by DF9CY Christoph Petermann ©

If you transmit a high frequency signal from an antenna of any kind, you will see that the field strength of the electrical (E-field) declines by the 1/(r*r) law, where r is the distance from the antenna. Only very close to the antenna we have things complicated. Here we can make a difference between the so called Near-Field and the Far - Field. Where does the Near - Field end ? A most common agreement is to say that the transition from Near-Field to Far-Field is somewhere between 0.5 and 1.5 wavelengths. The Far - Field is much easier to overview than the Near-Field.  

In general the Far-Field strength of the electrical field can be derived from Maxwell's laws and lead to: E = 1/ r * sqrt ( P * Z0 * Gt / (4*pi ))

Where r is the distance in meters, sqrt means square-root, P is transmit power in Watts, Z0 is the free space impedance and equals 377 Ohms, Gt is the gain factor (not dBi) over an isotropic radiator.

The result E is in V/m giving you the field strength.

I have made a little program in JAVA around this formula and took some care about the Near-/Far-Field problem. I integrated also the current standard from the VDE-DIN 0855. In November 1997 I received a new radio license - because I moved - and it is already pointed out in the letter, that I will have to show the conformance to the standards to the officials within the next 2 years. This VDE DIN 0855 is by the way different to the new regulations in the U.S.

Lately the new heart pace-maker restrictions came. They are in the program now. You will easily see that under normal amateur radio circumstances you must NOT make any changes to your radio system. The current values in the standards coming up now do not differ so much from each other.

So if you use this calculator, which bases on the valid German limits, you will have the field strength, you can look into your country's standards and see where you are. Do not get into panic, as I said. A good antenna is the best for DX and the best for solving ANY EMI problem.

Download the DF9CY JAVA EMI and RF-Field Calculator VERSION 2.02 | January 2000 for JAVA.

JAVA is available from WWW.JAVASOFT.COM

Once properly installed, this will enable you to run this and other JAVA applications in a comfortable 32-bit environment.

Unzip ALL files into one directory EMI.

DOS: Call RUNEMI.BAT from the Windows Explorer, or run JAVA EMICALC from the EMI directory.

LINUX: Open a terminal in /usr/java/emi directory (or where else you did install) and simply run java emicalc.

That's about it !

Dipl.-Ing. Christoph Petermann DF9CY, professional specialist on the EMI and conformance compliance.

Text and All Images are Copyright by Christoph Petermann DF9CY

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