For those starting to wade into radio as a hobby, one of the first real technical challenges is understanding trunked radio systems. On the surface, it seems straightforward: A control channel allows users to share a section of bandwidth rather than take up one complete channel, allowing for greater usage of the frequency range. In practice though it can be difficult to follow along, but now it’s slightly easier thanks to software defined radio.
This guide comes to us from [AndrewNohawk], who is located in San Francisco and is using his system to monitor police, fire, and EMS activity. These groups typically used trunked radio systems due to the large number of users. For listening in, nothing more than an RTL-SDR setup is needed, and the guide walks us through using this setup to find the control channels, the center frequency, and then identifying the “talk groups” for whichever organization you want to listen in on.
The guide goes into great detail, including lists of software needed to get a system like this started up, and since [AndrewNohawk] is a self-identified “radio noob” the guide is perfectly accessible to people who are new to radio and specifically new to trunked systems like these. Once you get the hang of it, it’s not too hard to scale up, either.
With the incredibly low cost of software defined radio (SDR) hardware, and the often zero cost of related software, there’s never been a better time to get into the world of radio. If you’ve got $30 burning a hole in your pocket, you’re good to go. But as with any engrossing hobby that’s cheap to get into, you run the risk of going overboard eventually.
For example, if the radio gear inside your car approaches parity with the Kelly Blue Book value of said vehicle, you may have been bitten by the radio bug. In the video after the break, [Corrosive] gives us a tour of his antenna festooned Hyundai Accent, that features everything he needs to receive and analyze a multitude of analog and digital radio signals on the go.
He starts with the roof of the car, which is home to five whip antennas (not counting the one from the factory installed AM/FM radio) and two GPS receivers. The ones on the rear of the car feed down into the trunk, where a bank of Nooelec NESDR RTL-SDR receivers will live in a USB hub. He’s only got one installed for test purposes, but he’ll need more for everything he’s got planned. Also riding in the back is a BCD780XLT scanner, which he got cheap on eBay thanks to the fact it had a dead display.
Luckily, where [Corrosive] is going, he won’t need displays. The SDR receivers and the scanner are all controlled from the driver’s seat by way of a Windows 10 tablet. This runs the ProScan software that provides a virtual interface to the BCD780XLT, as well as various SDR interfaces. He’s also got Gpredict for tracking satellites and ADS-B programs like Virtual Radar.
The car’s head unit has been replaced by a rooted Android entertainment system which supports USB host mode. [Corrosive] says it isn’t hooked up yet, but in the future the head unit is going to get its own SDR receiver so he can run programs like RF Analyzer right in the dashboard. We’re willing to bet that this will be the only car in the world that has both a waterfall display and the “Check Engine” light on at the same time.
Even if you aren’t ready to install it in your car, you might like to read up on using multiple SDR receivers for trunked radio or setting up your own ADS-B receiver to get a better idea of what [Corrosive] has in mind once everything is up and running.
Continue reading “Building An SDR Lab With Wheels” →
In the old days, if you wanted to listen to police, fire, or other two-way radio users, you didn’t need much more than a simple receiver. Today, you are more likely to need something a little more exotic thanks to the adoption of trunked radio systems. To pick up the control channels and all the threads of a talk group conversation, you might need a wide bandwidth receiver.
[Luke Berndt] found he needed 6 MHz to monitor the stations he wanted to hear. This is easily in the reach of dedicated software defined radios (SDR). However, [Luke] wanted to use cheap RTL-SDRs and their bandwidth is about 2 MHz. The obvious hacker solution? Use three of them!
If you haven’t looked at a trunked system before, it essentially allows a large number of users to share a relatively small number of channels. When someone wants to talk, they move to an unused channel just for that transmission. Suppose Alice asks Bob a question that happens to be on channel 12. Bob’s reply might be on channel 4. A follow up from Alice could be on channel 3.
In practice, this means that receiving the signal isn’t difficult to decode. It is just difficult to find (and follow as it jumps around). This is an excellent job for multiple SDRs and the approach even reduces the burden on the CPU, which doesn’t have to decode signals that aren’t essential to the conversation.
[Luke] includes source code and also notes how to change the serial numbers of the dongles since each has to be unique. We have seen so many great projects with the RTL-SDR that it is hard to choose our favorite. It is especially great knowing that the dongle was only meant to receive television, and all these projects are hacks in the best sense of the word.
Thanks [WA5RRior] for the tip.