A Guide to Listening to CB Radio with an RTL-SDR Dongle

In the June edition of The Spectrum Monitor, SDR enthusiast and ham Mario Filippi N2HUN published an article titled “Your New CB ‘Good Buddy’, the SDR Dongle”. While the CB radio heyday is well and truly over, Mario discusses how an RTL-SDR dongle can be used to have some fun listening to CB without needing to go out and buy a full CB radio. If you don’t know what CB radio is, Mario explains what it is, and its rise and fall in these excerpts:

In the mid-1970’s an early form of social media was sweeping across the country known as CB (Citizens Band) radio. In those years the FCC required CB radio operators to obtain a license, easily gotten by filling out FCC form 505, paying the fee ($20 or $4 depending on what year you applied), and waiting very patiently, usually two to three months for your license to arrive by mail with your call sign.

The concept of wirelessly communicating with others without studying for a licensing exam somehow caught on and was embraced by the American public. As a result, in the mid-70’s CB sets started flying off the shelves by the millions to appease this new insatiable appetite of Americans to talk over the air with their “good buddies” (CB slang for friend). Other major factors played into the oncoming tsunami of CB’ers: gasoline was getting scarce as a result of the recent oil embargo, prices were quickly escalating at the pump, and the Interstate Highway maximum speed was lowered to 55 MPH prompting drivers with heavy feet to communicate the whereabouts of radar-enabled local police (CB slang: Smokies or Smokey Bears) or the cheapest place to fill up. In addition, traffic information such as road conditions, accidents, speed traps and the best greasy spoon location was now available to the commuting public by simply turning on the CB radio and tuning to the trucker’s Channel 19, the epicenter for the latest road-related poop.

By the late ‘70’s there were so many CB’ers congregating on the air causing non-stop channel chatter and ignoring FCC regulations (C.F.R. Part 95) that Uncle Charlie (CB slang for the FCC) eventually dropped the license requirement. The American public now ruled the airways with expanded 40 channel radios and pandemonium. Call signs were replaced by nicknames or “handles” and everyone prided themselves with their own, unique self-descriptive moniker when “ratchet-jawing” (slang for talking a lot) on their CB radio. But when the early 80’s rolled around the public’s fleeting romance with this mode of communication had dwindled markedly and only the diehards remained on the air in happy solitude.

The article goes over several points which may be useful to those who did not play around on CB back in its popular days. He explains how CB radio exists on frequencies between 26.965 MHz to 27.115 MHz and how you should use an appropriate (large) CB antenna, such as an 43 foot S9 vertical antenna. He also notes how CB radio conditions can be affected by ionospheric conditions, and how on a good day (CB is usually open during the day as opposed to the night for the lower frequencies) you can actually receive CB radio from all over the world including Europe, the Caribbean and the US. 

As the article is a part of The Spectrum Monitor magazine it is not free to read, but each monthly edition only costs $3 USD, and comes with multiple articles from other authors too, which makes it quite a good bargain read every month. You can find the June edition at http://www.thespectrummonitor.com/june2015tsm.aspx.

CB Radio coming in with an RTL-SDR and CB antenna on SDRSharp.
CB Radio coming in with an RTL-SDR and CB antenna on SDRSharp.

What’s so special about 50 Ohms?

Hackaday contributor Al Williams has posted an article on Hackaday titled “What’s special about 50 Ohms”. Components such as coax, connectors and other components in RF circuits all have an important parameter called impedance. For good signal performance we want everything to have the same impedance. If wildly different impedances are used between components the signal will experience reflection (i.e. not all of the signal gets transmitted and some gets reflected back, causing a reduction in signal).

The standard impedance used in most radio equipment is 50 Ohms, however some applications like TV prefer to use 75 Ohms. The hackaday article discusses why 50 and 75 Ohms is the standard impedance used. Basically 50 Ohms is an compromise between the impedance of best power handling (30 Ohms) and the impedance of lowest loss (77 Ohms) of air dielectric coax cable.


RTLSDR4Everyone: Review of the Nooelec SMArt SDR, Direct Sampling and Generic vs Premium Dongles

RTL-SDR enthusiast and blogger Akos has recently uploaded three new articles. In his first article he discusses what he believes is the differences and advantages of Generic vs Premium branded RTL-SDR dongles.

In his second article he shows how easy it can be to perform the direct sampling mod on newer dongles, as most have the direct sampling break out pads. He shows how it can be as easy as sticking a wire into these holes. Please note that if doing this we would caution you to take decent ESD precautions as these pins are not ESD protected.

In the third article he reviews the recently release Nooelec SMArt dongle. The SMArt is a new RTL-SDR variant which comes in a smaller black case, cooling via thermal pads and with an SMA connector. With these modifications it is very similar to our RTL-SDR.com units, however the one advantage of the SMArt is that it is small enough to fit two side by side on closely spaced USB ports, like on the Raspberry Pi. In the post he shows what is inside the SMArt and discusses various points such as heat generated, included antennas and performance.

Inside the new Nooelec SMArt RTL-SDR dongle.
Inside the new Nooelec SMArt RTL-SDR dongle.

Live Right Now: Cyberspectrum 17 Software Defined Radio Meetup

Every month SDR evangelist Balint Seeber hosts the Cyberspectrum Meetup in San Francisco, where many SDR fans come together to listen to various presentations. This months meetup is live right now (at the time of this post) and you can watch it live now on YouTube, or delayed later now over, but the recorded stream is available for viewing on YouTube. If you are in San Francisco you can attend the live meetup, but if not you can watch the live stream on YouTube.

This time the talks include:

•”The Land Mobile Radio Spectrum: What is out there, how it works, and how you can hear it” with Desmond Crisis (@dcrisis)

Wireless two-way is the technology that keeps the world working in sync. I’ll explore the various public safety, private enterprise, and personal communications services from[masked] MHz.  We’ll discuss the occupied spectrum, modulation bandwidths, trunked radio schemes and digital transmission modes currently in use on the band as well as what lies ahead. Bring your SDR kit and play along!


• An Academic Look at Interference & Jamming

• Installfest / Hackfest / Debugfest

Decoding a Garage Door Opener with an RTL-SDR

After listening to dock workers with his RTL-SDR for a few days, RTL-SDR.com reader Eoin decided that he wanted to try a more practical experiment. He decided to see if he could reverse engineering the wireless protocol on his garage door opener. Upon opening his remote he discovered a bunch of DIP switches, which are presumably used to program the remote to a particular garage door. Eoin’s next step was to determine at what frequency the garage door opener was transmitting at. He made an assumption that it would be in the 433 MHz unlicenced ISM band as this is where many handheld remotes transmit at. He was right, and found the signal.

The garage door remote showing the DIP switches.
The garage door remote showing the DIP switches.

His next step was then to record the signal audio in Audacity. From the audio waveform he could see a square wave which looked just like binary bits. By manually eyballing the waveform and translating the high/low squarewave into bits he was able to get the binary data. He then confirmed this data with the dipswitch positions and discovered that a 010 binary code matched with the UP position on the dip switch and 011 matched with the DOWN position.

Having decoded the signal manually fairly easily, Eoin decided his next challenge would be to automate the whole decoding in GNU Radio. In the end he was successful and managed to create a program that automatically determines the position of the DIP switches from the signal. His post goes into detail about his algorithm and GNU Radio program.

Showing the decoded DIP switch positions from his GNU Radio program.
Showing the decoded DIP switch positions from his GNU Radio program.

SDRplay API Updated

The SDRplay API has recently been updated to version 1.94. They have also released version 3.9 of their EXTIO plugin. The changes include

  • 10kHz lower limit frequency support
  • 10MHz sample rate
  • Decimation of the sample rate for improved noise performance
  • Improved performance
  • Better function support for developers
  • Callbacks used for stream data and gain updates
  • Tuner AGC function moved to the API

The major changes for users appears to be the the 10 MHz sampling rate and the addition of decimation. The 10 MHz sample rate increases the visible spectrum, however it appears that the maximum IF Bandwidth is still only 8 MHz, meaning that the outer edges of the spectrum won’t show any signals. However,. but the IF filter roll off is not super sharp, meaning that the full 10 MHz should still be usable, with only minor attenuation at the edges. However, we note that in our testing we noticed some roll off at the edges, giving us about 9 MHz of usable spectrum. There should also be an improvement in SNR by using the higher sampling rate thanks to decimation.

Edit: Jon Hudson from SDRplay wrote in to let us know that our assumption of the outer edges being useless was incorrect (the crossed out text). He writes:

I noted one error in what you wrote there…..you suggest that the 10 MHz of visible B-W is worthless because the IF bandwidth is only 8 MHz max and hence you can’t see any signals beyond that 8 MHz window. This isn’t true. The IF filters start to roll off at 8 MHz, but they are not brick wall filters. The actual roll off is at +/- 5 MHz (10 MHz of bandwidth) so within that 10 MHz, at the edges, there are only a few dB of attenuation, and because the CNR has been defined by the front end circuits ahead of the IF filter, any filter attenuation will be applied equally to the signal and the noise leaving the Carrier to Noise ratio unaffected. As a consequence, whilst the user may seem some roll off, the ability to see and receive signals that may lie at the outer edges of the 10 MHz bandwidth is unaffected.

In this update they also added visual decimation controls. This is useful as this allows you to zoom into a signal without loosing resolution whilst maintaining a high sample rate. The decimation controls only appear to activate in the lower IF Bandwidth settings.

The SDRplay RSP is a $149 USD software defined radio with 10 kHz to 2 GHz tuning range, 12 bit ADC and up to 8 MHz of visible bandwidth.

The new RSP EXTIO control panel showing decimation and 10 MHz sample rate.
The new RSP EXTIO control panel showing decimation and 10 MHz sample rate.

RTL-SDR Blog SDR Unit Sale!

As many of you know we have been working on releasing a new revision of our RTL-SDR Blog SDR units for the last few months. We are a few weeks away from being able to release news about this new unit and begin sales.  At the moment we are still confirming the features and testing the prototypes so cannot release any news, but if you want a hint at what features might be coming you can take a look at our previous poll asking RTL-SDR users what they wanted in a low cost SDR. The new units may include some of these features/improvements.

For now we are selling off our current batch at reduced prices. The dongle only package is reduced from $19.95 to $17.95, and while the dongle + antenna kit is sold out in our international store, we have reduced its price from $24.95 down to $23.95 on our Amazon USA store.

You can purchase our units from our store at www.rtl-sdr.com/store.

These prices will only last until this batch of stock runs out, and there are only a few hundred units remaining. This special offer also combines with our 5% off deal if you buy more than two items from our store.


Fixing a long active USB Cable for RTL-SDR Use

Active USB cables allow cable lengths to be stretched to much longer than the maximum length of 5m allowed by the USB specification. However, although the packet timing requirements are met by the repeaters used in the active cables, there is still a significant voltage drop which can affect devices like the RTL-SDR.

Over on YouTube Shaun Dobbie discovered that his RTL-SDR would not run properly on his long active USB cable, and he suspected low voltage. After opening the case on the USB cable head he discovered two pins which allowed for external power input. By simply connecting an external 5V supply from a battery to the 5V input of the active cable he was able to fix the low voltage problem. If you’ve ever found that a long active USB cable doesn’t work then this may be the problem you have experienced. An alternative to this home solution might be to use an external powered USB hub, or buy an active USB cable that already has an external power input like this or this one.