Back in 2014 oh2ftg discovered that the RTL-SDR could actually be used to transmit databy modulatingleakage from its internal local oscillator. Now it seems that tejeez and oh2ftg have released a new program that makes transmitting with the RTL-SDR easy. The program is called rtl-trx. It runs on Linux and allows you to to transmit RTTY or a simple beacon with the RTL-SDR. The software is available on GitHub at https://github.com/tejeez/rtl-trx. About how it works, the readme says:
Local oscillator leakage from an RTL-SDR dongle can be used as a very low power FSK transmitter. This program transmits RTTY and also makes it easy to use the same dongle to receive RTTY in between transmissions. The goal is to make it possible to have a two-way QSO between two dongles.
Over on YouTube oh2ftg has also uploaded a video that demonstrates the software in action by doing a 1270 MHz RTTY QSO between two modified RTL-SDR dongles. He uses fldigi to decode the RTTY signal and the signal is sent with the following settings: 425 Carrier shift, 45.45 Baud rate, 5 Bits per character, none Parity, 2 Stop bits.
This previous post shows the hardware modification that can be done to improve the output power. Again, as with the Raspberry Pi transmitters, the output power is very low and probably won’t cause any trouble, but still please do take care if you intend on actually transmitting anything as the output spectrum is probably not very clean.
Oscar decided to take a standard RTL-SDR dongle and modify it so that it outputs a signal from the mixer output of the R820T tuner chip. To do this he removes some unneeded components from the PCB, and wires pin 5 of the R820T to the MCX antenna port through a 100pF capacitor. Pin 5 is connected to the mixer output from inside the R820T chip.
After performing the hack the RTL-SDR is able to output a signal anywhere between 500 MHz to 1500 MHz 1.8 GHz to 3 GHz (see why). To control the output frequency you simply need to tune to the frequency you want to transmit at in SDR# (after setting an offset to account for the R820T’s IF offset). This tunes the mixer in the R820T and causes the output frequency to change.
In the future Oscar hopes to take this idea further by creating a specific tuning application for the generator and finding a way to possibly FM modulate the output.
In our previous post we featured a video by OH2FTG which showed an RTL-SDR transmitting at 1270 MHz. Now OH2FTG has written in to give us some more information about the RTL-SDR transmitter. He has done a short writeup explaining how it’s done on his website. It turns out that the RTL-SDR is actually capable of transmitting a FSK morse beacon using it’s leaky oscillator.
In the video, code written by another ham OH2EAT is used. OH2EAT’s code essentially changes the frequency on the transmitting RTL-SDR at up to 300 times a second using a modified driver. This is used to create a Frequency Shift Keyed (FSK) transmission.
The modulating transmitter code is not yet available as it is not yet ready for release. In the future OH2FTG hopes to build an amplifier to boost the signal output for further experiments.