Receiving CB Radio with an RTL-SDR Dongle

Back in July we posted a story by Mario Fillipi (N2HUN) who wrote an article about using the RTL-SDR to receive CB radio, and how while the CB radio heyday is over, there are still opportunities for good listening available today.

Recently Mario has posted a new article on swling.com where he discusses his CB radio listening hobby further. To listen to the CB band at 26.965 – 27.405 MHz he uses an RTL-SDR dongle together with a ham-it-up upconverter. While an upconverter is not required since most RTL-SDR dongles typically tune down to 24-25 MHz, he finds that using one helps because it can help block out interference from the strong broadcast FM band.  We note that you could also use one of our BCFM Block filters for the same purpose.

Mario notes that recently he noticed the CB band was open during the night. Usually the frequencies that CB radio uses propagate best during the daytime, and poorly at night. But on some occasions it can open up at night as well. He writes that on some occasions during a winters night during a snowstorm he has been able to receive the world on CB, from Europe, the Caribbean and Australia.

CB Band Voice in SDR# with an RTL-SDR and Ham-it-up Upconverter
CB Band Voice in SDR# with an RTL-SDR and Ham-it-up Upconverter
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New Linux RTL-SDR Driver with Fully Exposed Controls

A new Linux based driver for the RTL-SDR has been released by Milen Rangelov (aka gat3way) which exposes all the adjustable settings on the R820T/2 tuner chips. This exposes adjustable sliders for settings like preselection, IF and notch filters and the multiple gain stages on the R820T/2. In the standard drivers the filters and gain settings are mostly set automatically, but manually adjusting them could yield better results. The filters are not very strong, but they could be used to help block out an interfering signal, increasing the dynamic range of the RTL-SDR.

Over on his post at r/rtlsdr Milen wrote the following about his driver:

The idea in brief is to modify the librtlsdr code so that an unix domain socket server receives i2c register set/get commands and executes them. It only works for r820t tuners. This modified rtlsdr library is then dynamically preloaded by means of LD_PRELOAD and used by the SDR software. This is actually what happens when you call r820tweak <program>. This way, no modifications to both gr-osmosdr source and the SDR program are required, instead of waiting for them to implement those controls, we kind of have a separate program that tweaks them.

The GUI program is a simple wxpython client, it currently exposes the following settings:

LNA, Mixer, VGA gain stages – the 3 variable gain stages

LPF/HPF filter cutoff – these control the “width” of the r820t2 lowpass/highpass filters, those filters are relatively sharp and this in turn is very useful to increase the dynamic range by fitlering out strong signals “close” to the weak signal you are hunting. Those are among the nicest features to play with, together with the gain stages.

LPNF cutoff – apparently there is also a low pass notch filter, however this doesn’t work as I expected. Anyway, still useful as a kind of variable attenuator.

Filter bandwidth – there is a bandpass filter which isn’t quite “sharp” at all, centered at the center frequency. Kind of additional filter, easier to manipulate than the rest of them, but not that powerful. May provide some extra dynamic range. Behaves kind of weird when gqrx decimation is used.

As far as the gain stages are concerned – the LNA gain is the first stage and thus the most important – it determines the SNR. Mixer gain is less important (unless the signal is too weak). The VGA gain should be almost always set to zero as it doesn’t contribute to the SNR at all while keeping the dongle warmer and so more thermal noise.

In the R820T2 i2c register specs there are some other interesting features, currently unexposed. One of them is the band selection filter (lo/med/high) which apparently is even used in the librtlsdr driver. I found changing that has absolutely no effect for me unfortunately. It might have provided opportunities for better reception around the “verge” where the librtlsdr driver switches them (approx. at 310MHz and 588MHz). But switching them has absolutely no effect.

The new fully adjustable driver by gat3way.
The new fully adjustable driver by gat3way running in GQRX.
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RTL-SDR.com V3 Stock Availability and Holiday Shipping Times

This is just an update post for those wishing to purchase or are waiting on delivery for items from our store at www.rtl-sdr.com/store.

Amazon USA

The RTL-SDR.com V3 has proven to be more popular than we anticipated and Amazon ran out of stock a few weeks ago. New stock should be arriving at the Amazon warehouses early next week, but we don’t know how long Amazon will take to process the stock and put it back on the shelves. It should hopefully be ready in under a week.

International

For international orders from our Chinese warehouse, please be aware that international shipping is quite slow at the moment, due to the peak season parcel rush. The international mail system struggles to cope with mail at this time of the year due to a vast increase in Christmas parcel volume. Delivery times are still typically under 4 weeks but for some parcels, and to some countries we can expect shipping times of up to 6 weeks or longer. 

Some people have noted that parcels ordered after a previous order have arrived earlier. We use the same postal system as everyone else (there’s only really one international air mail system!), but what can happen is that on some days the parcels may get a direct flight, and on other days they may get an indirect flight. Those indirect ones can end up taking much longer. Also planes can break down, parcels can miss a connecting flight or security can hold a container of many parcels for days just because they saw one suspicious parcel inside. Thus international parcel delivery times have a very large scattering, ranging from less than a week to six weeks or more. We thank you for your patience if your parcel happens to end up on the slower end of the scale. But if your parcel does end up taking over 6 weeks please let us know as we may be able to open a lost parcel investigation. If your parcel is lost it will be resent or refunded on your preference.

If you are tracking a parcel we recommend 17track.net, aftership.com and track-chinapost.com. Try all three as sometimes one has more up to date tracking than another. Please note that we’ve found that around this time postal workers will often not bother to scan the tracking label on parcels, since they are rushing so much. This can cause a lack of tracking updates for a while. Also if there are queues for customs checks in your local country the parcel can wait around in the container without a scan for a long time, until customs gets to it.

We have several shipping methods available. China Post/Swiss Post/HK Post are about the same speed once they leave. But generally Swiss/HK post are faster to leave China. China Post parcels can sometimes take a few extra days to pass through security. Some countries which support it, will automatically be upgraded to EMS ePacket priority mail. We also have some new special methods for some countries which are still air mail, but use a faster more direct flight to the destination. Of course the more expensive express mail services like DHL/UPS/Aramex are also available, but even they have about a 1 week shipping delay at the moment.

However, despite these shipping time uncertainties, the majority of parcels still seem to be making it through the postal system in a timely manner. Thanks to everyone for supporting the RTL-SDR.com blog!

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R820T2 Register Description Data Sheet Now Available

Recently Luigi Tarenga wanted to do some work on developing RTL-SDR drivers, so he emailed Rafael Micro requesting some additional documentation about the chip. Usually previous requests to Rafael Micro for such information seem to have gone unanswered, but this time it seems they have decided to publicly released the Register description document for the R820T2 chip.

Previously the R820T datasheet was leaked/released to the public, but the information in the datasheet did not help much with driver development. This register description document describes the function and configuration bits for the registers on the chip, and may be useful for people wanting to develop better drivers for the RTL-SDR.

We have uploaded a copy on our server here.

Description of some of the registers in the R820T2 chip.
Description of some of the registers in the R820T2 chip.
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SSTV From the ISS Scheduled for Dec 8 – 9

The International Space Station periodically schedules radio events where they transmit Slow Scan Television (SSTV) images down to earth for listeners to receive and collect. This time they have scheduled SSTV images for Dec 8 1235 – 1800 UTC, and December 9 1240-1740 UTC. The ARRL announcement reads:

Slow-scan television (SSTV) transmissions from the International Space Station (ISS) are scheduled for December 8-9. The SSTV images will be transmitted from RS0ISS on 145.800 MHz FM as part of the Moscow Aviation Institute MAI-75 Experiment, using the Kenwood TM-D710 transceiver in the ISS Service Module.

MAI-75 activities have been scheduled on December 8, 1235-1800 UTC, and December 9, 1240-1740 UTC. These times correspond to passes over Moscow, Russia. ISS transmissions on 145.800 MHz FM use 5-kHz deviation, and SSTV transmissions have used the PD120 and PD180 formats.

The ISS Fan Club website can show when the space station is within range of your station. On Windows PCs the free application MMSSTV can decode the signal. On Apple iOS devices, use the SSTV app.

These SSTV broadcasts can usually be easily heard with an RTL-SDR and appropriate satellite antenna such as a QFH, Turnstile or a hand held Yagi. Many listeners have reported in the past as being able to receive them even with non-satellite antennas such as discones, ground plane, rubber duck and long wire antennas, so try your luck even if you don’t have the right antenna.

We recommend using the Orbitron software to track the ISS, but you can also use the web tracker on issfanclub.com as recommended by the ARRL.

An SSTV image from the ISS sent last April
An SSTV image from the ISS sent last April from http://www.issfanclub.com/node/40913

 

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Building a Homemade FM Repeater with a Raspberry Pi, Rpitx and RTL-SDR Dongle

A radio repeater is usually a radio tower that receives weak signals from handheld, desktop or other radio, and rebroadcasts the same signal at a higher power over a wide area at a different frequency. This allows communications to be extended over a much greater area.

Repeaters are generally made from expensive professional grade radio equipment, however ZR6AIC has been experimenting with creating an ultra low cost repeater out of a RTL-SDR and Raspberry Pi. In his system the RTL-SDR dongle is set up to receive a signal on the 70 cm (420 – 450 MHz) amateur radio band, and then retransmit it using Rpitx on the 2M (144 – 148 MHz) amateur radio band.  He also adds a 2M low pass filter to the output of the Raspberry Pi to keep the signal clean.

RTL-SDR + Rpitx Block Diagram
RTL-SDR + Rpitx Block Diagram

Rpitx is software for the Raspberry Pi which we have featured on this blog several times in the past. We’ve also seen the qtcsdr software which also uses Rpitx and an RTL-SDR to create a transceiver. Rpitx allows the Raspberry Pi to transmit radio signals without the need for any transmitting radio hardware at all. It works by modulating signals onto a General Purpose I/O (GPIO) pin on the Raspberry Pi. If the GPIO pin is modulated in just the right way, FM/AM/SSB or other signal modulation approximations can be created at a specified frequency. The signal is however not clean, as this type of modulation generates many harmonics which could be dangerous if amplified. If you use Rpitx, always use appropriate filtering hardware.

ZR6AIC’s post goes into detail about how to install and set up the required software onto the Raspberry Pi and how to set up the script to piece all the programs together into a repeater. He’s also uploaded a video demonstrating the system in action on YouTube.

Raspberry Pi Fm Repeater

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Reverse Engineering Traffic Lights with an RTL-SDR Part 2

Back in September 2015 we made a post about how Bastian Bloessl was able to use his RTL-SDR dongle to reverse engineer and decode the signals coming from portable wirelessly synchronized traffic lights which are commonly set up around road construction zones.

Recently Bastian noticed that a new set of wireless traffic lights had been set up at his University, so he got to work on trying to reverse engineer those. He found that these new lights use the same frequency band, but work using a different modulation and frame format scheme.

The reverse engineered wireless traffic lights.
The reverse engineered wireless traffic lights.

To reverse engineer these new lights he made a recording of the signals in GQRX and then opened them up in Inspectrum, which is a very nice tool for helping to reverse engineer digital signals. Thanks to Inspectrum he was easily able to extract the preamble and decode the data in GNU Radio.

Bastian has also uploaded a video that shows him reverse engineering the binary frame format in the Vim text editor which may be useful for those wishing to understand how it’s done.

Reversing Frame Format with Vim

Once the frame format was reverse engineered, he was able to use the program he created last year which allows him to view the status of the lights remotely in real time.

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A Homemade SDR Frequency Controller Made From a PC Mouse

Over on YouTube user Mile Kokotov has uploaded a new video showing his SDR frequency controller in action. The controller allows you to tune the frequency with a knob, which is preferred over the keyboard and mouse by many. In his video he shows it in action on the SDRUno software.

The controller is made out of a simple rotary encoder and a USB mouse. The rotary encoder is an EC16 (with no detents/clicks). It is connected up to the mouse PCB instead of the scroll wheel. Mile mentions that the encoder only cost him about $2 from eBay, and the mouse only $3, bringing the total cost of the project to only $5. More information and images can be found on his project page.

The rotary encoder connected to the USB mouse.
The rotary encoder connected to the USB mouse PCB.
Homemade SDR Frequency Controller from PC mouse

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