Is 2.4 or 5.8 better when trying to go thru trees? We are working on possible link about 2-3km distance.
With the trees in front and the antennas on the roof we have LOS, but very small holes between a few of the eucalyptus trees. Are trees a total killer ? Those are the only obstructions on the link. We are getting close to being able to perform a test - but am wondering if 2.4 or 5.8 would be best to use in this situation.
Sam
Rule of thumb:
The lower the frequency the better the penetration
thus 900 Mhz > 2.4 Ghz > 5 Ghz when trees are involved
Although there is much more technical detail I could go into, a simple answer is 2.4 is going to be better through trees.
As a side note:
Personally I have noted on many occasions that trees can cause very unpredictable behaviour, an example would be a link with excellent signal strength, but erratic pings, and low speeds. Trees can cause massive amounts of retransmits, because of the way the signals are being reflected off the leaves (especially when wet).
To combat this problem I recommend using as narrow a beam width as possible on any links (this goes for both ends, even if one is clear of trees).
Its tough to say, 900MHz penetrates better, but 5GHz, with the tighter fresnel might work better through the holes between the trees.
I’d like to try both myself, just to see, but my money is on 900MHz.
I have found sometimes it can be a total killer, other times you can get away with it, but with reduced signal strength. It seams different types of trees have varying effects also… I think it maybe the amount of moisture in the tree itself. If you are talking such a small distance and have good power cards, and say a couple of 19db antennas it should work. Even do a test with a laptop at one end and a decent AP(at least 23db) at the other.
I think Kiwi-Chris is right when he says the type of tree matters.
Pine trees are a real stopper because of the density of their folliage, where as deciduous trees or trees with lower density folliage present less of a problem. Im not sure about pohutakawa Chris 
With narrow beam antennas, you have the possibilty of aiming past
a lower branch in an area clear of foliage. It might mean that 5ghz is better in such a case because of the lower beam width, but it also means that a single branch can stop 5ghz if its in the right spot.
Where you have go through foliage, the lower frequency the better.
Im in favour of low gain wide beamwidth antennas with higher power to try and scatter through.
The leaves change their position when the wind blows, and it is always better than still air.
Bear in mind though that the noisy signal will impact on your AP performance generally.