An experimental propagation beacon on 144.385MHz – part 2

An experimental propagation beacon on 144.385MHz laid out plans and some first test results.

This article explores a possible deployment scenario and likely paths over which it may be ‘heard' using Spectrum Lab or the like to dig the signal out of the noise.


Transmit end

Tx power is 10W with on-off keying (OOK or A1 CW).

Tx antenna system gain ~17dBi.

Receive end

Rx antenna gain is 9dBi.

Rx NF specification is 12dB (4236K).

Above are the results of an ambient noise test. In this case, the noise floor at the antenna is some 7000K, it is probably ~7dB higher than a modest weak signal station.

Path evaluation

Above is a screenshot of signals during that initial test. This chart was typical of activity through the day, a fine windy day of temperatures around 12° over the path.

Relying on this example, the noise floor is about -60dB and the aircraft reflections reach about -10dB at the mid point, >-30dB for some period around that mid point. So, there is a period of some minutes on each flight where the aircraft reflection is 30dB above the system noise floor.

We can make some adjustments for a deployment scenario:

  • omni tx antenna of 0dBi, subtract 17dB;
  • better rx antenna of 15dBi, add 6dB;
  • lower total rx system noise of 1500K, add 7dB.

So, we might expect over the 150km path length, that the aircraft reflection might be greater than 26dB over several minutes mid path.

Now let's extrapolate to a longer path. If the path was 1000km (eg Canberra to Brisbane or Adelaide), we might expect ~16dB more path loss, and that the aircraft reflection might be greater than 10dB over several minutes mid path. Figure out the case for Canberra to Perth… perhaps AE contacts from the East coast to Perth may be available to an intelligent observation and planning process.

There will be a limit to the radio horizon via an AE path, about 430km each way for an aircraft at 11,000m, a beacon such as this allows observation of that more easily.

A work in progress…