Originally posted by minilandrover
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Hi Colin,
Thank you for interest! Airplanes are passive radio wave reflectors which bounce radio signals back to ground greatly extending their range. I am listening to 6 m foreign TV transmitters and 2 m band ham radio beacons through Aircraft Scatter (AS). On 144 MHz / 2 m band the AS doppler shift is about +-200 Hz and on 6 m / 50 MHz band about +-70 Hz.
The AS flutter/swaying effect is produced by my observations as an interference of direct transmitter (Tx) frequency and the doppler frequency shift of the reflected aircraft scatter signal. I have also found that the momentary 'zero beat' of AS flutter happens at the moment when the plane is crossing the line between Tx and receiving station (Rx).
If you know with help of flightradar site or ADS-B receiver the position of the aircraft, this AS flutter zero beat effect can be used for direction finding of unknown Tx, for instance. On the other hand, if you know the Tx position, you can tell by the AS swaying zero beat the exact moment when an airplane crosses the line between Tx and Rx.
Two Way AS QSO?
Although I have had occasional AS QSO's on 2 m FM for decades ago there are yet very few active AS dedicated hams and I guess none within my range of two radio horizons = abt 840 km, so I have not had any 'real' schedule QSO's through AS. I guess that one could use MS call frecuencies for AS QSO's.
With help of aircraft locating sites like flightradar24 it is easy to look for an aircraft on course suitable for reflecting signals between two radio stations. If you have a rotatable directional aerial turn it towards the plane and follow its movement with the antenna direction.
The AS passive radio wave reflection is most intense when the aircraft is near the line between Rx and Tx stations, or above either station. If an aircraft passes above you a simple omnidirectional aerial is enough for observing AS or having a two way QSO through AS reflection. Because most aircraft reflecting surfaces are level, a horizontally polarized aerial is favored. However, if the plane is close to you the AS effect works quite well with vertical aerials, too.
Beacons and TV Stations
However, one can easily observe AS signals without help of other active hams. I listen to the 6 m QRP ham beacon OH5SHF with 20 W directive power (ERP) located 243 km away from my Rx station. It can be weakly but clearly heard on CW for a minute whenever a plane passes the line between Tx and Rx.
The OIRT 1 ch 50.750 MHz Russian 6 m band TV stations are so powerful that they have an AS range of up to the radio horizon of 420 km from Tx-Rx line. They can be heard also through MS easily up to 900 + km away but without the AS doppler swaying/flutter of course. The AS is said to be very clearly visible on upper HF bands, too, but I have not yet tried it.
Below are some screenshots about Rx and Tx positions and examples of 'Aircraft Scatter Direction Finding' (ASDF) of Tx stations.
Regards and 73,
- Juha -
Links
A Finnish AS thread with pic captions in English: http://oh7ab.fi/foorumi/viewtopic.ph...295&p=724#p721
Video about Doppler Shift of Aircraft Scatter on 144 MHz: http://www.youtube.com/watch?v=Iouk8hLckj4
Video of Aircraft Scatter Wobbling Flutter Sound on 49.750 MHz: http://www.youtube.com/watch?v=ANYf2kcTwME
PC sound card Audio Spectrum Analyzer "Spectrum Lab" software by DL4YHF: http://www.qsl.net/dl4yhf/spectra1.html#download
Attached images:
2012-02-08 49.757.844-1 Flight AFL321 Segezha Nadvoitsy TV Tx Aircraft Scatter Direction Finding Example (c) OH7HJ
2012-02-08 49750-3 Boeing 747 KAL510 direction finding example of KO59DX St Petersburg TV by crossing Rx-Tx line (c) OH7HJ
50.033.097 KP30HV Kouvola Kuusankoski OH5SHF 20 W dist 243 dir 170
2012-02-09 50.033.097-03 An example of using aircraft zero beat interference point for Tx direction finding (c) OH7HJ
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