QRN of the man-made sort. Part One GM1SXX 10 September 2006-09-10

Much has been made in the past about the suitability, or otherwise’ of the 2M band for satellite uplinks/downlinks. I won’t add to the debate here but I’ve noticed that the level of RF ‘fog’ on all bands from 144Mhz to 2.4Ghz has increased greatly over recent years in this heavily populated area. For those unfamiliar with Scotland, much of the population lives in the ‘central belt’ which straddles the country from West to East across the relatively low-lying central Scotland area.. As such, the population density is high with all the usual associated RF issues.

The Paisley area is blanketed from 145.785-145.830Mhz in a ‘fog’ of raspy buzzing carriers (read digital noise). I confirmed this for sure last week while I was out walking in the hills. At first I thought my shiny new FT817 was at fault... I could copy quite a number of more or less evenly spaced carriers at excellent strength on the rubber ducky and at first I thought the interference was local despite being in the countryside. Placing a 50R terminator on the aerial socket killed the interference stone dead while swapping to the HB9CV showed it was emanating from suburbia. Pointing my HB9CV away from the town, the noise drop is very noticeable.

I would very much like to hear reports from readers in other places about the prevailing situation where you live, especially for 2M and 70Cms.

To let you understand the geography of the area, the town of Paisley is in a river valley between rolling hills to the South (800 feet) and bigger hills to the North … actually more rolling hills on the North of the valley and 3000 foot ‘Munroes‘ further North still around the 30 mile mark. The city of Glasgow is 7 miles to the East of the town and as far as built-up areas go, the whole river valley is just one expanse of towns and habitation. VHF paths to the East and West are obscured in places by hills. The hills I did the experiment on were at 600-700 feet ASL on the South side of the river overlooking the valley.

I need to do some more tests using different radios (different first IF) and using a front-end tuned filter but I have no doubt at all in my mind that this severe QRN is local in origin. I have my suspicions about the origins of this noise but I want to do some more basic tests before I make further comment. The most interesting aspect of this QRN to my mind is that it is not as strong within the town as outside it on the hills.

To me at least, this suggests multiple low-level RF sources sharing common features.

On 70Cms, myself and others have noted a general increase in the background noise as if some sort of FHSS system is being used on the band. This in fact may be the case since 70Cms is a secondary allocation with the MOD being the primary user.

Of the 2.4Ghz band, I have to report that this band is heavily utilised by the usual users including many tens of thousands of wireless devices including WLANs, Bluetooth and countless leaky microwave ovens, to name just a few. In addition, there are industrial and medical users, both of which are perfectly entitled to use high power on the band in an unregulated manner. In short. 2.4Ghz is a free-for all.. Lest anyone shoots me down in flames over 2.4Ghz, I DO fully recognise the advantages of correctly polarised under-illuminated dishes VS helical antennas for sat-comms.. Nonetheless, several years ago, my employer had to axe a 2.4Ghz WLAN link between two buildings half a mile apart and replace it with a far more expensive (and far more reliable) LASER based system because of the radiofog present on 2.4Ghz. Our WLAN;’s used 14Dbd Yagis at both ends of an unobstructed path and we had frequent problems with the system that led to the demise of the system. The current level of radiofog on 2.4Ghz here will be typical of most large cities thanks in large part to the proliferation of licence-free communications systems for domestic and commercial use. A quick look around any high street computer shop will reveal the multitude of 2.4Ghz devices available to the public.

If you use a 2.4Ghz WLAN at home, it can be entertaining to do a search for local unsecured wireless nodes. From my back garden, I can usually connect to three different WLAN’s using a basic PCMCIA laptop card. With a small yagi, that goes into double figures.. Not one of these WLAN’s is secured. Plug and play is with us it seems! Nobody seems to have heard of WEP in these parts. I’ve even ordered an airline ticket in the recent past using a neighbour’s WLAN, just to prove it could be done.

The 5Ghz band has been suggested by some in the amateur satellite community as an alternative to the crowded 2.4Ghz band but the reality is that it will be no different from 2.4Ghz by the time any hardware makes it into orbit. 5Ghz is also an ISB band and as such is not a ‘protected’ band for amateur radio. In short, it too is a free-for all.

This will be part one of an ongoing ‘Observation’ since a lot of practical experiments are possible in this area of interference.

73 Al.

GM1SXX