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Physical Barriers to Wireless Buildout

Members of the ISP-Wireless list discuss how to overcome physical barriers when building out 2.4GHz wireless systems.

[May 26, 2000]

On the ISP-Wireless list in March, PF posted a question about a potential barrier to 2.4 GHz systems:

"All the manufacturers say the 2.4 GHz system will have a range of 100 to 600 feet indoors, but then say a tree would block signals outside. What does a tree have that my office wall doesn't?"

A number of respondents offered an explanation:

[PL wrote] "Water. That is why microwaves — which are 2.45 GHz — cook food. They excite water molecules because the water absorbs the energy and otherwise inhibits propagation. Unless you live on a delta somewhere, your walls should be pretty dry."

The same issue cropped up in a separate thread in May when MK wrote:

"We are about to set up a wireless system on a test basis and I'm wondering if anyone has some real life experiences on the impact of trees (or, more importantly, leaves) on 2.4 FHSS or DSSS signals. Since most of the trees in the Northern Hemisphere are now full with leaves, what impact is this having on running wireless?"

[MS noted varying seasonal effects on wireless] "I have one customer who had a church, one pine tree, three leafy trees and one tree yet to leaf out in his range. One week I was able to get about 100Kbps with my laptop and a Uni-16 antenna from his roof, but a week later, I got nothing. On the flip side, I have one customer working just fine while going through his neighbor's house and another one going through a grain elevator and a tree."

[EH added] "In training classes what you're told is this: 2.4GHz will not go through trees, you need to have clear line of sight and at least 80% of the Fresnel zone free. What we've found, however, is that sometimes you can go through trees, sometimes you can't. It depends on how much power is going into the trees, how dense the trees are and whether or not there's any line of sight out all. Trees can be tricky, and the best bet is to make sure you're high enough to clear them."

[editor's note: according to Wireless Infonet, the Fresnel zone refers to this: Radio waves spread out after they leave the antenna. The Fresnel (pronounced 'fre-nel' the "s" is silent) zone is the area around the visual line-of-sight that the signal spreads out into. The Fresnel zone must be clear of obstructions or the signal strength will be reduced because part of the signal will be blocked.]

[SW concurred] "Trees are a big problem for 2.4 GHz equipment. Even with an AMP (assuming you stay within the legal limits set by the FCC) you'll still find that trees will render your link quality unpredictable at best. The radio line of sight in that the first Fresnel zone needs to be relatively unimpeded because trees, buildings will absorb, reflect or refract the signal resulting in path loss."

[JH reported] "We have a POP-to-shop distance of one-quarter mile and to start, there were seven trees in full foliage blocking 100% of our path. We got a strong 2MB receive at the shop but no receive at the POP from the shop. We cut down 2 trees behind the shop and began receiving intermittently at the POP; cut down a third tree and the signal improved, but was still dropping intermittently. We cut down a fourth tree and the signal was constant at 2MB. The remaining trees are all oak and are blocking about 90% of the path. We'd cut them, but they're not on our property and the homeowners seem attached to them."

 —End