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):

• Set the waterfall scroll interval to less than half the length of the beacon's "key-down" interval
  (under "Options..Display Settings"). For example, if the carrier is "on" for 5 seconds, use a 1-second waterfall scroll interval.
• Set the frequency resolution (FFT length and audio sample rate) to an appropriate value. For shortwave observations, a frequency resolution of 1 Hertz is enough. See "important note" below.
• Set the "Write Interval" for the file export function to the beacons cycle time, for example 30 seconds.
  (in the "Text File Export" configuration dialog)
• Use the "peak_a(f1,f2)"-function to detect the amplitude of the carrier in a narrow frequency range. Allow a few Hertz of  'drifting range' for the parameters f1 and f2. This numeric expression must be entered in the export definition table. The "peak_a" function will look at the spectrum which has been recorded over a few seconds.

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) :

• Turn the checkmark "Use Write Interval" on, and set the write interval to 30 seconds
• Define the expression for one column of the export file like this (for example, column 2 of the export file = line 2 of the definition table):
  max(peak_a(795,805),export.value[2])
  (this expression detects the main peak between 795 and 805 Hz audio, compares it to the old value from column 2, and returns the maximum of both values which will be the "new" value for this colum. It you entered this in column 3, you would have to use index  [3] in the expression).
• In the edit field named "After writing a line to the export file, execute these commands", enter this:
  export.value[2]=-200
  (this sets the exported value in the second export column to a new starting value, as in the last step of the principle described above)

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