Whats up with trees blocking the signal

[hartz]

Hi,

Maybe one of you experts can answer this question: Why do trees affect receiption of Wireless signals?

All over the show you read about how it must be clear "Line-Of-Sight", and how people cut down trees to get a better signal, etc.

This is probably a total newbie question, but I'm sure I'm not the only one wondering how come TV and radio signals can enter into homes, Cell phones work everywhere, even my Wireless AP manages to work between floors and throughy closed doors in my house, but something as "flimsy" as a couple of tree leaves is able to severely degrade a signal.

I'd like to set up a simple Wireless network between myself and two friends living close by, one is easy LOS, for the other we will need to go over a roof-top that is between us.

I suspect that the answer will be in the same sort of space as the fact that some frequencies of light pass through clear glass, while others are blocked... maybe someone can make the explanation simple and understandable though.

Thanx,
_Hartz

 

[sawireless]

Transmission cell
Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts).

 

[Piesang]

I don't really understand it either. I have Wireless Broadband Router in my office. I can pick up the signal through walls.

Why does a tree make such a big difference?

 

[savage]

The output power is much higher on a cell tower / cb radio / etc... Because the output power is higher, the loss is less, and therefore they work in NLOS (Near Line of Sight).

Most access points has output power of only a couple of hundred of miliwatts... You do the math

 

[skydog]

I don't really understand it either. I have Wireless Broadband Router in my office. I can pick up the signal through walls.

Why does a tree make such a big difference?

It has to do with absorbing the waves rather then refracting. i'm not to technical on the exactly why.

I read somewhere a simple test is that if you put it into the microwave and it hs an effect on that substance then that material will affect wifi waves.

the last time i microwaved a wall it didn't have much affect on it

 

[oober]

The average resonance frequency of water is very close to 2.4-2.5Ghz.
Thus when our wireless signal passes through leaves that contain water the signal is scattered and absorbed by the water molecules and thus you have more signal loss than other signals with other frequencies.

 

[bboy]

its the water in the trees for 1

2.. wireless works at about 28 mw.... compared to a full on watt of other devices

3. the frequency of a cellphone or tv is much lower than the 2.4ghz of wireless..... a much lower frequency can bounce off the atmosphere/ground and pass through objects much easier

 

[Chaz]

The average resonance frequency of water is very close to 2.4-2.5Ghz.
Thus when our wireless signal passes through leaves that contain water the signal is scattered and absorbed by the water molecules and thus you have more signal loss than other signals with other frequencies.

Yep, what he said.

The higher the frequency, the more the path loss. Its based on Free Space Loss (FSL) and is easily calculated.

 

[Chaz]

Transmission cell
Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts).

I hope youre not one of the guys representing a local wireless company with an answer like that :-(.