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Comment: Re:Microwaves and 2.4 GHz (Score 1) 122

by dtmos (#47937203) Attached to: Scientists Twist Radio Beams To Send Data At 32 Gigabits Per Second

I think the special part is that 2.4 GHz is a convenient frequency where there is a balance between a larger amount of energy being absorbed by water and a smaller amount of energy being absorbed by glass and plastic.

No, 2.4 GHz was just one of seven convenient open frequency bands when, in 1947, the FCC assigned frequencies for the industrial heating, diathermy, and other RF sources that were causing interference on communication systems. These bands were scattered from 25 MHz to 20 GHz. See p. 8 and p. 50-51 of the Thirteenth Annual Report of the FCC, and the 1947 US Frequency Allocation Proposal to the Atlantic City International Radio Conference (see pdf page 464 of this pdf file). They were collectively called the "ISM bands", because the FCC aggregated Industrial heating, Scientific uses, and Medical heating (diathermy) equipment into bands that would minimize interference to communication systems. The microwave oven (called an "electronic cooker" in the FCC report) was so new that it was explicitly mentioned, and lumped in with other "industrial" heating systems.

Comment: Re:Spiral filter, and a Tardis (Score 1) 122

by dtmos (#47934759) Attached to: Scientists Twist Radio Beams To Send Data At 32 Gigabits Per Second

So let me ask: If the multiplexing is due to "the phase relationship between the oscillations of the field at different positions", may I assume that these systems would be very sensitive to multipath interference -- especially varying multipath interference, as in mobile devices? Is that why the only demonstrations I have seen involve point-to-point links in free space (where multipath would be minimized)?

Comment: Microwaves and 2.4 GHz (Score 4, Informative) 122

by dtmos (#47934723) Attached to: Scientists Twist Radio Beams To Send Data At 32 Gigabits Per Second

2.4GHz is perfect for heating anything with a high water content, like tissue. That's why microwave ovens use it.

This is a myth. There is nothing special about 2.4 GHz as far as water is concerned. There is a mild absorption peak at 24 GHz, but nothing at 2.4.

Comment: Incredibly wise advice (Score 2, Insightful) 120

by dtmos (#47760277) Attached to: The Grumpy Programmer has Advice for Young Computer Workers (Video)

Robin Miller: But age discrimination in employment, have you encountered?

Bob Pendleton: Oh, absolutely. I got laid off on my 49th birthday and haven’t been able to find a full time job since.

One piece of advice I always give younger engineers and programmers is to be increasingly vigilant about your career as you age. In the last decade or so before retirement one is very vulnerable to layoffs, because one's salary is high and one's formal education was a long time ago.

Comment: Re:Spherical Torus (Score 4, Informative) 147

by dtmos (#47746961) Attached to: Princeton Nuclear Fusion Reactor Will Run Again

Spherical Torus?

I wondered the same thing. However, the National Spherical Torus Experiment web site has this explanation:

The magnetic field in NSTX forms a plasma that is a torus since there is a hole through the center, but where the outer boundary of the plasma is almost spherical in shape, hence the name “spherical torus” or “ST”.

There are also some links to more detailed descriptions.

Comment: Velocity (Score 5, Interesting) 133

by dtmos (#46898509) Attached to: Star Cluster Ejected From Galaxy At 2,000,000 MPH

Two million miles per hour is less than 0.003c, but still quite a clip, even in astronomical terms.

Since they're discussing velocity (vector speed), and not just speed, the headline is correct in saying " -1000 km/s" when the measured value is -1025 km/s, but one can debate whether the abstract is correct in saying "an extraordinary blueshift of -1025 km/s", rather than "an extraordinary blueshift of 1025 km/s", since "blueshift" gives one the sign of the velocity already.

Comment: Most unlikely technology in 1981: Handheld GPS (Score 5, Interesting) 276

I always thought the most unlikely technological development in my lifetime was the handheld GPS device. It would be "most unlikely" because it required tremendous, simultaneous, and largely unforeseen advances in several different technologies, each of which was hard to predict in 1981. The list is at least:

1. Low power, low voltage, low noise L-band receivers, sensitive enough to be compatible with the weak signal coming from the internal antenna of a handheld device;
2. Stupendous amounts of digital signal processing, also at low power and low voltage;
3. Digital map databases of (substantially) every road in the world, accurate to a few meters;
4. A substantially world-wide, wideband wireless data link to get the digital map into the handheld device in the first place;
5. Low power, low voltage, high resolution, multicolor flat panel displays;
6. Gigabytes of low power, low voltage data storage memory; and
7. High energy density, high power density batteries capable of supplying the whole thing.

And, perhaps most impressive of all, the manufacturing technology to make all of the above small enough to fit in a handheld device, at a price low enough to sell by the zillions.

Of the list above, probably only #2 could have been predicted, and then only if one were willing to extrapolate the then-relatively-new Moore's Law by a very large amount. (Recall that Mead and Conway had only written their Introduction to VLSI systems the previous year; until then it was not clear that such complex chips could even be designed on human time scales, let alone built for a profit.)

The fact that a handheld GPS device is now an anachronism, since the technology is now small enough and low-power enough to be integrated into other handheld devices, like smart phones, pleases me no end.

One man's constant is another man's variable. -- A.J. Perlis