[SFDXA] SFDXA May Meeting - May 5th: RF Exposure, MPE and SAR

[Kai]

This is what I would have presented... if the audio had been better.

The topic is: RF Safety Assessment; May 5, 2021

We were always required to be compliant to the RF safety rules:
See Form 605 - the micro text was always there!
"Item (6) Amateur Applicant/Licensee certifies that they have READ and WILL 
COMPLY WITH Section 97.13(c) of the Commission’s Rules (available 
at web site 
http://wireless.fcc.gov/rules.html) regarding RADIOFREQUENCY (RF) RADIATION 
SAFETY and the amateur
service section of OST/OET Bulletin Number 65 (available at web site 
http://www.fcc.gov/oet/info/documents/bulletins/)."

We were just exempt from certain measurements under some conditions (but we were 
never exempt from compliance).

There is a sub-group of us in the ARRL RFSC who are now tackling a thorny 

problem regarding compliance measurements from which we were previously exempt, 
both MPE and SAR.

The standards have not changed; the "new rules" simply eliminate the ham exemptions.
We are working with the FCC and also cooperation with the RSGB in UK on this 
problem.

Where compliance is to Max Permissible Exposure (MPE) the determination can be 
simple:

    http://www.arrl.org/rf-safety-publications
    http://www.arrl.org/rf-exposure

RF Exposure MPE calculators: THEY All WORK THE SAME WAY!!
Make sure that your calculator uses the applicable US FCC Standard. Euros 
and 
most other countries use the ICNIRP standard, it may
be slightly different than ours.
Use the one with the simplest GUI. They all consider MPE ONLY. For example:

http://www.lakewashingtonhamclub.org/resources/rf-exposure-calculator/

You enter power, antenna gain, frequency and "yes" to include the 
ground/environmental reflection.

Calculators find power density in W/m, Pd = S*EIRP/(4 pi r^2) then compare Pd to 
the MPE standard to get r=compliant distance.
where:
EIRP is (Ptx) x (antenna gain); S is an average ground/environmental reflection 
power multiplier of 2.56 (an accepted EPA figure).

ONE CAUTION, despite what the calculator says to do, ALWAYS measure distance 
from the closest part of the antenna.

Result would be conservative in most cases. MPE compliance distance will be 
overstated in most cases.
But can be spot on in some cases, like the NZ5N EME station:
  2m, 21dBi, EME array in peak direction with 1500 W pointing at moon rise (i.e. 
on the horizon): D = 455 ft with ground reflection!!
  In other directions, we must guess ... assume side-lobe gain is 0 dBi in 
non-peak directions:
  D = 41 feet in all non-peak-of-the-beam directions!!

If you don't like the conservative result from the simple calculators, make a 
detailed model in NEC / EZNEC for a more precise answer, less conservative. Or 
use the Tables in:
OST/OET Bulletin Number 65 (available at web site 
http://www.fcc.gov/oet/info/documents/bulletins/).
And you can apply mitigating factors like modulation average power and duty 
cycle to the power you report.

WHAT HAS CHANGED:
(1) Hams are no longer exempt from an assessment in certain cases based on power.
(2) For distances less than D=WaveLength/2pi MPE is not used, but rather SAR 
(specific absorption rate, in W/kg).
That works out to be D = 20 - 40 cm (8 - 16 inches at VHF/UHF). Thus we 
have a 
thorny problem with hand-held radios, which now must be
assessed using SAR!!

The FCC limit for public exposure from radios and cell phones is an SAR level of 
1.6 watts per kilogram (1.6 W/kg). For aware hams it is less restrictive.

What is SAR? It was my first job at Motorola in 1979. I was to measure SAR on 
the head of a cadaver from UHF MX hand-helds.
SAR is a measure of how much energy is deposited INTERNALLY into a human body, 
as compared with MPE (maximum permissible exposure), which is a measure EXTERNAL 
to the body.
Those MPE calculators are useful at distances, but do not deal with the pesky 
near-in SAR cases. They deal only with a simplified MPE
compliance.

Do watch the presentation that Barry indicated: 
https://www.youtube.com/watch?v=_R6MQD-WZiA

Kindest regards,
Kai Siwiak, KE4PT
ARRL RF Safety Committee

QUESTIONS?? Please contact Kai at k.siwiak at ieee.org