Web based transceiver developped by F1ATB
Below is the connection diagram of the various modules making up the transceiver allowing links to QO-100 with the “Remote SDR” application. I have been using this system since May 2020 with satisfaction. It has since been reproduced by many other Hams.
The antenna is a dish 1.20 m in diameter with an LNB head which translates signals from -9750 MHz in frequency. QO-100 signals received in the band 10,489,500 kHz – 10,490,000 kHz are reduced to the band 739,500 kHz – 740,000 kHz.
A “Bias-T” splitter sends the 12v power supply to the LNB and the HF signals to the receiving SDR (Hack-RF One).
The Hack-RF One is an SDR that covers a wide reception frequency range up to 6 GHz with 8-bit dynamic range, sufficient for traffic via QO-100 where the different signals are of a similar level. It is possible to replace the Hack-RF One with a classic SDR key which perfectly covers signals around 740 MHz, but does not allow you to listen to your own transmission around 2.4 GHz. The Hack-RF is connected to the Raspberry Pi-4B by USB which provides it with 5V power.
The signal to be transmitted is generated by an SDR (Hack-RF One). It is mainly voice SSB for the QO-100 NB repeater. We can, with Remote SDR and other applications, generate other signals such as SSTV.
A band filter centered around 2.4GHz eliminates any out-of-band signals.
The generated signal being around the dBm, it is necessary to amplify with 2 amplifiers mounted in series (SPF5189Z), which are supplied with 5V. Each provides over 10dB of gain.
Finally, an amplifier (EP-AB003) from the WIFI field provides a few watts at 2.4 GHz. It is given for 8w but in practice, we must hope for about half.
The output signal is sent to a helix antenna in the center of the receiving dish.
Signal processing in reception and transmission is provided by a Raspberry Pi 4B with 2 GB of memory. A downloadable image of the Raspberry OS operating system and Remote SDR fileset is available on Github. The Raspberry also serves as a web server to send Remote SDR pages to the web browser like Chrome or Edge.
It is imperative to set up a device to avoid unwanted emissions in the event of a crash of the network, the Raspberry pi, etc., which would not allow orders to be placed.
The Python3 module which receives the audio samples from the microphone in packets of 256 bytes causes pin 26 of the GPIO of the Raspberry Pi 4 to change state each time a frame is received. This creates a square wave AC signal (0v,3.3v) that is passed through a 4.7uF capacitor to a rectifier diode and switching transistor device such as a 2n2222. At each half cycle, the relays on the collector are operated and held by the 47uF to 470uF capacitor until the next half cycle. These relays switch the 12v power supply of the transmitter output WIFI amplifier and the 5v of the pre-amplifiers. Thus, if the audio frames are received, the power supply to the amplifiers is established. If the frames no longer arrive or the system crashes, pin 26 in the high or low position, the power supply is cut.
An alternative to the transistor circuit is to use a monostable like the 4538 which provides the Q and /Q outputs to drive any type of relay.
All the subsets making up the transceiver are mounted in the case of an old PC. This allows for the necessary 12v and 5v power supplies.
Realization in a PC box
The Raspberry Pi 4B with 2 GB of memory can be purchased from a local retailer. SDRs, amplifiers, 2.4 GHz filter and Bias-T can be found at Aliexpress China. Remember to also supply cables with SMA connectors to interconnect the modules.