The OMs Chris DL1CR, Ben DK6OT and Juergen DF9OA have been dealing with Ham radio on the medium wave 630 m for some time. The idea came up to activate the QRG 828 kHz for the Maker Faire as part of an “event broadcasting”. The frequency choice is no coincidence, because until the beginning of 2015 there was a local station in Hanover. Meanwhile, all AM stations in Germany are completely shut down.
In order to use this AM radio frequency, it first requires the approval of the responsible officials BNetzA. This was granted surprise, but only with the allowed ERP of 100mW with an assumed efficiency of the antenna of 1%.
The transmission technology is built around a Direct Digital Syntheziser (DDS) of the type AD9850. An Arduino microcontroller controls everything, a microphone input with compressor and an MP3 module for the pause signal provide the modulation signal. A 5W-PA with harmonic filter and fitting means will (hopefully) reach the allowed 100 mW ERP.
Pause sign in AM https://youtu.be/3UrnGWDxwco
As a small highlight, our technicians, Chris have come up with shift of +/- 100 Hz with CW signals. This is not heard with a normal AM receiver, so do not bother. However, if you have a proper AFU-RX and set the receive QRG and bandwidth correctly, you can hear the CW. The signal (read: key pressed) is sent to 828 kHz + 100 Hz. 828 kHz - 100 Hz then represents the pause. Normally, the BNetzA prescribes a frequency constancy of the carrier of + - 10 Hz. The request to extend it to + -100 Hz to enable the CW broadcast was unbureaucratically complied with.
We will also run the station during the night of 17 to 18 August and probably also in the night of 18 to 19 August and are looking forward to receiving reports!
Frequency Shift Keying in CW https://youtu.be/WmW5eyIxiQ8
Based on our experience at 630m, the range of the CW signal with interference-free reception could be well over 100km.
Reception reports are very welcome!
Matthias Wendt, DL9MWE