Rob G3XFD – mystery transmissions on 7 and 10MHz

08:39 Fernando Luiz de Souza 0 Comments



From Rob Mannion G3XFD Tuesday March 25th 2014.
I've placed my video on to YouTube in the hope that other Radio Amateurs/listeners can help me identify the problems that this (very recent) interference source is causing me here in Dorset.
When I first heard the transmissions - I thought I had a problem with my Elecraft K3 transceiver. However, transmitting into a dummy load and other testing – with the help of Phil Ciotti G3XBZ - who assembled my rig and has one himself – at his QTH, proved there was no problem from my station. Additionally, he couldn't see or hear anything similar at his home about three miles away. 
When the offending transmissions first appeared (I was using c.w. on 7.020MHz at around the 10W level) the main offending transmission was about 600Hz below my own transmission) I thought someone might be causing interference or there were 'round the world echoes' of my transmissions (a weird but fascinating event that usually occurs on 14, 18, 21MHz, etc., in good propagation conditions). I was able to use the excellent Elecraft P3 pan adapter to 'see' the problem. Without the spectrum display unit I would't have 'seen' what was really happening.
I soon realised that the offending transmitter was automatic and was left switched on over a 24 hour period – I proved this with several 3am visits to the shack!). The higher the power radiated from my antenna system, the stronger the transmissions seem to be on my P3 pan adapter and they also are transmitted for a longer period (30 minutes plus). 
Another friend of mine who lives about five miles away has reported something similar on the spectrum 'scope screen on his Icom IC-756PRO.

My home in Bournemouth on the south west coast of England is surrounded on three sides by large houses converted in flats although (fortunately we are screened from them by tall trees) with many switched mode power supplies and 'internet over the mains' type units. My dipole antenna (mounted on a mast with a total height of 35ft. One leg of the dipole runs above our driveway and there are a large number of overhead telephone wires.
I've briefly seen the interfering transmitter operating on the 3.5MHz (80m band) but it's really strong on 7 (from 7 to 7.199MHz) and 10MHz (again over the whole band) . Incidentally, it doesn't actually transmit data (or whatever it's transmitting) continually but 'sits there' (somewhere) waiting to operate.
I would be most interested to hear from other Radio Amateurs who have identified similar transmissions. However, without a spectrum graphic display you would just think it was another one of those 'mystery' signals that appear to annoy us....but this one is different!  I hope you can help identify the source!
Regards to everyone,
Rob G3XFD



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