"I did some experiments with a RangeLAN2 PCMCIA card. A simple
spectral analyzer is among the utilities shipped with the card,[...]
I found a signal that doesn't have the character of a WiFi signal - it is
'sharp' and very strong. If the antenna is connected directly to the
card, the card's input is so overloaded with this signal that the
spectral analyzer shows nonsense.
After a while I found this to be a product of the microwave oven in our kitchen.[...] Then I tried other microwave ovens and found that this garbage is produced by more or less all of them.[...]"
The explanation is simple: "A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz [...], through the food." (Wikipedia)2.45GHz lies just in the middle of the band used by 802.11b and 802.11g radio LAN equipment.
Not only microwave ovens can be a source of interference in the 2.45GHz band: "The 802.11b and 802.11g flavors of Wi-Fi use the unlicensed 2.4 GHz spectrum, which is crowded with other devices such as Bluetooth, microwave ovens, cordless phones " (Wikipedia)
A 5.4GHz band radio network link was operated in Pilsen, Czech Republic. The link was running in a licence-free band within legal power limits. The link jammed the meteorological radar CHMI Brdy (Czech Hydrometeorological Institute), because the radar was operating on the same frequency 5.66GHz. Radar output from 28th September 2005 demonstrates the interference.
|Radar image with jamming||The same image with jamming highlighted in red|
This single incident has been resolved, but the problem was not fixed. With proliferation of 5.4GHz wireless network links, the situation gets rapidly worse. Currently it's possible to see interference on the radar images very often:
|I found this image that appears to show as many as 4 interferences at the same
time, but it is not the case. Note that Czech Republic is covered by 2 radars - Brdy 5.66GHz (left) and
Skalky 5.652GHz (right). I got an information from Czech Hydrometeorological Institute
that only (2) and (3) are real jammers. (1) and (4) are noise comming from
the Sun. This is verified by the fact
that both (1) and (4) point in exactly the same direction. The radar snapshot is from
4th January 2006, 15:00 UT
Other jamming examples follow. Note how the jamming stays at the same place, but it's intensity and gap pattern varies - this is caused by nonuniform nature of the data transmission.
Source: CHMI Recent Radar Data
There is a risk that radar jamming will lead to operation of radio LAN equipment being excluded from this band to protect the public service. This could make existing equipment useless. The technical nature of free space optics guarantee that nothing like this will happen to a Ronja based link, making it a safer investment.