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# Application note on using rpitx
+The recently released rpitx by Evariste, F5OEO allows us to transmit an RF signal over GPIO18 (pin 12) of a Raspberry Pi. The software can accept an I/Q signal as an input, so now a Pi can be used as a general purpose SDR transmitter.
+
+This document is about using GNU Radio and csdr with *rpitx*.
+
**Table of contents**
* [Using a Raspberry Pi as remote software defined radio peripheral from GNU adio](#using-a-raspberry-pi-as-remote-software-defined-radio-peripheral-from-gnu-radio)
@@ -12,7 +16,17 @@
![Warning](/images/lpf-warning.png?raw=true)
-
+
+The output of the Raspberry Pi GPIO 18 is a square wave, which contains harmonics.
+For example, if you transmit on 28 MHz, you will also transmit on:
+
+* 28 × 3 = 84 MHz
+* 28 × 5 = 140 MHz
+* 28 × 7 = 196 MHz
...
+
+...and actually you may distrub some important radio communication services. Please don't do this! You can get into trouble! You have been warned.
+
+If you ever want to connect this to a real antenna, you should use a low-pass filter, like the QRPi board. In addition, only transmit if you have a proper license to do so.
## Using a Raspberry Pi as remote software defined radio peripheral from GNU Radio