Introduction to RTL-SDR

RTLSDR Banner Graphic.jpg

What is it?

In the dark ages (more than 5 years ago), the only way to get a visual RF scanner was to shell out thousands for purpose built equipment. Then, through a series of regulatory changes and technological advances, digital TV USB tuners began being sold for less than 20USD online. Hackers more clever than myself quickly determined that the the RTL2832U chip could A) process a massive chunk of the RF spectrum and B) would happily spit that information out. (This is often referred to as Software Defined Radio or SDR. This abbreviation is combined with the RTL chip to give us the RTL-SDR name). One can now purchase a reasonably capable machine for less than even a cheap handheld radio. It requires a bit of DIY hacking and the ability to navigate open source software, but even a person with no amateur radio experience can get a rig up and running in a day. 

Why do you want one? Because it opens the door for all sorts of projects! Want to listen to the local police? Easy! Want to download images from a NOAA weather satellite? Can do! Just curious about what's floating the air around you? Take a look!

In this article I'll be covering how to get started and listen to your local police. In subsequent articles I'll cover plane tracking with ADS-B and NOAA weather satellite image downloading. We may even get to try some SSTV downloads from the ISS if they decide to do more!

RTL2832U with DIY Shielding

Stock Antenna

How do I start?

Hardware

Let's do this! At the very least you're going to need three simple pieces of equipment:

  1. RTL-SDR stick.
  2. Antenna
  3. Laptop

Presumably already have a laptop, or at least a computer. You can use any sort of computer with enough guts, but portability really helps when you want to take your setup somewhere. I strongly recommend this. Think of it like stargazing. You might be able to see things decently from your back yard, but if you move out of town things will improve greatly. In the case of some projects, moving to the roof might be enough. Either way, portability is good. Make sure you have free USB ports. I would also recommend a powered USB extension cable to help move your RTL-SDR away from your computer. This will help cut down on RF noise given off by your computer. 

I recommend this stick. It works great (24-1766MHz) and comes with a workable antenna. The pictures above are of mine, which should be the same model. (I coated mine in foil tape to cut down on interference). One thing to note is that this model has a MCX connector, which is small and uncommon. You can might want to consider finding a model with a larger connector (such as this) if you want to avoid having to build or buy an adapter down the road should you decide to build your own antennas. 

If you want more accessories or would like to geek out about other options, check out this excellent resource. 

Software

There are a many options when it comes to software for RTL-SDR. I am partial to HDSDR, but many other users use SDR#. They each have their strengths and weaknesses. My tutorials will generally involve HDSDR, but be aware that SDR# is an option. We may use it in future tutorials for its plugin abilities. For now we'll just need these two pieces of software:

  1. HDSDR
  2. Zadig

Download and install both programs. I recommend creating a folder where you install all your software and keep all your SDR bits.

HDSDR isn't going to be happy until it can recognize your RTL stick. You're going to need Zadig to properly set up your USB connection to the RTL stick. Follow these instructions to setup this up with Zadig. I've had this be troubling a few times, but stick with it and follow the instructions. Once you've got it going, make sure your RTL stick is plugged in and try opening HDSDR. If you don't get a pop-up complaining about a lack of RTLSDR device you've done it right!

Trying It Out

Once you've got HDSDR fired up and ready, click the Start button! You should see HDSDR spring to life! Let's cover a few basics. The red arrow shows the current tuned area. You can click around on the waterfall display directly, but for big tuning you'll want to use this area. You can change these numbers by click on them with your left and right buttons. 

Now, this is one place people get screwed up on with HDSDR. Look are the big red arrow on the picture below. You see how it says both LO and Tune? LO is the hardware oscillator frequency. There are technical reasons for having these separate, although some programs just take care of this automatically. For our purposes, don't worry about it. Just set your LO to 1 MHz higher than what you are looking for. You can also just look around with the red arrow buttons on either side of the waterfall display's frequency chart.

By now you're probably either hearing nothing or are sick of the constant static. Check out the dial by the big yellow arrow. This is your signal strength. Clicking on it will move the squelch. For those unfamiliar with the concept, a squelch mutes volume below the signal strength you set it at. Play around with this and you'll get a feel for it.

Listening to the Police

That's the basics. HDSDR is full of features and gives your direct control. It may be a bit overwhelming at first, but there's no practice like playing around with it. There are also many Youtube tutorials out there if you want some extra help. Once you feel comfortable poking around, let's try something!

Most smaller police departments in the US don't having anything too clever going on with their communications. Many parts of Europe encrypt their police comms and big US cities uses trunking to work with their limited bandwidth. But if you live in a place like I do (Boulder, CO) you can listen in without any trouble! Either look up the frequency online or poke around until you find it. Generally these things sit in the 130-150 MHz VHF area. More than likely there will be a lot of bands for different agencies. In this case, let's look for the City Police Dispatch, called Blue.

Note (2015/3/6): I recently found out that the designation for this channel has been changed to 'Police 1'.

I found mine at around 156.122. I know from personal experience that their Blue channel is actually 156.145, but RTL-SDR is often a bit inaccurate if you don't adjust the offset. Still, it was in about the right place. If you look at the above picture, the big green arrow is voice transmission on Blue, the city PD's main dispatch and comm channel. The little green arrow to the right is likely a Tac channel, with two units working some situation. Here's a recording from my living room! 

Pretty cool, huh? If you have a HAM radio licence or other handheld police scanner you can already listen to this stuff. But being able to see the whole spectrum allows one to get a visual picture of what's going on. You can find new channels and see where people are talking (or data is being sent) without manually scanning. You can also zoom out and get a visual demonstration of radio interference and harmonics. 

Further Experiments

Once you feel comfortable with the basics, you can move on to more exotic projects. One of my favorites is to download images from weather satellites. You can also track aircraft of all sizes. Some people even download images from the International Space Station. All these projects require a bit more software and custom antenna, but they are totally achievable! Stay tuned for the next tutorial. 


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