G8MNY  > TECH     11.04.25 09:00z 56 Lines 2397 Bytes #116 (0) @ WW
BID : 30031_GB7CIP
Subj: Truth test for EMF Calculators
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 250411/0848Z @:GB7CIP.#32.GBR.EURO #:30031 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                  (Updated Jul 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

The new guidance limits originate from ICNRIP, for measurable body effects
(often good for you, like hospital diathermal machines) & are many magnitudes
below any serious safety limits! These limits have now been set into law for UK
hams to comply with by UK regulator Ofcom.

Here is a simple exercise you can test an EMF calculator's grasp of reality.
(EMF does not mean Electro Motive Force, as all engineers were taught, but now
means Electro Magnetic Fields).


Example 1, 400W to                            Example 2, 200W to
a Resonant Dipole.                            half of a Dipole
                                              inverted L.
            High
           Current
            Middle
   ÄÄÄÄÄÄÄÄÄÄÄ¿ÚÄÄÄÄÄÄÄÄÄÄÄ                     High ÚÄÄÄÄÄÄÄÄÄÄÄ
 High    200W ³³ 200W    High                 current³ 200W    1 High
 Voltage      ³³        Voltage                      ³         Voltage
 End 1     No Feeder     End 2                coax===o)¿        End
           Radiation                                  ///

dBd gain = 0dB                             dBd gain = -3dB (-2.4dB?)
True ERP = 400W, Figure 8 pattern          True ERP = 100W (150W), Omni pattern


The balanced feed represents no hazard, but the unbalanced feed wire against a
good ground is a problem until signal is in an unbalanced coax feed.

From first principles, both of these aerials will have the same high voltage at
their ends (near field V/M field strength), but the true ERPs are nearly 6dB
different!

So for near field hazard results, both examples should produce the same caution
distance on a proper EMF hazard near field calculator, but a 6dB difference (2x
the distance @ 400W) in a FAR FIELD NON HAZARD calculator.

I always avoid the word "Safety" as the new regulations are based on
"investigation levels" principles that "just might have a detectable effect on
the body", & not actually "safety levels"!

N.B. Near fields (transformer & capacitance effects) drops off at the cube of
distance rate (to 1/8 power @ 2x distance) for about the 1st wavelength or so,
then far fields @ square of distance (to 1/4 power for 2x distance) after that.


Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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