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Re: [TowerTalk] RF Pattern

To: <>
Subject: Re: [TowerTalk] RF Pattern
From: "K8RI on Tower talk" <>
Date: Thu, 27 Apr 2006 16:41:02 -0400
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> 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 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 
average power.
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

Good Luck,

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
> Bill - W2WO
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