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Essay on Microwave Oven Effects On Wireless Lans
| Date: |
01-14-02 8:01pm |
| Subject: |
Business |
| Word Count: |
1170 |
| Page Count: |
4.68 |
Microwave Oven Effects On Wireless Lans
Microwave Oven Interference on Wireless LANs
Operating in the 2.4 GHz ISM Band
Abstract - Commercial microwave ovens as applied in
restaurants have two magnetron tubes and compared
to domestic kitchen counterparts they spread the
higher RF power and radiated heating energy more
evenly. The domestic kitchen or residential microwave
ovens have only one magnetron tube. The interference
from the commercial type of microwave ovens is more
difficult to characterise than the interference from the
residential ones. The commercial type of microwave
ovens radiate a CW-like interference that sweeps over
tens of MHz during the two bursts per mains power
cycle. The residential ones give a CW-like interference
that has a more or less stable frequency near 2.45 GHz
occurring once per mains power cycle. The impact of
the interference from the commercial type of
microwave ovens on wireless LANs conforming the
IEEE 802.11 standard for both DSSS (direct sequence
spread spectrum) and FHSS (frequency hopping
spread spectrum) has been evaluated.
I. INTRODUCTION
The release of the 2.4 GHz unlicensed band (2400 -2483.5
MHz) for ISM (industrial, scientific, medical
applications) prompted a significant interest in the design
of wireless LAN products. Interference from extraneous
sources (unintentional radiators) impacts the reliability of
communication in this 2.4 GHz ISM band. Sources of
such interference are the millions of residential
microwave ovens radiating at frequencies close to 2.45
GHz, and they have been described largely in the
literature. Commercial microwave ovens, based on two
magnetron tubes as used in restaurants, have been hardly
described in the literature. Since commercial ovens are
expected more often in the vicinity of office buildings
with a high population density of office equipment and
PCs, this type has been evaluated with respect to the
nature of the interference signal and the impact on
wireless LANs operating in the 2.4 GHz ISM band.
At first, published material on residential microwave
ovens, the reports from the NTIA (National
Telecommunications and Information Administration, in
the US) - [1] and [2] - are discussed. Next, the
commercial microwave ovens and the nature of their
interference is considered. The characterization of the
interference from such ovens requires a dedicated
measurement set up. Then the robustness of wireless
LANs based on DSSS and FHSS conformant to IEEE
802.11 against CW interference is discussed. At last, the
interference from the commercial microwave ovens on
wireless LANs based on DSSS and FHSS is measured
with a dedicated set up and the impact of the interference
nature is considered.
II. NTIA REPORTS
The NTIA makes [1] and [2] some pessimistic
conclusions about the possibility of sustaining highly-reliable
communication links in this band. The
ubiquitousness of these ovens and the wideband
interference picture that emerges from peak-power
measurements using, for example, conventional spectrum
analysers in max-hold mode and multiple sweeps, has led
to these pessimistic conclusions. The NTIA describes
measurement results for residential microwave ovens with
a maximum EIRP for these radiators that lies lay between
+16 and +33 dBm.
Some shortcomings in the NTIA measurement methods
are presented in [3]. The NTIA reports give results of
frequency- and time-domain measurements. Spectrum
analysers in max-hold mode were used to measure in the
frequency domain, which resulted in traces that capture
the peak emission, at each frequency sampling point,
occurring during the time interval of observation.
Spectrum analysers in zero-span trace mode were used to
find how the signal power around the selected frequency
varies over time. [3] mentions that the NTIA peak
spectrum measurements and frequency-domain
characterisation with time-domain plots show a pulsed III. RESIDENTIAL MICROWAVE OVENS
Microwave ovens have become more popular over the last
fifteen years and can be found in over two hundred
million home kitchens. The heating source of these
residential microwave ovens is based on a single
magnetron tube mostly positioned in an upper corner.
Without further provisions, such an oven would produce
an uneven heating effect, because of static stable standing
wave patterns inside the cavity of the oven. Therefore, the
usage of a rotating disk results in such a heating process
at which the different sides of the rotated food or drink
are “illuminated” more evenly. The power consumption is
mostly in the 600 - 800 Watt range.
2445 MHz
2452 MHz
2459 MHz
2466 MHz # RES BW 10 kHz
# VBW 10 kHz
# SWP 15.0 sec
Peak 10 dB/div
2.41 GHz 2.48 GHz Frequency
Fig. 1. Max-hold spectrum for residential microwave
oven.
# RES BW 3.0 MHz
#VBW 1 MHz
# SWP 30.0 msec
Peak 10 dB/div
fcentre 2.456 MHz
Time 0 30 msec
Fig. 2. Zero-span spectrum for residential microwave
oven.
NTIA measurement approach. With a high speed digital
oscilloscope it can be shown, that during the active period
the emitted signal is a CW with a frequency that moves
over a few MHz. The beginning of the burst looks like a
pulsed CW of which the frequency can vary more, and the
radiated signal strength is lower. The end of the burst
looks like the pulsed beginning and also has a lower level.
Although there are many differences between the
emissions from ovens of different manufacturers, the
centre burst frequency is mostly somewhere around 2450 -
2460 MHz, and the sweep goes over 2 - 6 MHz. Likewise,
the total active period is about 8 msec (out of the 20 msec
mains power cycle at 50 Hz, or 16 msec at 60 Hz) of
which the first and last 1 msec of the burst considered the
beginning and end, have a pulse nature.
IV. COMMERCIAL MICROWAVE OVENS
Microwave ovens which are used for commercial
applications, are based on two magnetron tubes which are
alternately active during one half of the mains power
cycle of 20 msec. As illustrated in Fig. 3, the radiated
electro-magnetic waves from the l/2 waveguides that are
mounted on the two magnetron tubes are reflected by the
rotating disk with metal mirror plates. This type of oven
has a power consumption in the 1200 - 2500 Watt range
and the cabinet is a more solid one of stainless steel.
In an attempt to measure the max-hold spectrum for a
commercial microwave oven, we found a characteristic as
shown in Fig. 4, which occupies a much wider spectrum
than the one found for a residential oven as illustrated in
Fig. 1. To characterise signals from a commercial oven
we used an approach based on down mixing with a 2450
MHz carrier and filtering with a steep low-pass filter 1 to
provide a baseband signal that is offered to a digital
oscilloscope (a 125 MHz dual digital oscilloscope).
In order to capture the oven activity over a full mains
power cycle, we have selected a lower sampling resolution
for the digital oscilloscope. Fig. 5, which is obtained in
this way, illustrates the variation in the envelope of the
CW-like microwave oven signal.
A pulsed behaviour similar to the one found for a
residential oven, is observed during the beginning and the
end of the burst. The commercial oven shows a random
variation in frequency over tens of MHz, meaning that it
covers a considerably wider frequency band than the
domestic brother. The commercial oven gives a large
power variation, as illustrated in Fig. 5.
V. WIRELESS LANS BASED ON DSSS AND FHSS
Currently available wireless LAN products employ either
Bibliography
REFERENCES
[1] Gawthrop, F. H. Sanders, K. B. Nebbia, J. J. Sell,
“Radio spectrum measurements of individual
microwave ovens,” NTIA Report 94-303-1.
[2] P. E. Gawthrop, F. H. Sanders, K. B. Nebbia, J. J.
Sell, “Radio spectrum measurements of individual
microwave ovens,” NTIA Report 94-303-2.
[3] S. Vasudevan, J. Horne, M. K. Varanasi, “Reliable
wireless telephony using 2.4 GHz ISM band: Issues
and solutions,” in Proc. IEEE Fourth International
Symposium on Spread Spectrum Techniques &
Applications, Sept. 1996, (ISSSTA‘96), Mainz
(Germany), pp. 790-794.
[4] IEEE P802.11, Draft Standard for Wireless LAN
Medium Access Control (MAC) and Physical Layer
Specification, P802.11 D5.1a, Jan. 1997.
[5] A. Kamerman, “Spread spectrum schemes for
microwave-frequency WLANs,” Microwave
Journal, vol. 40, no. 2, Feb. 1997, pp. 80-90.
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