Tag Archives: SDR Applications

Ohio Section Journal – The Technical Coordinator – April 2023 edition

One of the responsibilities of the Technical Coordinator in the Ohio Section is to submit something for the Section Journal. The Section Journal covers Amateur Radio related things happening in and around the ARRL Ohio Section. It is published by the Section Manager Tom – WB8LCD and articles are submitted by cabinet members.

Once my article is published in the Journal, I will also make it available on my site with a link to the published edition.

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Now without further ado…

Read the full edition at:

Jeff Kopcak – TC

Hey gang,

For my January article, when I saw KE8VIY’s presentation demonstrating receiving and processing ADS-B aircraft broadcasts using SDRs, it inspired me to explore mine again. I’ve had a request or two to cover them here in the OSJ. There was also discussion about them after my recent presentation for Cuyahoga County ARES.

Software defined radio, or SDR, is a technology that replaces analog components such as mixers, filters, amplifiers, and detectors by implementing them in software. Like most communication systems, an antenna receives the signal passing it to the RF hardware. RF hardware, on receivers especially, is a wide-band VFO with a typical range from a couple MHz up to about 2 GHz. The IF, or baseband, is sampled by an analog-to-digital converter converting the signal to bits. Software is used to control frequency range received and process the digitized signal. Signal processing is often handled by a PC, smartphone, or tablet. However, some have powerful on-board processing. The processed signal is then handed over to the sound interface for listening with ordinary speakers or headphones.

Signal processing could be as simple as selecting AM or FM and listening to an analog radio station. Processing could include a chain software to further process a received signal. Examples would be to decode digital modulation, such as P25 or DMR, then pass human intelligible audio to the sound interface. Other processing may turn received data into text in cases such as ADS-B, POCSAG (pagers), APRS, and packet reception.

Most common are SDR receivers. There are transceivers, like the HackRF One, and many ham radio offerings. Probably the most well-known vendor is Flex Radio because they’ve been doing government work and have been in the ham radio market for a long time. Other ham offerings include Elad, Xiegu, and the big-name radio manufacturers like ICOM, Kenwood, and Yaesu.

I’ve used the “RTL-SDR” for about ten years. These use the low-cost Realtek RTL2832 chipset and commonly sold under the name “NooElec R820T SDR & DVB-T NESDR mini.” Originally designed for DVB-T reception (European digital broadcast TV standard), it was found these could be hacked into a general 3 MHz wideband receiver. These were a focus of my series of articles called “Dongle Bits” published in 2015.

NooElec RTL-SDR dongle (nooelec.com)

NooElec SDRs were inexpensive at $20 each. At that price, I bought a bunch. A pair were almost always in use on my main desktop PC for trunked radio decoding of the statewide MARCS and regional GCRCN radio systems. A couple were in the shack for playing around doing radio things and another couple were in the travel bag mostly for trunked radio decoding while traveling.

A problem with early NooElec SDRs, they were not frequency stable. “Warm-up” time was needed in order to stabilize frequency reception. Sounds like tube radios. Some recommended 45 minutes to an hour for them to warmup. Seemed excessive. I didn’t see much change after about 15 minutes.

PPM offsets for my dongles ranged from the lower 40’s to upper 50’s. When these dongles are used (as intended) to receive an 8 MHz (or so) wide DVB-T broadcast, a couple PPM isn’t going to make a huge difference. Receiving a 12.5 kHz wide P25 signal, 50 PPM will put the receiver on a different frequency.

If I heard police or fire sirens and wanted to fire up Unitrunker with the RTL-SDR dongles, it would be 2-3 minutes before signals could be decoded intelligibly. It would take that long for the dongle to warm up enough to fall within range of its determined PPM. I lived with adjusting PPM/offsets because ten years ago, I was too cheap to buy the TXCO controlled dongles.

Since whenever I started with RTL-SDRs, a number of mine have gotten destroyed over the years. One shattered when I dropped it after Dayton a number of years ago. Another was no longer detected when plugged in. Most recently, one had some kind of firmware corruption which caused the device ID to change making it no longer recognized by drivers.

RTL-SDR blog v3 dongle (rtl-sdr.com)

After Guy’s talk on ADS-B, I went and looked at the prices of RTL-SDR dongles. The price of TXCO controlled dongles had fell to $33. These are quality ones released by the RTL-SDR Blog site available through Amazon. TXCO were $60+ ten years ago when I went the cheap route. TXCO RTL-SDR dongles have an oscillator to stabilize the tuner to within one or two PPM. I bought a round of frequency stable dongles to replace the RIP ones and remaining working dongles. Another advantage to these RTL-SDR.com dongles, they come with an SMA connector as opposed to the MCX connector on the NooElec dongles. SMA is a standard antenna connector used by many ham radio manufactures for handhelds.

TXCO dongles are worth it. Since they’re much lower in price than they used to be, there’s no reason to screw around with ones that don’t have a stable receiver. Now, starting Unitrunker it takes seconds to start decoding P25 signals. Product description indicates there is passive cooling. I will say they do get quite warm.

Some programs I’ve been mentioning throughout: Unitrunker is a program for decoding trunked radio protocols through discriminator (tap) audio or an SDR. It can decode APCO P25, EDACS 4800/9600, Motorola Type II, and MPT1327 systems. One device monitors the control channel, another device is sent to receive voice traffic on other frequencies. Unitrunker can’t decode voice traffic itself. Another program, such as DSDPlus, decodes voice frames.

DSDPlus (often stylized DSD+) decodes multiple digital audio formats including P25 Phase 1, ProVoice, X2-TDMA, DMR/MotoTRBO, NXDN, and D-STAR (no audio). At one point, DSD did decode D-STAR voice frames because I played with it on a D-STAR net and the net controls were quite impressed. Maybe it was an addon or beta that never became part of a final release. I remember compiling it in CygWin, a Linux terminal on Windows before Windows had WSL. DSDPlus offered a “Fastlane” program which afforded more frequent updates and faster access to new features for a small donation.

Both programs have not had releases in years. Unitrunker’s last release was in 2017 and DSD’s last official release was in 2015 – with the last Fastlane update in early 2020. Unitrunker has, what I call an “eternal beta,” version available in a Google Group. I haven’t seen any reference to that Group except in the Radio Reference forum. One must be accepted to the group in order to access the download. When I played with the beta a few years ago, I had problems decoding the MARCS system which I didn’t have using the latest available legacy version.

Another program I started using which does the job of Unitrunker, DSDPlus, some recording & streaming addons is called sdrtrunk. It has a really nice feature set. Though, I think it’s really setup to be a different program than Unitrunker. It was overwhelming first listening to a radio system.

Different talkgroups were coming out of the left and right channels, it doesn’t follow conversations on successive transmissions, displayed talkgroup labels use the Radio Reference Alpha Tag, not Description tags which Unitrunker uses. When a transmission on a talkgroup ends, Unitrunker will linger for subsequent transmissions on the same talkgroup. If there are no more keyups on that talkgroup within a few seconds, it will move on to another active talkgroup. Sdrtrunk will immediately jump to another active talkgroup when a transmission completes – making it hard to follow back-and-forth conversations. It seems like only talkgroups of interest should be loaded and not a whole statewide system. Alpha Tags are shortened descriptions intended for radio displays versus the full Description of a talkgroup. Both are fields in the Radio Reference database. An example of an Alpha Tag is “18-CCAN” which is short for “Community Care Ambulance – Cuyahoga,” the description. Description means a whole lot more to me about the agency. I didn’t find system technical details such as peer sites listed in the program.

sdrtrunk (github.com/DSheirer/sdrtrunk)

One thing I did notice right away, the decoder built into sdrtrunk is much better than DSDPlus. I’ve heard no garbled transmissions. Whatever they are doing to auto adjust for PPM and to decode voice frames works much better. I still like Unitrunker and it’s recommended in the Radio Reference forum for system mapping. If I can figure out these issues, sdrtrunk seems like a better option especially since it is multiplatform and more of an all-in-one solution.

I posted a number of projects using RTL-SDRs on my site over the years, including ones for receiving ADS-B, APRS, and P25 trunked radio. There have been minor software changes to Unitrunker in later versions, but it still works.

Coming up on May the 4th be with you, Technical Specialist Jason – N8EI will be giving a presentation on the GAP Titan vertical antenna. The Lancaster and Fairfield County Amateur Radio Club – K8QIK will be hosting the meeting at the Fairfield County EMA building. Jason will be there virtually but stop by if you’re in the area. Jason has many excellent presentations including ones already presented for the Ohio Section.

Thanks for reading and 73… de Jeff – K8JTK

Dongle Bits: Settings, Programs, & Apps for Software Defined Radio

This article appeared in the The Lake Erie Amateur Radio Association newsletter The Spirit of ’76 and ’88 October 2014 edition and The Wood County Amateur Radio Club newsletter CQ Chatter November 2014 edition.

Read the rest of the series in the Dongle Bits articles category.

Last time on Dongle Bits, I talked about the $20 European TV tuner dongle that was hacked allowing direct access to the signal data. The result is a cheap wideband receiver for your computer. We’re going to take a look at key settings you should know about when using these devices. Then look at some software and projects that transform these into systems that would have cost hundreds or thousands of dollars!

PPM and Settings

An important thing to know about these dongles: they are cheaply made and not tested for accuracy. They are designed to receive DVB-T signals at a bandwidth of 6 – 8 MHz where a few KHz error doesn’t matter. This is obviously not true when you’re dealing with FM signals that are 16 KHz wide or digital at 12.5 where a few KHz will put you on a completely different frequency or channel.

PPM stands for parts per million and is the difference in received frequency vs. frequency shown. To visualize this, use SDRSharp to receive a known FM signal. The center frequency shown will be different from the signal on the scope. Typical PPM offset is anywhere from 45 – 65 and will be in the programs settings. The dongle will drift another 2 – 5 PPM over the next 20 – 45 minutes as it warms up. Gain is obviously another setting that will help you receive signals. The RTL AGC setting works but will err on the side of too much gain. Manually, using more than 32.8 dB will overload and produce duplicate signal spikes. The Correct IQ setting will get rid of phantom spikes at lower gain settings.

PPM at 0
Dongle with no frequency correction. The actual 162.550 frequency is just to the left of the displayed frequency. 162.550 is one of the NOAA Weather Radio frequencies.
RTL-SDR Settings (PPM corrected)
Shows the gain and PPM frequency correction of 55 for the dongle I’m using.
PPM Corrected
Shows 162.550 centered with frequency correction applied.

The crystals on the RTL-SDR dongle can be replaced with higher accuracy temperature controlled crystals (TCXO) that have a variance of 1 ppm! These crystals are $10 but you have to wait for them to ship from China. Pre-modified dongles are available but you will pay three times the price for the dongle.


PCs aren’t the only place these SDRs can be used. They can be plugged into an Android device too. You will need a USB OTG cable (on-the-go) and Android 3.1 or later. Search Amazon or EBay for “USB OTG.” OTG is a standard for plugging in USB keyboards, mice, and thumb drives into mobile devices. Running external USB devices off the internal battery will drain it much faster. A powered USB hub would off-load the dongle power consumption. Apps include SDR Touch (wideband receiver program), ADSB Receiver, and SDRWeather for monitoring NOAA weather alerts on your device.

This is the RTL-SDR running on my Android Nexus 7 tablet with SDR Touch receiving the 146.880 repeater in Lakewood, Ohio. It is connected with a USB OTG cable to the RTL-SDR dongle, then to an MCX to SMA, and then SMA to PL259 adapter.
This is a screenshot of the above setup with SDR Touch.

What can I do with this thing?

The definitive source on all things RTL-SDR is at the appropriately named www.rtl-sdr.com website. This site has it all. They regularly post software, updates, projects, and new developments. There is something new just about every week.

Some features of RTL-STR.com are The Big List Of RTL-SDR Supported Software. This is the list of software packages that support RTL-SDR on all platforms. Software ranges from wideband receivers to single purpose programs. This will give you some ideas of things to try with RTL-SDR. SDRSharp was written to have plugins extend the functionality of the program. These include plugins that make SDRSharp scan frequencies, add an audio FFT, scope, level meter, or CTCSS (PL) detector.

There is an extensive list of projects and write-ups including an Amateur Radio category. Some interesting ones are receiving live NOAA satellite imagery, analyze cellular phone GSM signals, radio astronomy, signal strength heat mapping (foxhunting?), and how Brazil uses our military satellites to transmit SSTV images.

With the onset of many digital standards and narrowbanding, there are more digital signals out there you may not be able to identify by hearing them or seeing them on the waterfall. This Signal Identification Guide has known types, frequencies they may be heard on, mode, bandwidth, sample audio, and waterfall image. I find myself using the Radio Reference database search utilities to help identify signals and their owners (a premium account maybe needed for some features).

My first SDR project was to use the Raspberry Pi as a SDR remote network server. The Raspberry Pi could be placed in an attic or basement connected to an antenna and controlled by another computer.

Audio can be piped from one program into another using Virtual Audio Cable (VAC). Some time ago, during one of the digital nets on the .76 repeater in Cleveland, I used SDRSharp and VAC to receive the FLDIGI messages being passed on the net. The signal path looked like this: received RF signal (146.760) -> RTL-SDR (signal data) -> SDRSharp (audio out) -> Virtual Audio Cable -> FLDIGI (audio in) -> message decoded on screen. If I had a HackRF, I probably would have been able to transmit messages without using any “ham” gear.

The next and probably final article, I will demonstrate tracking airplanes equipped with ADS-B transmitters and listening to trunked P25 public service radio systems for under $100.