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LF beacon hunting: first steps

This is an outline of my first LF beacon-hunting experiments. The most comprehensive source of information is Alan G4TMV's Beaconworld website. This includes a detailed guide to reception techniques and frequently-updated beacon lists. By comparison my initial attempts are very primitive, using only an HF transciever, longish random wire and a (by modern standards) slow PC with an assortment of audio freeware.

The aeronautical beacons which mostly operate between 200kHz and 400kHz are particularly good targets: many operate 24/7, they are well-documented and their distinctive 400Hz keyed modulation and slow morse ID mean they can be readily identified without special software.

Getting started

The logical place to start was with my local beacons, so I used location data from one of the Beaconworld lists to generate a list of British beacons ordered by their distance from my station. The nearest was BRI (Bristol Airport) on 414kHz.

BRI was very strong, and its morse ID clearly audible. Speclab clearly showed both carrier and 400Hz sidebands, but equally clear is the action of the receiver's AGC (visible as pronounced horizintal banding on the waterfall chart) and spurious signals (numerous vertical bands on the waterfall and spikes on the spectrogram).

Using the receiver's RF attenuator reduced the input below the level where the AGC and spurious mixing products were obtrusive and produced much more useful results.

Looking further afield

I then started to look for more distant beacons. Some I could identify aurally; others were only audible as carriers while the waterfall clearly showed the characteristic pulsed sidebands, for example OX (Oxford Airport) over 100km away. The signal on 367.6kHz is the lower sideband of a beacon on 368kHz.

Only the three nearest beacons required attenuation; clearly the amount of attenuation necessary depends on the efficiency of the aerial system and sensitivity and selectivity of the receiver as well as the strength of the incoming signal.

The 368kHz beacon mentioned above is a puzzle. Listings suggest it could be WTD (Waterford) or UW (Edinburgh), both of which are well down my list by distance. Waterford would seem to be the more likely of the two, though the signal is far too weak to identify its ID aurally.

Potential improvements

Doing more than I've described here will need significant hardware improvements, viz

• A better aerial for LF.
• A LF pre-amp.
• A faster PC. Speclab uses all my available CPU power to run fairly slowly. A fast PC could enable visual identification of morse IDs.
• Better IF filters.
• Better AF filters.
• Long-period FFT integration (of the type use to produce the spectrograms on this page) could perhaps be used to find (though not necessarily identify) extremely weak or interfered signals. Unfortunately Audacity's abilities in this are limited to little more than I've already done here; I'm unaware of any other free software which even does this well.
• Calibrate the RF attenuator and AIP. This would make possible stitching together narrow spectrographs. Probably more trouble than it's worth.

Software notes

The waterfall charts (showing frequency, time and amplitude) are produced using Speclab (Fmin = -500Hz, Fmax = 500Hz, Foff = -800Hz, scroll interval = 1s). The spectrograms (showing frequency and amplitude) are generated using Scilab to process and plot the data generated by Audacity's "analyse" function (spectrum, Hanning function, 16384, linear frequency) from a 60s WAV recording (16 bit, 8kHz sampling). Both Speclab and Scilab were configured to record the 1kHz wide slice of the 2.4kHz passband where the filter response appeared to be flattest. The list of beacons by distance was generated using OpenOffice.org Calc. Stations with spare CPU cycles may also find Ham Radio Deluxe useful for PC receiver control. Links to sources for all of this (free) software are on my software list.

Document history

2007-04-17: first version