A glance into the RTL-SDR equipment (software program outlined radio with dipole antenna equipment)

If you’ve taken an amateur radio exam in the past few years, you’ve likely come across the topic of Software Defined Radio (SDR) in documents several times. The inexpensive RTL-SDR (Software Defined Radio) with dipole antenna kit is an experimental platform that enables SDR reception with practical hardware.

If you’ve taken an amateur radio exam in the past few years, you’ve likely come across the topic of Software Defined Radio (SDR) in documents several times. The inexpensive RTL-SDR kit is an experimental platform that enables SDR reception with practical hardware.

The term SDR as an acronym for Software Defined Radio shouldn’t need any further explanation today. The principle is that parts of the signal processing that were previously carried out in hardware (electronic components) are carried out digitally in SDR. In times of ever cheaper DSPs and powerful microcontrollers, this is an all too sensible line of thought.

What do we have here?

The story of this radio module system, which is known from the RTL-SDR blog, began, as so often, with a coincidence. Rumor has it that a Linux kernel developer found out that the RTL2832 IC manufactured by RealTek is more than just a classic DVB-T decoder. It can collect I / Q samples and send them directly to the host – a feature that the chip manufacturer intended to decode FM radio. The SDR kit described here it’s about reception. It is not possible to send with the module. So it is a radio in the sense of the word use.

Within the kit According to the documentation and the information in the data sheet, the chip can monitor a spectrum of up to 3.2 MHz in “real-time operation”. Practical experience shows that samples are lost when this bandwidth is fully used. A realistic bandwidth is therefore a restriction to a maximum of 2.8 MHz or a little less. In the documentation Elektor even advises not to process more than 2.4 MHz at the same time.

The IC RTL2832 itself does not have an integrated tuner that takes into account the restriction of the recorded frequency range. Something like this can therefore also be found in USB sticks based on the RTL2832. The USB module supplied by Elektor is a tuner manufactured by Rafael Micro with the designation R820T, the interior of which is shown schematically in the following circuit.

A block diagram of the R820T tuner. Unfortunately, the Formosa-based manufacturer does not publish enough information about the behavior of its chip. The above illustration comes from an unofficial data sheet.

Getting started with the SDR

Such SDR USB sticks are available in different versions. The “luxury version” was included in the Elektor package. In addition to a tripod and a suction cup holder for the antennas, only a dipole T-piece is (unfortunately) included – however, two extendable antenna pairs of different lengths can be used. The shorter antennas are 5 to 13 cm long and cover the frequency range from UHF to approx. 1.5 GHz. The longer samples are 23 to 100 cm long and cover the frequency range from VHF to UHF. Note that these antennas are only intended for reception. Adjustments for a transmitter would also be required at this point.

I performed the following steps without the included extension cord. For my experiments I screwed the two longer antennas (as shown below) onto the dipole adapter and connected the connection cable directly to the USB dongle.

My setup: Dipole with the two longer antennas on the tripod, which are connected directly to the USB stick.

The inexpensive dipole adapter for the antennas works in practice without any problems. A perfectly fitting Phillips screwdriver should be used to tighten the screws if you do not want to damage the screw heads.

Before connecting the USB stick to a PC, the required software should be installed. A PC with Windows 10 was developed as standard for the operation of an SDR. Linux works too! This is a great thing for me as I usually work on a Linux PC. For more information on this topic, see the quick start guide.

For information on software, contact AirSpy, as they offer software for various SDR dongles. Downloading the Windows SDR software package works well. The downloaded archive is then completely unpacked into a local working directory. This is necessary because the installation process will not run if you run the exe file directly from the archive. For legal reasons, AirSpy cannot involve the drivers. Instead, they are downloaded directly from the provider’s server using the install-rtlsdr.bat batch file. Note that the zadig.exe file is approximately 5 MB in size. This isn’t actually much these days, but downloading from the vendor’s server proves to be unreliable at times.

After installing the software and the driver, the dongle is connected to the PC via USB. This is where the plug-and-play device detection included in Windows should start working. The next step only takes place when Windows displays the message “The device is ready for operation”. However, the driver downloaded in batch is not yet functional. For this reason, the zadig.exe file should be started in the next step by clicking the right mouse button with administrator rights.

Zadiq is a tool that specializes in replacing the drivers for USB devices that Windows provides by default. The first task is to find the USB dongle. It is usually called “Bulk-In, Interface (Interface 0)”. However, this task is made more difficult by the fact that the tool filters the USB devices that it shows. To find the device, you need to check the> List all devices option and uncheck the> Ignore hubs or compound parents option. Then click the big button to replace the driver. Possible security warnings are easy to ignore.

One, two, AirSpy over

The AirSpy software is written in C #. Double-click the SDRSharp.exe file to start it. They just ignore any error message. In the next step you have to select the USB dongle in the Source field. The correct type is RTL-SDR USB.

As a precaution, turn the volume down so that you are not surprised by loud noises. In theory, you could now click the play icon to refresh the real-time display and track the spectral components towards the audio output. Usually, however, the gain is set very weakly, which is why you have to click on the little gear icon. The software responds to this by showing the popup shown below where you can use a slider to increase the gain.

A screenshot of the pop-up dialog with slider to increase gain.

You can now search for a known RF frequency. The next figure shows what I was able to capture near my lab.

Screenshot of my successful reception: Gigi D’Agostino via SDR!

SDR extension

When I had a spectrum analyzer from an American manufacturer demonstrated to me a few years ago, I discovered that it was more of a computer than a classic spectrum analyzer. A large part of the device’s intelligence was provided by software plug-ins that interpreted the data supplied by the measurement system. This SDR system is very similar: the toolbox displayed on the left side of the screen contains several dozen plug-ins, the settings of which can be expanded with a click. For example, I’ve also experimented with audio FFT. The result can be seen in the following picture.

If the setting is correct, the analyzer performs an FFT of the audio signal supplied. The possibilities of “programmatic signal processing” did not bypass the user community. There is extensive additional software that, in addition to various amateur radio protocols, also decodes aircraft transponders.

If you want to code yourself with Visual Studio, this introduction will help you. However, basic knowledge of C # and Visual Studio 2017 is required.

The RTL-SDR kit as an inexpensive alternative

If you have always been looking for a real-time spectrum analyzer, the “RTL-SDR (Software Defined Radio) with Dipole Antenna Kit” is an inexpensive alternative. The tool is suitable for amateur radio operators as a kind of automated receiver with extensive visualization functions. What you do with it is up to your imagination. If you use your kit in a particularly innovative application, the Elektor editorial team would appreciate your feedback!

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