[TowerTalk] FCC RF Safety Regs Info Sept 1
Sat, 2 Sep 2000 03:19:51 EDT
I have serious concerns about the 'dummy' compliance calculators that are
making the rounds.
I do not know the formula they are using (although I DO know the one they
should) and it is extremely inaccurate and misleading as presented.
The inputs include frequency, power, and antenna gain (as well as distance).
Some modest range of parameters reveal 'cutoff' distances WELL WITHIN the
NEAR FIELD at some freqs and WELL WITHIN the FAR FIELD for others. This
Because the gain value is being used to get an ERP. However, this 'value' is
ONLY ACCURATE at a specific solid angle value or subset of values IN THE FAR
FIELD. One simply CANNOT use a dBmax value in the far field and apply it
globally to both near field and far field power densities at all solid angles
Furthermore, the power pattern (as it were) in the near field need not in any
way be the same as that in the far field. I am further concerned because the
average ham has no idea what the far field and near field are nor where the
transition region lay for a given aperture (why would they need to?)
So, BEWARE! The dummy calculator is ONLY ACCURATE as a WORST CASE estimate IN
THE FAR FIELD. It has no accuracy whatsoever for MF/LF antennas that I can
UNLESS you know the ACTUAL GAIN at a particular distance for an antenna at a
given frequency (and I do not mean PEAK FAR FIELD GAIN necessarily) the dummy
calculator is useless and misleading.
The good news is that putting the peak gain of the far field in the equation
DOES provide a worst case scenario estimate. But I cringe when it comes back
with values of distance very far within the NEAR field.
A best rule of thumb is KEEP OUT of the near field. Period. Depending on the
aperture, this may crudely be viewed as of order 1/8 wave or more.
I have no doubt that most hams are easily within compliance. I have serious
doubts if the dummy calculator is used to determine it; BE CAUTIONED THAT IT
ONLY GIVES A WORST CASE SCENARIO in the far field and DOES NOT actually
calculate the value accurately within the near field.
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