> The RF patterns (for safety) discussion is interesting. What seems to be
> missing in much of the discussion is a definition of "average power". How
> is this determined? For example, if I am a "hunt and pounce" CW DXer who
http://n5xu.ae.utexas.edu/rfsafety/power.shtml gives a good explanation and
It's pretty straight forward.
It's based on a 30 minute period over an uncontrolled area and 6 minutes
over a controlled area.
> transmits less than 1% of my operational time --- but might hit 20% for
> short periods in a pileup ---- how do I use the formulas and compute
You use 30 minute and 6 minute periods where you transmit the most.
First you take the duty factor which is 40% for CW, 20% for SSB and 100% for
FM, RTTY/Digital/ and AM. Calculate how much time you are transmitting
during your busiest period and calculate the average transmit time for both
30 minutes and 6 minutes. Multiply the results by the duty factor.
> "average power"? Is "average power" computed the same way for controlled
> and uncontrolled areas? If my operational time varies from 0 to maybe 2
Average is average, but the times are different.
> hours a day---with a long term average of 30 minutes a day---how is this
> averaged for these computations?
It doesn't matter how much you are on during any particular day. The only
periods used ate the busiest 30 minutes and the busiest 6 minutes (periods
with the most transmit time.
> I assume that actual CW transmission might have 50% average key-down power
> and SSB somewhat less.
40 and 20% respectively. Multiply the average transmit times to get the
For an example on FM. You are running 300 watts, you transmit 2 minutes,
listen 2 minutes and transmit for another 2 minutes. So your average is
66.66... or just round to 67%. So your average transmit time is 67% and on
FM the duty cycle is 100% * 67% * 300 watts = an average of 201 watts.
or take 1500 watts of SSB with a 20% duty cycle. Chasing DX you are going to
be transmitting less and listening more. So say you only transmit 2 minutes
total out of 6. That is 33.333, or 34% rounded up.
That makes 20% * 34% * 1500 for an average of only 102 watts.
> For operators in my general category, where the long-term transmitting
Long term does not enter into the calcs.
> average time is a few minutes per day (CW or SSB) it would seem that these
> averages will dominate any of the exposure computations. Really heavy use
> of RTTY, FM, etc, is a completely different matter and would appear to be
> so different that they should be computed differently. For my type of
You use the duty factor for these which is 100%, but the transmit time is
still the average for the busiest 30 minute and 6 minute times.
> operation, it would seem that (provided someone does not lean his head
> against the center of a beam or dipole) any reasonable operation is safe,
> even with a KW transmitter.
> Am I way off the mark here?
Not too far
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
> Bill - W2WO
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