Practical Wireless

Ofcom EMF Rules


Dear Don,

From the Ofcom EMF exposure document: “A reduction in the transmit time over any six-minute period will give a correspond­ing reduction in the average power.”

This phrase ‘any six-minute period’ confuses me. ‘Average power’ is not unambiguou­s either.

Imagine I have two scheduled QSOs a day, each lasting six minutes. For one I use 100W and for the other I use 1W. The rest of the day my radio is off.

‘Any’ means ‘any’, presumably. When a magician says ‘pick a card, any card’ he or she means one card out of a fanned-out pack. Whatever I pick, the other 51 cards are not represente­d.

Now 1120 to 1126 is a 6-minute period, one of 240 in a day. It qualifies as ‘any six-minute period’. The (peak) power in this - in ‘any’ - period could therefore be 100W (1 chance in 240), 1W (1 in 240) or 0W (238 in 240).

This does not help. Ofcom must want some kind of average six minutes.

The arithmetic mean is what people usually mean by an average. This is 101/240 which is 420mW, much less than the 10W EIRP lower limit.

If Ofcom meant any ‘active’ six-minute period - one in which I actually transmit - the average peak power is 50.5W.

Now assume I operate 12 hours in the day. Six minutes QRP, the rest on 100W. I’m clearly putting out a lot more RF.

‘Any six-minute period’, by my magician’s argument, is unchanged and the peak power could still be 100W, 1W or 0W. The distributi­on of powers has changed, but the values have not.

Average peak power in any six-minute period is now 11901/240 = 49.59 W and for any ‘active’ six minutes we get 11901/120 = 99.18W.

So, what precisely do Ofcom mean? The assessment power, in these two rather extreme examples, could be anywhere from zero to 100W.

And I still haven’t allowed for the nature of our operating. Assuming a 50:50 transmit to receive cycle, each active six minutes has only three minutes of actual transmissi­on. So, should all this be based on 50W and 500mW?

I’m not writing this to stir things up - would I do that? I really don’t know how Ofcom meant me to work this out. Tony Jones G7ETW Harrow

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