Become a fan of Slashdot on Facebook


Forgot your password?

Comment Re:Things that I wish wouldn't keep getting repeat (Score 1) 336

So Potassium is a great example of internal radiation, which is in biological equilibrium with your body almost always.

What really freaks laypeople out is when you tell them that radioactive potassium in their body gives off anti-matter. For the curious, K-40 sometimes decays to Ar-40 by emitting a positron and a neutrino.

Comment Re:Physics puts enormous limits on using 30-300GHz (Score 1) 33

Indeed. In that paper I linked to they specifically mention 60 GHz as a great re-use candidate exactly because the attenuation due to atmospheric gases/water is so horrendous.

I don't think your assertion that "2.4 GHz was opened up because of its high absorption by water molecules" is entirely true. It's a fact that water does absorb some power from 2.4GHz RF, but the reason microwave ovens are there is because it's dead center in an ISM band where things like that are allowed. There is no absorption peak for water at 2.45 GHz. The caption for figure 1 in the second link sums it up nicely: "The frequency for maximum dielectric loss lies higher than the 2.45 GHz (wavenumber 0.0817 cm-1, wavelength 12.24 cm) produced by most microwave ovens. This is so that the radiation is not totally adsorbed by the first layer of water it encounters and may penetrate further into the foodstuff, heating it more evenly; unabsorbed radiation passing through is mostly reflected back, due to the design of the microwave oven, and absorbed on later passes." If there were a peak at 2.45GHz, you'd boil the water off the first few mm of food and progressively leatherize the food all the way to the center.

Comment Re:Physics puts enormous limits on using 30-300GHz (Score 1) 33

Too cool! I visited the VLBA radio telescope on St. Croix when I was there and had a long, interesting chat with the tech on duty. I love how all the LNAs are liquid helium cooled in the broiling heat of the tropics. He did mention that they didn't bother with 96 GHz (I think it was) due to the extreme absorption there.

Comment Physics puts enormous limits on using 30-300GHz (Score 4, Informative) 33

The FCC has a publication on the behavior of RF in the 30-300GHZ range, and the outlook is not rosy. Atmospheric gases, water vapor, rain drops, foliage, and other attenuation and noise sources make these frequencies problematic for medium- to long-range, high speed comms.

Using 60 GHz is interesting because it's attenuation is so high it can be reused every 4 km.

Comment Re:bay of thieves (Score 2) 81

Also, be aware the gigabit ethernet does use all pairs. If you liven up the unused pins for your 10/100 (especially at higher voltages) dont accidentally plug it in to a gigabit port. Damage to the port will likely follow.

If you read the PoE spec, you'll see that the power is sent on both wires of at least two pair, and because the electrical spec calls for transformers on the receive end, the power will cause no issues. Since there's no voltage difference between the wires in the same pair, there's no problem with core saturation. For non-PoE devices, there's no difference. For PoE devices, the receive transformer has a center tap to access the power.

Slashdot Top Deals

Computers can figure out all kinds of problems, except the things in the world that just don't add up.