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Comment Re:"It support" (Score 1) 198

a newspaper which makes no bones about it support for an independent Scotland

Maybe you should call "it support" to fix your apostrophe problem.

Sorry, no. The GP poster's statement is merely missing an "s" -- an apostrophe would be incorrect there. (It should read, "a newspaper which makes no bones about its support for an independent Scotland.")

Remember it this way: his, her, its. If you can replace "its" with "his" or "her", it does not need an apostrophe.

Comment At my office (Score 1) 320

The engineers at my office wanted to watch the launch, so we invaded the accounting office that had the windows facing the Space Center. It was a beautiful launch, up to the time the exhaust trail forked, forming a "Y". The accountants all said, "Oh, look how beautiful!" The engineers all said, "Uh-oh. That's not supposed to happen. . . ."

Comment Congratulations! (Score 1) 160

Congratulations! You hit my other favorite myth, that there is something special about water at 2.4 GHz. There isn't. Water vapor in the atmosphere has less than 0.001 dB/km (yes, kilometer) specific attenuation at 2.4 GHz. The first significant resonance for water isn't until 22.3 GHz, and even then it is less than 0.2 dB/km. It's a myth!

Water had nothing to do with the creation of the 2.4 GHz ISM band, or the placing of microwave ovens at that frequency. Amateurs do moonbounce communication in the 2.4 GHz band -- that should tell you all you need to know about propagation there.

Comment Re:It's also due to the scale of the obstacles. (Score 1) 160

To be sure, but the relevant scale is the wavelength, which is about 5.5 cm at 5.5 GHz, and 33 cm at 900 MHz. There's nothing magic about those lengths -- a forest, or a building, is just as likely to have structures of either length, or any other length, or neither length. Certainly one can create frequency-selective surfaces that pass one band of frequencies and reject others. The statement to which I objected was that it was always true that 900 MHz signals travel farther than 5 GHz signals, due to some perceived difference in radiowave propagation. That is simply a falsehood -- a myth.

Please don't carry the discussion on any further. Instead, take a pair of parabolic dishes and run the test yourself -- through a forest, through a building, through the path of your choice -- then repeat the process with resonant dipole antennas.

Comment Re: Myth? (Score 1) 160

I'd expect them to diffract around typical household corners differently...

They do, and you'll find that shorter wavelengths get through smaller openings that longer wavelengths cannot. It's a complicated field to model, and one can find cases where either is superior, but shorter wavelengths do have advantages in a scattering environment.

Comment Re:Myth? (Score 1) 160

I design antennas and wireless links for a living, sir. Yes, I have done this. Since 1984.

Of course 900 MHz has better range, if one is using dipole antennas. The point is, the apparent difference in range is due to the antennas used, not some intrisic property of the propagation medium. (It's also likely that the 5 GHz transmitter has lower output power, and the 5 GHz receiver a higher noise figure, than the 900 MHz versions, making the 5 GHz range even less, but I'll ignore those factors for now.)

Give me parabolic dishes, and 5 GHz will go where 900 MHz will not. Give me resonant dipoles, and 900 MHz will go where 5 GHz will not. The behavior follows the antenna selection. It has nothing to do with "propagation," whether through trees, concrete block, or anything else.

Maybe numbers will help. Since the effective area goes as the wavelength squared, the amount of power captured from a resonant 5.5 GHz dipole, compared to that from a resonant 900 MHz dipole, from a given power flux density, is (900/5500)^2 = 0.027, or 2.7 percent. This means that, with identical efficiencies, matching loss, etc., the signal the 5.5 GHz receiver will get from its antenna is down 10*log10(0.027) = 15.7 dB from the signal the 900 MHz receiver gets. That's why your range at 5 GHz is less than at 900 MHz.

Now, let's move to parabolic dish antennas. Since the effective area of the dish antenna is the same for both bands, for a given power flux density the same signal level will be presented to each receiver at the antenna terminals. However, the power flux density generated by the transmit parabolic dish antennas will vary, but this time by the frequency squared, so the power flux density at the receivers on 5500 MHz will be 15.7 dB stronger than it will be at 900 MHz.

You can see where this is going. One has four possibilities:

1. Tx dipole, Rx dipole: 5.5 GHz signal 15.7 dB weaker than 900 MHz
2. Tx dipole, Rx dish: 5.5 GHz signal the same as 900 MHz
3. Tx dish, Rx dipole: 5.5 GHz signal the same as 900 MHz
4. Tx dish, Rx dish: 5.5 GHz signal 15.7 dB stronger than 900 MHz

You may ask how much range change is implied by a 15.7 dB change in signal strength. One simple range model is the use of a loss exponent; a typical value for the loss exponent might be 3 or 4. If we are optimistic, and take 3 as the value for the loss exponent, and assume a 15.7 dB change in loss, one can determine the ratio of the two distances as

10 * n * log10(d1) - 10 * n * log10(d2) = loss(dB)

30 * log10(d1/d2) = 15.7

d1/d2 = 10^0.524 = 3.34.

The band with the 15.7 dB strength advantage (the 900 MHz band, when dipoles are used, or the 5 GHz band, when dishes are used) has more than three times the range of the alternative. (If one assumes a loss exponent of 4, as might happen in an ugly indoor environment, the ratio works out to be about 2.47.)

Comment Re:Myth? (Score 3, Interesting) 160

when there are obstructions(i.e. real life) 900MHz reaches further than higher frequencies. It ain't no myth it's physics

No, it's a myth. If your 2.4 GHz radio had an antenna the size of your 900 MHz radio antenna, the performance would be the same. But because the 2.4 GHz dipole is smaller, the 2.4 GHz range is less. But it has to do with the antennas used, not any propagation phenomenon.

Resonant dipole antennas are constant-gain antennas, meaning that their gain is constant with frequency, while their effective area varies inversely with frequency squared. There are also constant-aperture antennas, in which their effective area is constant with frequency, while their gain varies. A parabolic dish antenna is an example of the latter; its gain varies with the frequency squared. If you take two parabolic dish antennas, fit them with 900 MHz feeds, and then take the same dishes and fit them with 2.4 GHz feeds, you'll find that the 2.4 GHz antennas have (much) higher gain and the resulting system, much greater range than the 900 MHz configuration.

It's also possible to set up a link with a constant-gain antenna (e.g., a dipole) on one end and a constant-aperture antenna (e.g., a parabolic dish) on the other. In this case the two effects cancel out, and the user does not see a difference in range between the two frequencies.

You'll find, if you actually do this experiment, that it does work this way -- regardless of whether the path goes through a forest, a house, or both. It's physics, period.

Comment those signals propagate better NOT (Score 0) 160

I am so tired of this myth.

900 MHz signals do NOT "propagate better." They propagate in free space just the same as 2.4 GHz signals and, in the presence of scattering (e.g., small openings in otherwise shielded areas) not as well as 2.4 GHz.

What people fail to realize is that these systems typically use some variant (often a physically shortened variant) of a dipole antenna, and the 900 MHz antenna is physically larger than the 2.4 GHz antenna. It therefore has a much larger effective area (the effective area being inversely proportional to the square of the frequency of operation), and so captures much more of the incoming energy. To the user, it looks like the propagation is better, but in reality the device just has a physically bigger antenna.

Look at it this way: If propagation got worse as the frequency went up, we'd never see light from the sun!

Comment Nomograms (Score 5, Interesting) 132

I was musing just the other day about a related calculating method that has fallen into disuse, the nomogram. Nomograms always impressed me as an especially clever way to perform specific mathematical tasks.

When I was young, and dirt was still sparkling and shiny new, nomograms were in every engineering textbook, handbook, and reference book. Their demise in engineering applications seems to have come with a whimper, not a bang, as no one seems to have noticed it.

Comment Tossed Salad (Score 1) 258

The comment has been made before that the better analogy is to a tossed salad (or salad bowl), rather than a melting pot, as the ingredients tend to maintain their individuality, rather than becoming homogenized. A lot of it is bland lettuce, but one also has everything from olives to jalapeños to spice things up a bit. (Extra credit for identifying a good analogy for the salad dressing.)

Comment Range (Score 1) 120

From TFA:

[. . .] According to this prediction, the possible attack distance is approximately 16.78 cm using the same sound source that we used for the real-world attack with the maximum volume (113 dB). This attack distance range might not be sufficient for a malicious attacker. However, attackers can overcome this distance limitation by using a more powerful and directional source (e.g., a loudspeaker array) than the single speaker used in our experiments. For instance, SB-3F from Meyersound can generate sound of 120 dB at 100 m, and 450XL from LRAD and HyperShield from UltraElectronics can produce 140 dB at 1 m, which is equivalent to 108.5 dB at 37.58 m. Therefore, the possible attack distance is 37.58 m, if an attacker uses a sound source that can generate 140 dB of SPL at 1 m.

Comment What *is* every little app doing? (Score 2) 129

There may not be any need to panic and start fixating on what every little app is doing.

But then again, there might. How is one to know? That's the biggest problem I have with the mobile telecom computing model. I have no idea what the apps do, and no way, other than make it my life's work, to find out.

I hate having to trust the OS provider that everything is properly sandboxed, that none of the apps in their stores are malware, etc. What's going on, inside this box?

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