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Ok... I can feel the suspense over the Internet. :)

The answer is indeed D. All frequencies attenuate equally through free space. What??

I know that most everyone, including myself, were taught that higher frequencies attenuate faster than low frequencies. However, this isn't true.

Antennas are designed for a particular size, base on number of wavelengths. For example, a 1/4 wave dipole is an antenna type. Now, let's think about that for two different frequencies. Which 1/4 wave dipole antenna would be larger, a 900 MHz antenna or a 5 GHz antenna? 900 MHz of course, because each wavelength in 900 MHz is quite a bit larger than 5 GHz. So, the smallest antenna possible for 900 MHz is much larger than the smallest antenna possible for 5 GHz.

What it comes down to is actually quite simple. The larger the antenna, regardless of frequency, will receive more signal. A larger ear hears better. A larger antenna has more surface area to "grab" the signal and thus, you have more received signal. A 5 GHz antenna that is the exact same size (it would be difficult to make them exact) as a 900 MHz antenna, would receive the same signal strength if the source was of equal power.

More detail below:

Attenuation occurs because of the inverse square law. [url=http://en.wikipedia.org/wiki/Inverse-square_law]Link[/url] Basically, when any source of energy (sound, RF, light) is emitted from a source, it begins to spread apart. This is the basic definition of attenuation. This is why for every doubling of the distance, you lose 6 dB's of signal [b]no matter the frequency[/b].

Now, let's apply that to Free Space Path Loss. FSPL does take into account frequency. In fact, if you do the math, it will show that higher frequencies have a higher FSPL than lower frequencies. So, what gives? Keep in mind, FSPL and attenuation are different things. The math behind FSPL (which is beyond my education) actually includes the fact that higher frequency antennas of the same gain are smaller.

This enlightenment is courtesy of Joe Bardwell. He invited me to speak at a gathering he put together and he spoke on this topic. I may have butchered the answer so hopefully he can come back and give us a blog to help with the explanation. Thanks Joe!

GT