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Re: [TCML] Maximum Field
On 12/21/12 7:07 AM, Derek, Extreme Electronics wrote:
Jim,
I agree with your assessment of the radiated energy, but if you
design a coil to give a high field without any mitigating sparks you
will radiate more RF than usual.
Even if you only radiate 0.01 % of say 5KW, that is enough to
interfere with radio communication in a large area, my warning was
purely to take this in mind when building such a coil.
Luckily radios tend to use much higher frequencies than most of our
coils these days, so this is becoming less of an issue admittedly.
Derek
0.01% is an amazingly high radiation efficiency...
Let's put some real numbers in...
Consider the TC as a 3 meter long antenna carrying an RMS current of 8
mA (this is about what you'd see in a 5kW big coil... 42 mH Lsec, 0.1uF
CPri, 33 uH Lpri, etc.)
The standard far field equation for a "small" dipole (L<<lambda)..
This coil is at 84 kHz, so lambda (wavelength) is about 3.6 km..
field at distance r = I*L/4pi * 377 * 2pi/lambda * 1/r
so we have...
8E-3*3/2 * 377/3600 = 0.0013/r V/m
Let's assume you're at 1000 meter distance so the radiated field is
1.3uV/meter.
As a comparison, a 1 watt transmitter perfectly radiating, at the same
1km would have an electric field of 0.07 V/m
That is, your 5 kW coil produces an electric field at 1km that is the
result of radiating about 20 microwatts.. (i.e. you're radiating 0.002%)
And in practical terms, atmospheric noise has a higher power density
than your 5kW TC
AM broadcast stations have fairly decent radiation efficiencies.. well
above 50%, and they produce fields of millivolts/meter.. KNX, a 50kW
clear channel station some 50 miles away from my house produces a field
of 15 mV/meter.
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