Joe,
If the transmitting antenna (your product) and the receiving antenna were in
free space, you pretty much could assume that the radiation falls off at 1/r^2,
and thus use a 10.5dB correction factor between 10m and 3m measurements. (You
might have to worry about near-field effects and antenna interaction at low
frequencies.) But Radiated Emissions measurements for equipment are done over a
ground plane. Thus the receiving antenna sees:
* Direct radiation from the equipment.
AND
* Radiation that has bounced off the ground plane.
Because of the difference in path lengths, these signals may sum anywhere from
exactly in-phase to exactly out-of-phase, depending on the frequency and antenna
heights. For horizontal antennas this turns out to be just a small disturbing
factor, less than 1dB or so. But vertical antennas can see anywhere from 200%
to 0% of the free-space voltage for that same position of the antennas.
Because the FCC and CISPR regulations require you to vary the receive antenna
height between 1m and 4m, you will see lobes in the vertical pattern because of
this constructive/destructive interference. After having some of our products
pass easily in our 3m chamber, and then fail miserably on a 10m test site, our
EMC folks came up with an additional correction for:
* Transmitting antenna height of 1m (tabletop product on 0.8m high table).
* Receive antenna height of 1-1.75m in our 3m chamber.
* Receive antenna height of 1-4m on a 10m site.
* Frequencies from 30MHz to 1GHz.
This Vertical Correction Factor (VCF) is:
* About 1dB at 30MHz.
* About 7dB at 200MHz.
* About 1dB at 1GHZ.
Thus, if I am testing a product in our 3m chamber, and want to be sure that it
will pass the official tests at 10m, at 200MHz I had better see vertical
emissions no higher than 3.5dB (10.5dB for 1/r^2 minus 7dB VCF) above the 10m
limit. Because of Murphy's Law, and to protect us from slight variations in
production, our EMC folks like us to have 4dB margin against this corrected
limit. If we are within 2dB of this corrected limit, we may pass Radiated
Emissions tests on the initial units, but will have to rerun A-B Radiated
Emissions tests in our 3m chamber for *any* contemplated changes to the product,
and may have to test production units regularly to make sure that we stay legal.
This is not a fun way for us Design Engineers to spend our time... Thus we tend
to overdesign the products, which adds cost.
We have had a 10m Open Air Test Site (OATS) here for a number of years. But
because of Kentucky weather, we could only count on being able to use it about
5-6 months per year. For another couple of months per year we could hope/pray
for a warm day to run 10m tests, but expected to have to travel to a closed-in
10m test site. But, in late October we started construction of a new lab
building that will have a completely-equipped 10m semi-anechoic/anechoic
chamber. It's supposed to be completed in late summer. Yeehah!
John Barnes Advisory Engineer
Lexmark International
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
[email protected]
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: [email protected]
Michael Garretson: [email protected]
For policy questions, send mail to:
Richard Nute: [email protected]
