQ.com or the QRZ Callsign Database.

Locating the Source of a Signal

The above information on Identifying the WebSDR Radio above is not going to tell you where a signal in the waterfall is transmitting from. This only tells you the location and information of the radio that is recieving the signal. Trying to find where a signal is transmitting from is tricky and may even be impossible in most cases. However, you could get some clues as to the general location in the following ways:

Locating the source of a signal may be useful if you are trying to relay to first responders the location of someone calling for help (we don't always know if someone else is listening in and hearing if nobody is responding to that person's transmissions). However, it can be quite frustrating and not a good idea to jump the gun if you are not at least 95% sure of where the transmission is coming from. First responders will even tell you that they can't go on just a signal off a radio on the internet.

In emergency situations, always be sure you know for certain you have all the exact details needed to get help to the person who needs assistance. In the USA, it can be a crime to call for help on the behalf of someone who doesn't need it or if you don't know where to send the help. Be careful in these situations!

Outside of general curiosity, there really shouldn't be any reason to know the exact location of a transmitter or receiver. And in that case, just the country (or maybe state or general part of the country) should be sufficient.