How to Choose an SDR for Recording Public Safety Radio

If you live somewhere with an active police, fire, or EMS radio system, you can listen to it and record what happens there. The hardware costs less than most people expect, and the technical bar has dropped considerably over the past few years. This guide walks you through how to assemble the right setup, in plain language, without selling you anything you do not need.

One thing worth knowing before you start: if you stand up a feed and contribute it, you get free access to Roger Plus for as long as your feed stays active. Your captures also surface in the Roger app, where you can see what is happening in your area in plain English, with the location and context already worked out. That perk is real and worth knowing about. The rest of this guide is about the hardware, so you can make a sound decision either way.

What you are trying to do

Most police, fire, and EMS agencies in the United States communicate over a radio standard called P25. Older systems behave like a conventional radio: one frequency, one channel, tune in and listen. P25 is different. It is a trunked system, which means a set of agencies shares a small pool of frequencies, and a control channel acts as a traffic director, assigning a frequency to each conversation as it occurs.

To record from a system like this, your equipment has to do two things simultaneously:

• Monitor the control channel continuously, so it knows what is being assigned where.
• Follow each assignment to the correct voice frequency quickly enough to capture the beginning of the conversation.

That is the entire job. Everything in this guide concerns selecting gear that performs both functions reliably.

There are two flavors of P25 you will encounter. Phase I is older and simpler. Phase II is newer, more spectrally efficient, and more demanding on your computer. You do not need to know more than that right now. The distinction starts to matter when we reach the section on choosing a host computer, because Phase II asks more of the machine performing the decoding.

Why not just buy a police scanner?

This is the first question most people ask, and it deserves an honest answer. You can absolutely buy a Uniden SDS100 or SDS200, or a Whistler TRX-2, and listen to your local public safety traffic. Those are capable products and they do their job well.

But they are built for a person sitting in a chair with a speaker. They are listening tools.

What you are building is a sensor. It runs continuously, captures every transmission rather than only the ones you chose to monitor, tags everything with timestamps and talkgroup information, and uploads it. You can technically rig a conventional scanner to do some of this with extra cables and software, but the audio path is constrained by the scanner's own hardware, and you spend the entire process fighting the device's intended use. The software-defined radio setup in this guide costs roughly the same as a good scanner and does a far better job at this specific task. You are not buying something to listen to. You are building something that watches and remembers.

The parts you need

Here is the whole signal chain, top to bottom:

Five hardware choices, in descending order of how much they matter: antenna, cable, SDR, computer, software.

A warning that catches many beginners off guard: the antenna and cable matter more than the SDR. A cheap SDR with an excellent antenna will outperform an expensive SDR with a window-mounted whip. Spend your attention in the right order, top of the chain first.

Picking the SDR receiver

An SDR, or software-defined radio, is a small USB device, usually about the size of a flash drive, that converts radio waves into data your computer can process. Four products merit your attention, and one to avoid.

RTL-SDR Blog V4 — the standard starting point

The RTL-SDR Blog V4 runs $39.95 with the antenna set, or $29.95 dongle-only direct from the manufacturer (price as of May 2026). It has a feature called a TCXO that keeps it from drifting off frequency, which is critical for trunked systems. Without it, you lose the control channel and start missing calls. Most beginners run two of these on a powered USB hub: one stays on the control channel, the other follows voice channels. This handles most local police, fire, and EMS systems running P25 Phase I. The one genuine limitation is that in dense urban environments with many strong nearby signals, the cheaper hardware can be overwhelmed.

Nooelec NESDR Smart v4 — the equivalent alternative

The Nooelec NESDR Smart v4 is essentially the same device as the RTL-SDR Blog V4, priced in the same $35 to $45 range, with a slightly different community and documentation around it. Either one works. Pick the one whose guides you find easier to follow.

Airspy Mini — a real step up

The Airspy Mini runs $99.00 (price as of May 2026, official ITEAD US store). It can observe a wider slice of the radio spectrum at once, which means a single Airspy can do the work of two RTL-SDRs and handle busier systems better. A trunked system often spreads across several megahertz of spectrum, and the Airspy can watch all of it at once where two cheaper dongles cannot. If your local system is P25 Phase II or covers a large metro area, this is worth the step up.

SDRplay RSP1A — the weak-signal specialist

The SDRplay RSP1A sits in a similar tier to the Airspy Mini at roughly $109 to $120, slightly better at pulling in weak signals, but the software support for trunked decoding is a bit less polished, and its drivers are closed source, which matters for a Linux setup you intend to run untouched for months. If you also want to use the same device for shortwave listening, the RSP1A is a good pick. For a dedicated feeder node, the Airspy Mini is the simpler choice.

HackRF One — skip it for this

You will see the HackRF One recommended online, but for P25 it is actually worse than the cheaper RTL-SDRs. The way it samples produces worse performance in exactly the situations public safety monitoring puts it in, and its wide front end is easily overwhelmed by strong nearby signals. It is an excellent tool for other radio projects. Not this one. Anyone recommending it for a P25 feed has not run it against a busy system.

The short answer. Two RTL-SDR Blog V4 dongles if your local system is Phase I and not too busy. One Airspy Mini if it is Phase II or covers a major city. If you are unsure which one your area runs, look it up on RadioReference, covered further down.

Antennas and cables

This is the part most people skip and then regret. The receiver is the cheap part of this project. The antenna and the cable connecting it to the receiver are where the real quality lives or dies.

Antenna types

For most beginners covering a mix of frequencies, a discone antenna is the right pick. A discone covers a huge range of frequencies on one antenna, which is exactly what you want for a system that might use VHF, UHF, and 700/800 MHz all at once.

• The Tram 1411 is the standard discone in the hobby, around $79.95. Roughly 44 inches tall, covers approximately 25 to 1300 MHz, holds up well outdoors. A solid first antenna.
• The Diamond D130J is the higher-end version at $129.95. Stainless steel construction, tighter build, costs more but lasts years longer outdoors.
• The Tram 1491 "Super Discone" is larger and picks up the lower VHF frequencies better. Worth it only if your local system still uses VHF-low.

If your local system runs only on 700/800 MHz, which is increasingly common with modern police and fire systems, a small antenna tuned specifically for those frequencies will actually beat a discone on those bands by a meaningful margin. These run roughly $25 to $40 and stand about three and a half inches tall with four short radials. The trade-off is that you give up everything else. If you know your target is 700/800 MHz only, the tuned antenna is the better radio choice. If you want flexibility, the discone is correct.

Cables

The cable matters more than you would think. At the high frequencies most public safety systems use, cheap cables such as RG-58 or RG-6 lose a great deal of signal over even short runs, enough to cancel out everything a better antenna gained you. If your cable run exceeds 25 feet, use LMR-400, which runs around $45 for a 50-foot N-male assembly. For runs over 100 feet, look at LMR-600. Every adapter and connector adds a little loss too, so keep the chain short and clean. Plan the cable path before you mount the antenna.

Lightning protection

If the antenna goes outside, you need a lightning arrestor in the cable line before it enters the building. This is not optional. Electrical code requires it, your insurance company expects it, and a small inexpensive part genuinely prevents a house fire. The PolyPhaser brand is the standard; a suitable coaxial arrestor runs roughly $120 to $135, though budget gas-tube arrestors from other makers start near $22. Bond the antenna mast to your building's ground. Do not skip this.

Filters and amplifiers

If you live close to a cell tower, a public safety bandpass filter covering roughly 769 to 872 MHz will clean up your reception considerably by blocking the cellular signals trying to overload your receiver. An amplifier, called an LNA, placed right at the antenna can help recover signal lost over a long cable run, but used incorrectly it makes overload problems worse rather than better. If you add filtering and amplification, put the filter before the amplifier. If you are unsure, leave the amplifier off until everything else is working.

The computer that runs it

The SDR plugs into a small computer that performs the actual decoding. A modern four-core processor with 8 GB of memory handles most P25 systems with two cheap dongles. The decoder software needs a recent version of Java. A solid-state drive helps with writing audio logs. Beyond those basics, the real choice is between a Raspberry Pi and a small mini-PC, and it comes down to how busy your local system is.

Raspberry Pi 5 (8 GB) — the default

A small, low-power computer that works well for most home setups. It reliably handles two to three simultaneous conversations, which covers most suburban and small-city systems. Get the active cooler (a small $5 fan), because it runs warm under continuous load. Power draw is low, under 15 watts. The downside: if you live somewhere with a genuinely busy system, a major city with many agencies and six or more conversations running at once, the Pi can fall behind and start dropping calls. Step up to the 16 GB board only if you intend to run several services on the same machine.

Intel N100 mini-PC — the performance option

A small desktop computer, from brands such as Beelink or Topton and typically $140 to $180, comfortably handles six to eight simultaneous conversations within a tight power budget. Worth the additional money if you are capturing a major metro area, or if you want to monitor more than one system from the same machine.

Do not use a laptop as your permanent setup. Laptop USB ports often cannot reliably power multiple dongles, laptops sleep at the wrong times, and they are not built to run continuously for months. A laptop is fine for the first day or two while you test. After that, move to dedicated hardware.

The software

The software is free. You have a few choices, and each has its own character.

• SDRTrunk. Free, runs on Windows, Mac, and Linux. Has a graphical interface, extensive documentation, and a large, active community. Decodes P25 Phase I and Phase II, plus DMR, NXDN, and several others. This is what most people should start with.
• Trunk Recorder. More of a command-line tool, free, and very efficient with system resources. The right pick if you want a node that simply runs and uploads without you ever looking at it. A little harder to set up the first time, easier to maintain once it is working. This is the kind of tool that runs unattended for months.
• DSD+ Fastlane. Paid, Windows only. Good for desktop listening, not really built for an always-on feeder node.
• OP25. Free, Linux only, runs from the command line. Powerful and efficient, but the setup has a steep learning curve.

For a first build, use SDRTrunk. If you later decide you want something leaner that just runs in the background, you can switch to Trunk Recorder without buying any new hardware.

Finding your local frequencies

You need to know what frequencies your local agencies actually use before any of this will work. Three places to look:

• RadioReference.com is the main one. It is a user-built and user-maintained database covering most of the active systems in the country. A free account lets you look things up; a paid account adds bulk export and a few extras. Search your county, find the trunked system, and look for the control channel frequencies, marked with a "c." Note the system type as well, since that tells you whether you are dealing with Phase I or Phase II. We have no formal relationship with RadioReference, but it is the most complete database that exists, and you can use it freely for your own purposes.
• FCC ULS is the government's licensing database. It is the most authoritative source, but harder to search than RadioReference. Useful for double-checking when a system changes and the community database has not caught up yet.
• Local ham radio clubs. An underrated resource. People who have been listening in your area for years know which talkgroups are still active, which ones were encrypted last year, and which sites have coverage gaps. A coffee with one of them can save weeks of guessing.

What if your area is encrypted?

This is the question worth answering before you spend anything, and most online guides duck it. We will not.

Some police departments have moved to encrypted radio over the past few years, and encrypted traffic is unreadable. No software, no antenna, no amount of hardware changes that. Modern P25 encryption is genuinely uncrackable for anyone outside law enforcement. If your target is fully encrypted, you should know that before spending a dollar.

A few honest points about this:

• Check first. Look up your local system on RadioReference. If everything you care about is marked "encrypted," a feeder node will capture metadata but no voice from those channels.
• Encryption usually is not total. Most agencies that encrypt police patrol channels leave fire, EMS, public works, school district, transit, and inter-agency channels in the clear, because too many partners need to talk across them. Those unencrypted channels carry a large share of what is actually happening in your community on any given day, and they are often the channels first responders themselves monitor outside their own department.
• Even encrypted systems leak useful information. The control channel itself is not encrypted. You can see which talkgroups are active, when activity spikes, and how long calls run, even when you cannot hear the voice. That pattern data is more useful than people give it credit for, and it is part of what the Multido pipeline derives value from.

Is this legal?

Yes, in all 50 states, with some local fine print.

Federal law, the Communications Act of 1934, makes the radio spectrum a public resource and allows you to listen to anything broadcast in the clear. The Electronic Communications Privacy Act of 1986 makes it illegal to intercept encrypted traffic or cellular calls, but everything broadcast openly is fair game.

A few states add restrictions on using a scanner in a moving vehicle. Florida, Indiana, Kentucky, Minnesota, and New York require a permit to have a scanner in your car. These laws are old and mostly unenforced against ordinary hobbyists, but they are on the books. If you live in one of these states and want a mobile setup, look up the permit process or stick to a fixed home install. California, New Jersey, Michigan, Oklahoma, Rhode Island, South Dakota, Vermont, Virginia, Nebraska, and West Virginia add extra penalties for using a scanner while committing a crime. These do not restrict normal, lawful use.

A fixed setup at your home, listening to unencrypted public safety traffic, is legal everywhere in the United States. This guide is general information, not legal advice. If you have specific questions about your state, talk to a lawyer licensed there.

What happens when you start contributing

Once your node is up and feeding, several things happen automatically.

Every transmission gets transcribed, located on a map where possible, and matched to its talkgroup. Patterns get recognized: a fire department working a structure fire, a police pursuit crossing jurisdictions, an EMS call escalating into a major incident. All of this surfaces in the Roger app, both for you and for everyone else in your area using it.

We are live across four states and five active systems, processing more than 45,000 transmissions a day. First responders in those areas use what gets captured while they are on the job. That is not a marketing line; it is simply what is happening, and it happened organically, without anyone formally inviting them, because the feed is faster and clearer than what they had. When you stand up a node, your area becomes part of that.

You also get free Roger Plus access for as long as your node is actively contributing. Run a node, get Plus free on that basis. Plus gives you the full feature set in the consumer app: deeper search, broader area coverage, and the more detailed incident timeline.

The bottom line

If you want to learn how the whole stack works and you have a few weekends to spend on it, this path is genuinely rewarding. Start with two RTL-SDR Blog V4 dongles, a Tram 1411 discone, a 25-foot run of LMR-400 with a lightning arrestor, an 8 GB Raspberry Pi 5 with the active cooler, and SDRTrunk. Spend the most time getting the antenna placed right. Everything else follows from a good antenna.

Either way, your feed makes the picture clearer for your community. That is the real point of all this.