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Re: RF field strength - SSTC coils



Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


I found a post of mine (From Jan 14th) that didn't show up in the archives...

I just was sent a copy of IEEE Std C95.1, 1999 edition:
IEEE Standard for Safety Levels with Respect to Human Exposure to RF 
Electromagnetic Fields, 3kHz to 300 GHz..

This is an update of the earlier 1991 standard.  I suspect that most of the 
changes have to do with exposures in higher frequencies than we fool with 
in tesla coils, but, there was some interesting stuff.

I leave it to others to evaluate what sorts of fields result from the 
typical TC...

They have limits (MPE = Maximum Permissible Exposure, which includes some 
safety factor) for grasping contact. (where sparks,startle, etc. aren't an 
issue)

The spec calls for a maximum RF current (averaged over 1 second) of 1000 f 
mA, where f is in MHz, and this applies from 3 kHz to 100 kHz. From 100 kHz 
to 100 MHz, the maximum current is 100 mA.  For induced currents, it's that 
value per foot (i.e. if the current passes through both feet, you can be 
exposed to 200 mA).  The current can be calculated (or measured) using a 
model of the human body impedance. They have charts, and there's a lot of 
variability (50%), but here's some numbers (error bars on Zmag are about 60 
ohms and 2 degrees for phase).  The first number is for males (where it's 
the top of the error bar, the second for females (where it's the bottom of 
the error bar).

Frequency(kHz)  Zmag    Phase
10              500/630 12/11
30              480/580 10/9
100             420/540 9/8
300             400/500 7/7
1000            380/480 5.5/6
3000            350/450 6.5/7


For the field limits.. in controlled enviroments
3-100 kHz, 614 V/m E field, 163 A/m H field, 6 minute averaging time
100-3000 kHz, 614 V/m E field, 16.3/f A/m H field, 6 minute averaging (f in 
MHz)

For pulsed fields, in the range 100 kHz-300 GHz, peak E field must be < 
100kV/m and for pulses <100 ms (i.e. TC output), the MPE is the value from 
above (614 V/m) adjusted as follows:

Peak MPE = MPE * Averaging time in seconds(i.e. 360)/ (5 * pulse 
width(seconds))

A max of 5 pulses calculated as above with a time between pulses > 100 ms 
are permitted in the averaging interval. For higher PRF you just average 
over the interval (6 mins), except that in any 100 ms period the sum of the 
peakMPE*pulsewidth = MPE *averagingtime/5

-----------------------------------------------------------------------
And... another post of the same day:
I'm looking for a report by S.J. Rogers (USAFSAM-TR-81-28) called 
"Radio-Frequency Burn Hazards in the MF/HF Band", and I'll report when I 
find it.  But, in the mean time, there's an interesting report at:


http://www.brooks.af.mil/AFRL/HED/hedr/reports/bioeffects/3-1-4-4.htm

which discusses shock and burn hazards from RF.  Lots of interesting data. 
(The table of contents for the whole report containing the part above is 
at: http://www.brooks.af.mil/AFRL/HED/hedr/reports/bioeffects/87-3con.htm )


There were some studies by Gandhi and Chatterjee (in 1982) that did some 
modeling and then measurements for threshold perception and let-go.  They 
mention that sensation changes from tingling to internal heat at around 
100-200 kHz.  Perception threshold linearly runs from 0.4 mA at 10 kHz to 
14 mA at 150 kHz, and then to 100 mA at 20 Mhz.  Let go is 6.4mA to 85mA to 
200 mA. (those are currents through the muscle/skin)..

They then worked out what E field corresponds to that for a metal roof, a 
50' fence, a car, and a forklift...
for 10-100 kHz, perception threshold was 250,160,80,20 V/m for the objects 
respectively. Let go fields were 1050, 850, 550, 110 V/m.

These are pretty low fields... the Efield program can calculate the 
expected field given topload voltage, etc. (I'd use a working number for 
max topload voltage of 500 kV, or, perhaps 30 kV per cm smallest radius of 
the topload) Something made with, for instance, 4" dryer duct would be 150 
kV (5 cm radius...)


The above report also cites some stuff from Guy and Chou more relevant to 
TCs because they were worried about LF and MF signals including signals at 
23 kHz and 146 KHz   They found that fields as low as 97 V/m could induce 
currents in the body, and the highest absorption would be at the ankles and 
would have a Specific Absorption Rate (SAR)of 8 W/kg (the maximum allowed 
by the ANSI standard, providing whole body averaged SAR is <0.4W/kg, and 
averaged over 6 minutes (0.1 hour).  The real problems were with people in 
contact with a vehicle (standing outside and touching the vehicle).

Fascinating stuff...


OSHA's site also has some interesting stuff..
http://www.osha-slc.gov/SLTC/radiofrequencyradiation/rfpresentation/healtheffects/mainpage1.html 




At 02:54 PM 5/28/2003 -0600, you wrote:
>Original poster: "Jan Wagner by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><jwagner-at-cc.hut.fi>
>
>Hi,
>
>On Wed, 28 May 2003, Tesla list wrote:
> > Original poster: "Steven Ward by way of Terry Fritz 
> <teslalist-at-qwest-dot-net>" <srward16-at-hotmail-dot-com>
> > Dan,
> >
> > It seems that maybe since you use a signal generator to feed your SSTC, you
> > are slightly out of tune and this causes your coil to radiate so much
> > power.  My SSTC does not give me any zaps or anything when i touch ground,
> > even with a 14" spark firing off of it.  Its a feedback driven coil, so its
> > always perfectly in tune.  Maybe thats your problem?
>
>Shouldn't the radiated power be highest at resonance? As the secondary
>draws the highest power there... Same for an antenna fed by a tuned tank,
>transmit power is max at resonance. Of course, when there's no streamer
>breakout, more power is radiated, but this isn't related to the drive
>being in-tune or not. ?
>
>But anyway, it would certainly be interesting to know whether or not a
>1..10 kW SSTC could exceed the FCC specsed SAR of 4W/kg (6 minutes
>exposure?) in typical spectator distance. Not that there are any effects
>noticeable, in practice, though... ;-)
>
>regards,
>  - jfw
>
>--
>*************************************************
>  high voltage at http://www.hut.fi/~jwagner/tesla
>  Jan OH2GHR