Figure 1. Test Setup With 2.4 GHz Antenna and Jingling Change
Abstract: ESD events between small pieces of metal, such as coins,
at low voltages produce intense EMI with bandwidths into the tens of gigahertz.
Response of a commercial Wi-Fi (802.11b) antenna to such EMI is measured.
The results provide insight into one kind of noise Wi-Fi receivers are subjected
to. The results also suggest that such antennas might be used for ESD event
Discussion: ESD between small pieces of metal can produce strong electromagnetic
interference and such events are common in the work environment. Measurements
of the output of a wireless LAN antenna in the vicinity of small metal ESD show that the amount of voltage delivered
to a receiver can be significant.
Figure 1 shows a commercial antenna used in wireless LANs. About 30
cm (one foot) above the antenna, a plastic bag with coins can be seen. When
the bag is shaken, the electromagnetic fields from the small ESD events between
the coins result in significant voltage at the antenna output. The antenna
used for this experiment has a gain of 10 dBi (10 dB above an isotropic radiator
and about 7 dB above a dipole, see the article on LAN transmit power linked
at the end of this technical tidbit for more information on what this means).
There is also an article linked on patch antennas, of which this antenna
is an example. The net result is that this antenna, in the direction of its
maximum radiation, increases a received or transmitted signal by about 4
times (~6 dB) the power compared to a dipole.
Figure 2. Antenna Output from Test Setup of Figure 1
Figure 2 shows a typical output from the
antenna and test setup of Figure 1.
The waveform in Figure 2 exceeds the range of the scope and is "off the scale"
at more than 4 Volts! Note that one Volt per division is the least sensitive range of the scope.
Figure 3 shows the waveform expanded out to 2 ns/div
from the 10ns/div used in Figure 2.
Figure 3. Trace of Figure 2 Expanded at 2 ns/div
In the first 5 ns of Figure 3 there is a very sharp peak exceeding 4 volts.
The oscillations preceding it are between 3 and 4 GHz, well past the
1.5 GHz bandwidth of the scope, and reflecting the high bandwidth of low
voltage ESD. It is likely that that first peak is greater than 10 Volts in
reality. Just to the left of the center of the screen, a series of
peaks occur with a frequency of about 1.5 GHz and with amplitudes well over
4 Volts. Judging by the appearance of the peaks and the 1.5 GHz frequency response
of the scope, I would estimate the peaks are nearly 10 Volts.
If the waveform of Figure 3 were further expanded, the plot would look jagged
because some parts of the waveform are very fast and require a sampling rate well
above the 8 GSa/sec of the scope used here in order to accurately display
the waveform. Sin(x)/x interpolation
is turned off for these measurements.
With peak voltages on the order of 10 Volts (substantially
larger than the transmit signal normally used in 802.11b), there is the possibility of overloading
of the receiver front end. If this occurs, receiver performance will be degraded
for sometime after the event until the receiver recovers. If sensitive devices
are used in the receiver front end, protection may be required depending
on the likelihood of nearby ESD and the front end design. If the jingling
change were held very close to the antenna, the resultant voltages would
be substantially greater as well.
Another interesting case is that of a mobile phone placed in a pocket
or handbag with loose change. The phone would be imbedded with the ESD events!
This very close range ESD could result in large signals picked up by the
phone's antenna. The amplitudes may be significantly greater that those shown
in Figures 2 and 3 raising the possibility of receiver front end damage. In a related, although somewhat different situation, I
know of one case where ESD occurring within a mobile phone by static induction
damaged the receiver front end and reduced the receiver sensitivity permanently by a significant amount.
ESD event location techniques often use calculations based on the time of
arrival of an ESD generated wavefront at 3 or 4 antennas. By using sensitive
directional antennas, like the one used in this experiment, one can better
define the area of interest and achieve better signal to noise ratios compared
to dipoles or omni-directional antennas. Research into ESD event location
by time calculations was done at Bell Labs about 8 years ago and more recently
at Ion Systems
in Berkeley, California.
generated EMI can have strong effects on systems. Data presented here raises
the possibility that such EMI may also have an effect on high frequency
RF systems such as wireless LANs. RF designers may find the data presented
here useful in their designs.
Discussion of wireless LAN transmit power can be found at: http://www.wi-fiplanet.com/tutorials/article.php/1428941
A short description of a patch antenna can be seen at: http://home.iae.nl/users/plundahl/antenne/patchant.htm
Additional information on this site regarding ESD effects on systems includes:
November 1999: Transient Suppression Plane
May 2001, Hidden Threats
to Electronic Equipment
- June 2001, A Static
Field Powered EMI Source
January 2002, Cable Effects Part 1: Cable
May 2002, Printed Wiring Board Coupling to
a Nearby Metal Plane, Part 2: ESD Immunity
- February 2003, Crossing Ground Plane Breaks - Part 3, Immunity to Radiated EMI
May 2003, Signal Paths Passing Through Ground and Power Planes, Effects on Immunity
ESD Immunity in System Designs, System Field
Experiences and Effects of PWB Layout (~950K)
(2000 EOS/ESD Symposium paper)
Thanks to Agilent Technologies
the scope for this experiment. The model used for this article was an Agilent Technologies 54845a, an 8 GSa/sec
unit that is now replaced by a much faster scope, the 54853a.