Hi Iain,
How the field rolls off is a strong function of the antenna geometry so
"typical" might be a hard case to crack. As you point out, for a small dipole
E field roll off can be easily determined, but for HF PLC the antennas are
typically very large, and with undefined and complex geometry. I think you
will have to study some simplified cases to get a better understanding of this
subject.
A rule of thumb for when the E field behavior becomes "far field like" (with
1/d roll off, for large antennas) is when the observation distance is equal to
the total extent of the antenna. Wavelength/2pi is good for a dipoles up to a
halfwave but longer ones or not so well behaved. I would imagine that for the
work you need to do a number of NEC antenna models would give you the data you
quickly and accurately. Search for "NEC and antenna" on the web and you will
find more references than you will ever need to find codes and examples.
good luck,
Colin..
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of PLC Webmaster
Sent: Sunday, December 14, 2003 8:04 PM
To: [email protected]
Subject: E-field drop off with distance (1-30MHz)
I am currently researching the radiated emissions from High Frequency Power
Line Communications (1-30MHz).
I am trying to obtain information on how the e-field typically falls of with
distance. I have read books that have made claims that this is anywhere in the
rang 1/d to 1/5d. I am trying to make a statistical analysis to explain to
those who claim that in the near field it is (1/d^2) which they derive from an
infitesimally small dipole.
Does anyone know of any papers, experiments or actual EMC compliance
measurements in the range 1-30MHz that I could use to make an analysis of the
drop off rate.
Thanks,
Iain
