Stefan's Radio Blog

APRS Windows Phone - Working on version

I finally found again some time to work on next version of the APRS Windows Phone app. Since I got a new notebook I had to prepare the development environment first. That took quite some time today. As a first step I implemented a bug fix for the reported speed and improved some input validations for the settings. Tomorrow I will test the reported speed and begin to work on APRS monitoring.

Frank's Sputnik Replica - The Transmitter

In the Januar edition of the RAZzies magazine Frank Waarsenburg PA3CNO continued to report about building a replica of the original Sputnik 1 transmitter. In his interesting article he writes about the difficulties and obstacles he faced: Missing data for winding the coils, errors in 2p19b datasheets and a critical bug in the schematics. He described his approaches to tackle these problems and I was happy to read that he found my blog providing some useful information for him.

I think for everyone who is interested in understanding the Sputnik 1 transmitter or who wants even to build a replica Frank's article is a must read. Please note that the RAZzies is a Dutch HAM magazine. If you don't understand Dutch simply use the Google translator or similar tools in order to get a translation.

The image above shows Frank's version of the Sputnik 1 transmitter and a switching power supply build on a Veroboard.

Hacker Attack

Yesterday this blog has been attacked by a hacker. He or she succeeded to post an article and to create a new administrator. Today I removed the user and the article again. Please leave me a message if you encounter any weird content.

The software in SDR - how does it work?

Last year I have spent some time trying to understand how the software in SDR works. After reading some books about digital signal processing and also articles about SDR published in the internet I found the project This project offers a really nice book giving a great introduction into SDR with a lot of experiments using Matlab and the RTL-SDR. If you want to understand the software part of SDR I would recommend to start with their book. You can download either the free PDF version from their website or buy the printed version.

Yesterday a small article in German about this topic has been published on

Using Raspberry Pi as SDR Transmitter

When reading the CQ DL magazine 1-2016 I stumbled over an interesting article: Evariste F0OEO developed a software called PiTX that implements an SDR transmitter for the Raspberry Pi. With PiTX it is possible to transmit in AM, FM, SSB, SSTV and FSQ in a frequency range between 130 kHz and 750 MHz.

As an input signal the software take an audio file and generate modulated RF using pin 12 (GPIO 18) on the GPIO port. The produced output is a very low power square wave signal. So in real applications one might want to add a filter and an amplifier.

The source code is open source licensed under GPL and available at GitHub. There you can find also a short installation guide. The software supports IQ signals in the Stereo WAV files as input signal. This opens interesting options for experimenting with SDR: You can generate the IQ signal with your own software, transmit it over the air using PiTX and listen to the signal with your radio.

More about the PiTX can be found Or you might want to look at QTCSDR from HA7ILM - an experimental SDR transceiver based on the RTL-SDR stick, an Audio-Card and a Raspberry Pi.