HF Spectrum Monitor Technical Information
Location : Central Southampton, UK
September 2015 : The receiver has been re-boxed in a larger enclosure with built-in mains power supply with better connectors and earthing.
This website allows you to connect to and control a wideband software defined HF radio receiver connected to this server.
It offers a full 30MHz instantaneous view of the HF spectrum, which can be zoomed to show bands in detail.
You have tuning control of one of the receiver's DDC tuned channels. You can view the narrowband spectrum
in this channel, and listen to the output of a demodulator via streaming audio.
All the data collection, processing and graph creation is performed on the server, with the browser acting as a
display and request interface. The system is free to use and no login details are required.
When you load the viewer, you will be allocated a temporary ID that allows you to tune one channel and view the
tuned and wideband spectra.
The HTML5 viewer has been tested with Firefox 12.0, Internet Explorer 9.0, Chrome 18.0 and Opera 11.62
(audio stream has extra delays and gaps in Chrome, and seems a little erratic in Opera).
Queries and suggestions may be sent to
Note that occasionally some man-made interference (QRM) is seen from nearby IP over mains signalling.
Main Flash Viewer
(See current 30MHz spectrum, tune and listen live audio)
Main HTML5 Viewer
Note that the MP3 streaming may not work behind a Proxy/Cache (e.g. at work), and in this case, a small 10s live sample MP3
can be played instead.
[To see if you have the Flash Player (Version 9 or later), click here]
[To download the latest Flash Player click here]
A 7 day 24 hour zoom-able waterfall display shows a chosen section of the spectrum built up from snapshots
taken at 2 minute intervals. The "live" page shows data over a rolling24 hours (data from 00:00 UTC to "now" from today, data from "now" to 23:59 UTC from yesterday). Previous days completed spectra may be selected for comparison. Fully zoomed resolution is approximately 380 Hz / bin (windowed).
Hardware and software details
This HF Spectrum Monitor is based around a commercial Digital Wideband Receiver. This digitises the antenna input using a 16bit
ADC at 80 MHz, providing a wideband signal snapshot together with four 48kHz narrowband tuned outputs. The receiver output is
via high speed USB2.
A precision low phase noise crystal frequency reference from
Precision Test Systems
provides a stable 80MHz clock for the receiver.
Although the receiver itself has a good on-board crystal, this reference provides a stable signal with around 0.01ppm accuracy.
The receiver is connected via USB2 to the 1U web server (AMD Athlon 5050e Dual Core 2.6GHz with 4GB RAM).
This allows multiple users to view the wideband spectrum, and each user may tune one narrowband channel and view a
narrowband spectral plot or provide audio output.
A separate 24 hour waterfall plot may be viewed and zoomed independently by each user.
The server runs CentOS Linux. The connection to the internet is via 4 Mbit ADSL modem / router.
The antenna is a 1.1m active loop made by
It is mounted on a vertical metal pole (also used for a washing line) around 2.5m above the ground.
Data collection, receiver control and image generation software is written in "C".
The data collection program runs continuously, saving sections of wideband and narrowband
data to temporary RAM files. All images are produced using the GD library in PNG format on demand from the user's web browser.
The main web page contains dynamic HTML or Adobe Flash viewers that display these graphs and allow receiver control.