Ask Dr. SETI ®

Greg, Sweden

The Doctor Responds:
There are three different mechanisms at work here, Greg, all of which conspire to obstruct microwave signals when foliage is present. The most obvious of these is water attenuation. Trees are water-based lifeforms (as, in fact, are we). Their leaves are especially adept at storing water. Water molecules exhibit several different natural resonant frequencies in the microwave spectrum. When signals at these frequencies interact with water molecules, some of the photons transfer energy to the water by exciting various vibratory modes in it. If energy is transferred this way, some of it is obviously lost by the photon stream, resulting in a reduction in signal strength. You've probably already observed this effect when viewing Ku-band satellites. Obviously, dry leaves are less deleterious than wet ones.

Another mechanism is thermal in nature. On Earth, trees are generally warm bodies, stabilized somewhere near 300 Kelvin. Planck tells us that any object warmer than absolute zero radiates a predictable blackbody spectrum. There will be thermal radiation components present at whatever frequency you tune. When radio telescopes are used as total-power radiometers, these thermal emissions look like signals, and in fact can obscure the actual signals we seek. Remember, thermal noise is just that -- noise, and when looking for weak signals (the name of the game for SETI), signal-to-noise ratio is critical. In the cold of the arctic (or even the Swedish) winter, this effect is somewhat reduced.

The third consideration with respect to foliage is physical resonances. Consider a pine needle, for example, that's exactly one-half wavelength at the frequency of an impinging signal That needle is going to act rather like a half-wave dipole antenna, absorbing energy. At the familiar 21 cm hydrogen line, a 10.5 cm pine needle (fairly typical of the trees around my dish) is going to raise havoc with any incoming signal.

Obviously, it's best to chop down all the trees in the vicinity of your SETI dish. If this is not an option, you can only hope for dry trees, cold winters, and non-resonant leaves.