Saturday, May 22, 2021

Aircraft Scatter on the 40 MHz band

 A few days ago during the Sporadic-E backscatter tests on the 40 MHz band, I noticed some curious little ticks on the waterfall display. These short lines were moving slowly downwards in frequency and lasted for about 30 seconds, a tell tale signature of aircraft scatter.

On the 21st of May, the band seemed to be quieter with less Sporadic-E about so I tried some aircraft scatter tests with both of the Irish 40 MHz beacons (EI1KNH 40.013 MHz & EI1CAH 40.016 MHz).

Both beacons are about 200kms from my location and I used the FlightRadar24 website to track the various flights. This was the strongest signal that I got...

The image above shows the carrier & CW portion of the signal appearing at about 11:06 UTC (12:06 Summer time). As the aircraft was moving at several hundred kph, there was some doppler shift in the received signal and this can be seen in the signal slowly drifting downwards in frequency.

There is a gap at around 11:07 UTC when the beacon was sending out PI4 signals and it then returns again.

This was one of the longest signals I got as the vast majority last maybe 20-30 seconds.

The image below shows what happened....

The EI1KNH beacon is about 200kms from my location. A Delta Airlines plane heading from London to Seattle crossed over the mountains to the south-west of the beacon and it reflected the signal.

While the beacon has an excellent take off to the East, it is badly blocked to the West by the local mountains. I have a pretty good take off in that direction which means I am probably close to line of sight to planes at an altitude of 35000-40000 feet (7-8kms) over the mountains near the beacon.

Despite the doppler shift, I managed to get a successful PI4 decode during this crossing and a small number of other ones.

Using the FlightRadar24 site as a reference, I could correlate a lot of the doppler tracks seen with passing aircraft near both of the beacons. Nearly all of the signals were incredibly weak and were not audible to the ear. Most of the tracks were also about 10-30 seconds in duration. 

It's also worth noting that not all passing aircraft resulted in me hearing a signal. One large airplane went right over the EI1KNH beacon and I saw nothing. Obviously the orientation of the aircraft with respect to the beacon and my location are a factor here. It needs to be close but not too close.

I also got some reflections from aircraft that were on the far side of the beacon. I got one trail from a Ryanair flight just off the coast of Wicklow and heading north for Dublin as well as tracks from trans-Atlantic aircraft passing just north of the EI1CAH beacon in the west of Ireland.

FT8: Considering how often I was seeing these little doppler tracks at 40 MHz, I suspect a lot of people are getting FT8 decodes on the 28 MHz and 50 MHz bands from stations in the 100km to 250km range via aircraft scatter without even really realising it. I often see this, I get one FT8 decode from a station and get nothing else.

Other bands: Needless to say, aircraft scatter isn't confined to the 40 MHz band. If you or a distant beacon or station are blocked by a local hill then you might see aircraft scatter on 28 MHz or any of the VHF and UHF bands.

In the tests that I carried out above, I was 200kms away from the distant beacons and aircraft and the signals were weak as a result. If you try this and the distance are much shorter then you may experience much stronger reflections and signals.

1 comment:

NN4DX said...

Hi John, When I lived in Southern California in the 80's airplane scatter was prevalent on 50 MHz. Scatter could be detected by fluttering audio and an increase in signal strength as jets left LAX north over the San Fernando Valley from my location in Simi Valley. There were a number of 6-Meter repeaters north of LA along with a great deal of FM simplex work.

73, Don