What is a WebSDR and why make one?
SDR means “software defined radio” – and the basic premise is that instead of homing in on one signal using a classic receiver, you can sample a substantial “chunk” of radio spectrum and then do whatever processing you like with it using software (all of the demodulation/filtering etc is now software-based). Web-enabling this means that the “chunk” becomes accessible for anyone! In fact, it is available for multiple users concurrently who can each choose what to do with it.
Over at WebSDR.org this concept has been running since 2008, and allows anyone to jump on a particular node somewhere in the world and tune around (click on the link and try it!). You can:
- Listen to CW, SSB, AM or FM within the passband of the SDR.
- Surf the bands without having to put up an antenna (great for units!)
- Listen to HF from anywhere using a modern smart phone!
- Check whether propagation seems open (can we hear DX on a certain band)
- Check your own station for performance (audio tests as well as propagation)
However, when you look at the node list there isn’t that much available ‘down under’: at present there is 1 node in Melbourne and 1 node in New Zealand. So, having another node in Sydney would certainly help!
What are we building?
We are building a 5-band WebSDR receiver containing the following items:
- HF loop antenna (active wideband loop)
- Active multicoupler
- 5 x Softrock RX III
- 5 x USB sound cards (192kHz sampling rate – Asus Xonau U7)
- Micro-sized “NUC” PC
The flexibility of this solution allows for 5 x 192kHz “chunks” of RF to be delivered anywhere in the HF spectrum (from 1.8MHz to 30MHz). Initially, we will configure this to deliver coverage on 80-40-20-15-10m. As seasonal or solar cycle characteristics change the propagation we may review the band coverage. Changing frequencies is a simple software reconfiguration!
Why are we doing it this way?
The HF loop antenna is compact, optimised for receive performance, and will deliver good performance across the HF spectrum. The multicoupler ensures we aren’t throwing away any signal as it is split between the 5 Softrock Receivers. The Softrock receivers are a tried-and-tested way of accessing a “chunk” of RF spectrum, and they output an audio signal containing the quadrature ‘I’ and ‘Q’ samples that let the PC do its magic. This is fed through high-resolution Asus Xonar U7 sound cards that have a 192kHz sampling rate (this is the limiting factor for how much RF bandwidth we can sample). The PC runs WebSDR node software that samples the audio and makes it available on the ‘net.
In short, we are aiming for a quality reproduction of 5 segments of HF. All of the specified parts have great dynamic range, and therefore the solution should not be compromised by any nearby strong signals. The wideband loop antenna and RX III receivers are very flexible options that allow future reconfiguration easily!
What’s the current status?
The most significant labour cost is building the Softrock kits. Club members Phil Angilley (VK2BDF), Matt Maguire (VK2RQ) and Geoff Van der Wagen (VK2AVR) are currently building the 5 Softrocks. Currently we have finished 3 out of 5. Once these are built and tested we will bring the other parts of the project together in another article!
Who else is helping?
This project is generously sponsored by ARNSW for the benefit of amateurs everywhere!