
Sat, 11 Jul 1998 10:46:30 EDT

Todd W.

A comment on your comment about antennas and their electrical length required
to radiate.

"As for antenna effects, remember that the signal's wavelength determines
how long the antenna needs to be to begin radiating. For a sine wave, lamba
about 3 feet at 300 MHz....."

Most EMI radiated from PCB's do not come from 1/4 or other fraction wavelength
antennae structures of the traces. There are of course some that do (like
resestive trace lengths, stubs, improperly terminated nets etc.), but in my
experience, most EMI comes from unequal or unmatched image current paths, and
also from loop antennae that are formed when currents are forced to change
direction. (somewhere in my thoughts is that when you solve for current
divergence you get a magnetic field generated, the H part in EM radiation)
There are many such instances such a X degree turns, going around a power
plane splits, and also going through vias. These effects, are of course
insignificant at low frequencies but become real in the RF and uWave

Just a different way of looking at things!

Hans Mellberg,
EMC Consultant