Mare & Gal Electronics

In past I assembled small SDR receiver based on Raspberry Pi and SDR USB dongle based on software developed by PA3FWM. The problem was with limited usability by multiple clients connected to the Raspberry pi at the same time. The old SDR was retired and put aside for few years.

Then András Retzler ha7ilm developed (and stopped developing) OpenWebRx, which was the base for now regulry maintained and further developed OpenWebRx.de.

I recently built homelab server based on “proper” server infrastructure with plenty of RAM and lots of processing cores. I decided to setup OpenWebRx in one linux virtual machine with raspberry pi only as remote receivers serving single user (server itself).

For start please visit the web page with this SDR receiver:

https://sdr.v4.si/

Open many receivers…

Usually, the Web SDRs will show some error like:

This is what I would like to avoid and let as much users as possible connecting to the service.

What is needed

1. Some machine based on linux (own homelab server or remote)
2. at leat one raspberry pi, rpi 4, 8G RAM can handle 4 receivers
3. at least one SDR USB dongle based on RTL2832U & R820T2 with TCXO
4. Receiving antenna
5. Transverter for HF bands
6. Splitter for mutiple bands
7. Power supply
8. Accessories (USB extensions, housings, SMA pigtails, …)

The architecture

This article covers setup based on USB RTLSD dongle(s) plugged in raspberry pi close to antenna. The R.pi feeds complete receiver bandwidth to OpenWebRx server via network. The raspberry is off-loaded from serving multiple users. The server hardware is located in another place with more suitable environment, while raspberry pi can be placed very close to antenna. Another advantage is when the lightning strikes near or directly into antenna, there will be less damage. I recommend placing optical connection between antenna and server.

The VHF and up RF signals received by multiband antenna A2 is split to several RTL SDR dongles via splitter S2. The SDR receivers covers VHF & Up bands directly.

Receiving HF bands require some up-converting with the transciever. The input frequency range of the SDR dongles is 25MHz to 1750MHz. The transverter converts input signals up for 125MHz. The input+125MHz is then split to several RL SDR dongles the same way as for VHF direct reception.

The raspberry pi runs Soapy remote server package feeding the received streams to OpenWebR server.

OpenWebRX installation

Setting up the linux server is out of the scope of this presentation. Just follow some of the installation instructions for Ubuntu or Debian. Please keep in mind that for larger audience (more than e.g. 10 clents connected at the same time), the raspeberry pi has not enough resources.

The OpenWebRx has installation instructions for the Debian and Ubuntu Linux distributions that contain packages for OpenWebRX and all dependencies.

Try manual setup instructions when above installation instructions are not successful. Keep in mind this will guide you through compilation process from sources.

Remote receiver station

Remote receiver runs on Raspberry pi. I tested installation with Banana Pi M2+ EDU (quad-core ARM H3 with 512MB RAM) and Orange Pi Zero (512MB RAM Quad-Core H2). Both worked but on long run they crashed once or twice per week. Finally I decided to use raspberry pi 3 B for two VHF/UHF bands and rasberry pi 4 B with 8G RAM for four HF bands.

The most commonly used open-source SDR server applications seem to be SoapyRemote and rtl_tcp. SoapyRemote provides access to supported SDR dongle.

Installing soapy remote from sources

First, install git, gcc, g++, make and cmake:

sudo apt-get install git gcc g++ make cmake

Compile and install SoapySDR

git clone https://github.com/pothosware/SoapySDR.git

cd SoapySDR

mkdir build

cd build

cmake ..

make -j4

sudo make install

cd ..

sudo ldconfig #needed on debian systems

cd ..

Install SDR hardware drivers

sudo apt-get install rtl-sdr librtlsdr-d

Compile and install SoapySDR adapter module

git clone https://github.com/pothosware/SoapyRTLSDR.git
cd SoapyRTLSDR
mkdir build
cd build
cmake ..
make
sudo make install 
cd ../.. 

On orange/banana pi, install nano

sudo apt-get install nano  # not needed for R.pi

Blacklist existing driver for RTL devices

sudo nano /etc/modprobe.d/no-rtl.conf

add following contents to the file

blacklist dvb_usb_rtl28xxu
blacklist rtl2832
blacklist rtl2830

Reboot the system

sudo reboot

After reboot, plug in the first USB SDR dongle. Run the rtl eeprom tool:

rtl_eeprom

The output will show current SDR dongle configuration stored in EEPROM.

What could be changed is only the serial number. Don’t mess with any other settings than serial number.

rtl_eeprom -s 012345

Repeat same process for all SDR dongles and use different serial number for each one. This will provide simple diffirentiation between dongles.

sometimes the output may look like this:

Found 1 device(s):
   0:  Generic RTL2832U
 Using device 0: Generic RTL2832U
 Found Rafael Micro R820T tuner
 Current configuration:
 Error: invalid RTL2832 EEPROM header!
 Error: invalid string descriptor!
 Error: invalid string descriptor!
 Error: invalid string descriptor!
 
 Vendor ID:              0xffff
 Product ID:             0xffff
 Manufacturer:           ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
 Product:
 Serial number:
 Serial number enabled:  no
 IR endpoint enabled:    yes
 Remote wakeup enabled:  yes

If such (messed-up) configuration is shown, you should load the eeprom with the content from this hex file.

wget http://pavlin.si/tmp/nesdr.hex
rtl_eeprom -w eeprom_original.bin

then follow instructions to set the serial number above.

If USB dongle is not found, try the following procedure:

1. unplug the dongle
2. short the SDA and SCL on the EEPROM
3. plug the dongle while keeping the SDA/SCL low
4. remove the short to ground from SDA/SCL
5. load default EEPROM content

rtl_eeprom -g realtek

Now check that SoapySDR can find and access SDR hardware

SoapySDRUtil --probe

#
 Soapy SDR -- the SDR abstraction library
 #
 Probe device
 Found Elonics E4000 tuner
 [INFO] Opening Generic RTL2832U OEM :: 00000030…
 Found Elonics E4000 tuner
 
 -- Device identification
 driver=RTLSDR
   hardware=E4000
   index=0
   origin=https://github.com/pothosware/SoapyRTLSDR
 
 -- Peripheral summary
 Channels: 1 Rx, 0 Tx
   Timestamps: YES
   Time sources: sw_ticks
   Other Settings:
      * Direct Sampling - RTL-SDR Direct Sampling Mode
        [key=direct_samp, default=0, type=string, options=(0, 1, 2)]
      * Offset Tune - RTL-SDR Offset Tuning Mode
        [key=offset_tune, default=false, type=bool]
      * I/Q Swap - RTL-SDR I/Q Swap Mode
        [key=iq_swap, default=false, type=bool]
      * Digital AGC - RTL-SDR digital AGC Mode
        [key=digital_agc, default=false, type=bool]
 
 -- RX Channel 0
 Full-duplex: NO
   Supports AGC: YES
   Stream formats: CS8, CS16, CF32
   Native format: CS8 [full-scale=128]
   Stream args:
      * Buffer Size - Number of bytes per buffer, multiples of 512 only.
        [key=bufflen, units=bytes, default=262144, type=int]
      * Ring buffers - Number of buffers in the ring.
        [key=buffers, units=buffers, default=15, type=int]
      * Async buffers - Number of async usb buffers (advanced).
        [key=asyncBuffs, units=buffers, default=0, type=int]
   Antennas: RX
   Full gain range: [0, 96] dB
     IF1 gain range: [-3, 6] dB
     IF2 gain range: [0, 9] dB
     IF3 gain range: [0, 9] dB
     IF4 gain range: [0, 2] dB
     IF5 gain range: [3, 15] dB
     IF6 gain range: [3, 15] dB
     TUNER gain range: [-1, 42] dB
   Full freq range: [51.999, 2200] MHz
     RF freq range: [52, 2200] MHz
     CORR freq range: [-0.001, 0.001] MHz
   Sample rates: [0.225001, 0.3], [0.900001, 3.2] MSps
   Filter bandwidths: [0, 8] MHz

Compile and install SoapyRemote

git clone https://github.com/pothosware/SoapyRemote.git

cd SoapyRemote

mkdir build

cd build

cmake ..

make

sudo make install

Start SoapyRemote server

SoapySDRServer --bind

To enable the server startup automatically on boot:

sudo systemctl enable SoapySDRServer

Reboot the system. After reboot check if the soapy remote server is running.

journalctl -u SoapySDRServer

Install soapyremote – client side

This part runs on the machine where OpenWebRx is running. It provides connection to the soapyremote server running on raspberry pi.

The installation from source only requires CMake and a compiler. The apt-get install instructions for Ubuntu, which installs all required tools is:

sudo apt-get install cmake g++ libpython3-dev python3-numpy swig

Clone the SopaySDR repositor

git clone https://github.com/pothosware/SoapySDR.git
cd SoapySDR

Build and install SopaySDR

mkdir build
cd build
cmake ..
make -j4
sudo make install
sudo ldconfig #needed on debian systems
SoapySDRUtil --info

Now check connection to remote server (on raspberry pi). My Raspberry pi has IP address 44.150.168.87. Replace this with the IP of your remote sopay SDR server.

SoapySDRUtil --find="remote=44.150.168.87"

#
 Soapy SDR -- the SDR abstraction library
 #
 [INFO] [UHD] linux; GNU C++ version 9.2.1 20200304; Boost_107100; UHD_3.15.0.0-2build5
 Found device 0
   default_input = True
   default_output = True
   device_id = 0
   driver = audio
   label = PulseAudio
 Found device 1
   driver = remote
   label = Generic RTL2832U OEM :: 000122
   manufacturer = Realtek
   product = RTL2838UHIDIR
   remote = tcp://44.150.168.87:55132
   remote:driver = rtlsdr
   serial = 000122
   tuner = Rafael Micro R820T
 Found device 2
   driver = remote
   label = Generic RTL2832U OEM :: 000123
   manufacturer = Realtek
   product = RTL2838UHIDIR
   remote = tcp://44.150.168.87:55132
   remote:driver = rtlsdr
   serial = 000123
   tuner = Rafael Micro R820T
 Found device 3
   driver = remote
   label = Generic RTL2832U OEM :: 000120
   manufacturer = Realtek
   product = RTL2838UHIDIR
   remote = tcp://44.150.168.87:55132
   remote:driver = rtlsdr
   serial = 000120
   tuner = Rafael Micro R820T
 Found device 4
   driver = remote
   label = Generic RTL2832U OEM :: 00000001
   manufacturer = Realtek
   product = RTL2838UHIDIR
   remote = tcp://44.150.168.87:55132
   remote:driver = rtlsdr
   serial = 00000001
   tuner = Rafael Micro R820T

As shown above, the four dongles with different serial numbers are listed. Now do same check with other raspberry pi servers in your system.

OpenWebRx configuration

The OpenWebRx repository has excellent guides:

1. User management
2. OpenWebRx configuration
3. Background decoding guide

The device setup for HF band with 125MHz up-converter and SoapyRemote:

The transverter frequency offset can be entered for the whole device or within the single profile. We are using one device with one profile. The frequency offset can also be used to trim the actual frequency of the receiver in absoulte Hz instead of “ppm” units.

For the VHF or UHF receiver, the offset is not needed:

Enjoy:

https://sdr.v4.si/

Final installation of the remote modules

Short reception video:
https://youtu.be/ywgDHMhMieo

Some ideas for future improvements:

• add some storage for all bands like SDR buffer TU Graz
• add some more bands
• add CW skimmer