SvxLink is an EchoLink and general purpose voice services system for controlling ham radio repeaters. A repeater is a radio tower that receives a weak transmission from a handheld or remote radio and then repeats the same message with greater power over a wide area. With repeaters radio communications can cover a much further distance.
Ham radio enthusiasts often set up repeaters for their own frequencies, so that they can be heard over a wider range. To control the repeater software like SvxLink is required. In the latest software update of SvxLink they added RTL-SDR support. They write:
The biggest news in this release is the support for RTL2832U based DVB-T USB dongles. This make it possible to use such USB dongles as cheap SDR (Software Defined Radio) receivers. This will open up the world of cheap receiver hardware to all SvxLink users. It will for example be very cheap to set up an extra receiver with local coverage for a SvxLink based repeater, as long as there is a network connection to the repeater. The modulation forms supported are: FM, FM narrow, AM, AM narrow, USB, LSB, CW, CW wide and wideband FM (broadcast). Running multiple receivers on the same dongle is supported as well as using multiple dongles.
Over on YouTube user Java’s Toys has uploaded a video showing a demo of his reception of a BPSK63 signal using his RTL-SDR and the Ham-it-up upconverter. BPSK63 is a text based digital communications mode used by ham radio enthusiasts to make contacts. It is twice as fast compared to the more commonly used BPSK31 mode.
Java’s toys used HDSDR together with Fldigi to receive and decode the signal.
D-STAR or Digital Smart Technologies for Amateur Radio is a digital voice and data protocol used in amateur radio. I was tweeted a link earlier which shows how the RTL-SDR can decode D-STAR text messages and headers (link is in Italian but Google translate can help, and the pictures show more than enough information). By using SDRSharp and stereo mix you can tune to a D-STAR signal, and pass the audio to a command line based decoding program (dstar.exe) which can be downloaded from the above link, which will then decode D-STAR text messages.
Here is also an older video showing D-STAR decoding with HDSDR in action.
For people already with expensive ham radios, the RTL-SDR can be used as a cheap panadapter. A panadapter is device that allows you to visually see the RF spectrum and waterfall being received by the ham radio. There are multiple (expensive) commercial panadapters available, but combined with a PC or laptop, the RTL-SDR will work just as well.
In this video YouTube user akdude47 shows a tutorial on setting up the RTL-SDR as a panadapter for a Yaesu FT-857. The setup involves connecting the IF output of the radio to the RTL-SDR, and putting in some settings into HDSDR.
YouTube user opilarczyk has posted a video comparing HF performance of the rtl-sdr with the direct sampling mod at 7MHz with WebSDR, an online SDR streaming site. The comparison shows how the direct sampling mod is extremely susceptible to broadcast FM interference.
Digital Radio Monodial (DRM) radio is a type of digital shortwave radio signal that is used by international shortwave radio broadcasters. It provides superior audio quality compared to AM signals by using digital audio encoding. With an upconverter, good antenna, and decoding software the RTL-SDR software defined radio can receive and decode DRM signals. This tutorial is also applicable to other software defined radios that can receive HF with or without an upconverter, such as the HackRF, Airspy, Softrock and Funcube dongle.
Examples of DRM Decoding
YouTube user Superphish shows DRM reception with his Ham-it-up upconverter, and rtl-sdr.
The RTL-SDR software defined radio can be told to run in a mode called “direct sampling mode”, which with a small hardware mod allows the dongle to tune to the HF frequencies where ham radio and many other interesting signals are found. This means that no upconverter circuit is required.
However, the difficulty with direct sampling is that a hardware modification to the dongle is required. Also, the performance can not be expected to be as good as an upconverter without the addition of extra filtering circuits.