Spectrum Lab Application: Beacon Logger

By Wolfgang Buescher, DL4YHF
Last modified: October 12, 2001.
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Measuring the signal strength of low-duty-cycle beacons

This application is used for propagation studies on shortwave. Radio amateurs all over the world operate becons on certain frequencies (like 14.1 MHz). These beacons periodically transmit a carrier and their identifier in Morse code. Some of these beacons have a very low duty cycle but a precise interval, for example 1 second carrier and 3 minute interval. This application takes precise readings of the signal strength, and writes the result into a text file which can be post-processed with a spreadsheet program etc.

The settings for the two beacon-logging applications (presented below) can be loaded from configuration files which are part of the installation package. They are called "Beacon1.usr" and "Beacon2.usr". You can load them from Spectrum Lab's file menu as explained here, or add new entries to SL's quick settings menu as explained here. The latter allows you to quickly switch between different settings.

A more recent beacon-logging example (from 2005) for the experimental MW beacon DI2AG can be found in a separate document (only in german language so far).

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The precisely timed approach

If the beacon you want to monitor has a very stable interval, try this (or load the BEACON1.USR example into SpecLab from the "Quick Settings" menu):

There is one problem with this approach: you must precisely start the export function to catch the key-down phase. If your PC's clock or the beacon's clock is drifting a bit, the "sample slot" will slowly move away from the beacons key-down phase, no matter how precisely you measured the beacon's cycle time.

Important Note:
To get an appropriate frequency resolution (not too high and not too low), you must carefully select the FFT length and audio sample rate, for example: FFT length = 8192 samples; no decimation;  ADC sample rate = 11025 samples/second gives a resolution of about 1.4 Hz.
The FFT length and sample rate also set the time required to collect samples for a "new" spectrum. In this example, it takes about 0.75 seconds to collect samples for a completely new spectrum (without "overlap" or "sliding average"). Overlap would spoil the accuracy in this case. You can notice the overlap effect by a "smeared" waterfall along the time axis, if the waterfall scrolls much faster than the time required to collect samples for a "completely new" spectrum).

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The peak-collecting approach

To catch the carrier amplitude of a beacon with low duty cycle without the need for precise timing, there is a different solution which will result in a very clean exported file. Principle:

  1. Measure the carrier's peak amplitude in a 1-second interval. Compare this to the previously detected peak (which is saved in a variable). If the new value is lower than the old, forget the new one.
  2. Repeat this for 30 consecutive 1-second intervals. If the beacon transmits a 2-second carrier every 28 seconds, we can be sure that we have the correct peak value
  3. Write the maximum peak value into the export file. After this, set the variable for the peak value to a negative value. Start again with step one.

In Spectrum Lab, the steps described above can be performed simultaneously for many different frequency ranges. The following interpreter commands must be entered in the export configuration dialog to do the job (or load the BEACON2.USR example into SpecLab from the "Quick Settings" menu) :

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