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Comment: Re:No they don't (Score 1) 221

by MattskEE (#49374113) Attached to: Chinese Scientists Plan Solar Power Station In Space

That does not change the fact that the math is pretty solid and it would work.

You haven't done the math or cited any.

There is a big efficiency problem in space-based solar with the double conversion from DC to RF/laser and back to DC. Total efficiency neglecting transmission loss is about 64% (80% twice). Then you have to invert it back to AC usually, but that is also needed for terrestrial solar.

So 35% power is thrown away, but since the sun is stronger in space without atmospheric attenuation, and there are no clouds, some of that will be made up.

So to a first approximation the power per unit of panel area is the same from space as it is on earth.

Now factor in launch costs which are tremendous (even if SpaceX succeeds in yet another 10x reduction in launch costs as claimed) and the difficulty performing maintenance in space versus on land, and I don't see a strong case. Installation costs on land are around $1/watt, I'd like to see you come within a factor of 10 of that in space.

Comment: Re:Waste of time (Score 1) 51

by MattskEE (#49269089) Attached to: Fujitsu Could Help Smartphone Chips Run Cooler

In undoped semiconductor resistance will usually decrease as temperature is raised because a higher temperature excites more electrons into the conduction band where they can carry current.

But for cases where there is already a lot of charge the opposite usually applies. In something like a MOSFET the electrons are supplied by the source contact or in a doped bulk semiconductor there will be lots of charge from dopants. In these cases increasing the temperature doesn't significantly increase the charge. However raising the temperature does increase the electron scattering rate which reduces the electron mobility and slows electrons down, so the resistance actually increases.

Comment: Re:I'm disappointed in my fellow geeks (Score 1) 214

by MattskEE (#49235755) Attached to: Billionaire Teams Up With NASA To Mine the Moon

I'm all for exploring space, but the premise of mining the moon sounds pretty shaky and its totally fair for geeks to want realistic goals because that's what attracts sustainable capital investment from public and private sources that are needed to realize the goal. It's not a zero-sum game, but putting private and public money into crazy unworkable ideas takes some money away from realistic ideas.

Moon mining sounds to me a lot like orbital solar power - it sounds great and cool until you actually think for a moment and realize that it has very little to no net benefit per panel area compared to terrestrial solar after you transmit the power back to earth, and the cost is astronomical to boot. What is the benefit of mining the moon aside from sounding cool? It doesn't seem like there's a particularly high concentration of any of these purported mineral riches and with the expense of mining the material in space I'm highly suspicious of it being in any way economical after shipping back to Earth. He-3 is a bust since there's currently no major use for it. Maybe moon mining would be viable for obtaining materials on the moon without launch costs, but the supposed mineral riches are mostly high priced specialty materials and not the boring metals like iron and aluminum that would be needed for building a spaceship or lunar colony.

Fuck off, some of us have dreams.

Some of us like to support our dreams with back of the envelope analyses to ensure viability before getting too invested in them.

Comment: Re:Spectrum is measured in Hz? (Score 1) 91

by MattskEE (#48943725) Attached to: US Wireless Spectrum Auction Raises $44.9 Billion

Nope, reread your link. Channel capacity (at a given signal to noise ratio) is proportional to bandwidth alone. 1.000GHz to 1.065GHz is as good as 20.000GHz to 20.065GHz.

But as you say higher frequencies often have worse propagation characteristics, especially through buildings, which reduces channel capacity by reducing the signal to noise ratio.

Comment: Re:TWT based (Score 1) 54

by MattskEE (#48880641) Attached to: TWEETHER Project Promises 10Gbps MmW 92-95GHz Based Wireless Broadband

To get tens of watts from solid state in W-band yes you need either spatial or corporate (or both) combining of individual chips. So the amp ends up being of similar size to a mm-wave TWT but you don't need an expensive and large HV power supply or water cooling. Chip level output powers in GaAs I think have been done up to about 500mW, and 1-2W in GaN.

GaN for mm-wave still has some yield and reliability problems (pick at least one depending on supplier), and performance is not yet up to the ideal levels. But that's the same thing that GaN went through at lower frequencies, and mm-wave GaN is improving.

Comment: Re:TWT based (Score 2) 54

by MattskEE (#48879835) Attached to: TWEETHER Project Promises 10Gbps MmW 92-95GHz Based Wireless Broadband

Solid state GaAs is slowly catching up to TWTAs at this frequency. They're not common but it is possible to buy a 30 watt solid state amplifier, probably for the same price you can get a TWTA that has a little bit more power. GaN still has lots of problems at this frequency but it's improving and will likely be competitive with tubes within 5-10 years.

But yes it seems like it would be much easier to do this at Ka band where solid state amps are now a better value than tubes for communication applications.

Comment: Re:Physics doesn't work like that. (Score 2) 54

by MattskEE (#48879743) Attached to: TWEETHER Project Promises 10Gbps MmW 92-95GHz Based Wireless Broadband

In addition to penetrating solids the range is challenging (or expensive anyway) just because of limited transmit power levels. Power is important because that gives you your range, your cell phone and home wireless router transmit up to about 1 watt. A 1 watt output solid-state power amplifier at this frequency would cost $5-10k, or at least that was the case about a year ago. This project seems to propose using travelling wave vacuum tube technology which provides lots of drive power (50-100 watts) but at a high price (over $100-250k).

Comment: Re:kW or kWh? (Score 1) 245

by MattskEE (#48756419) Attached to: Bill Gates Endorses Water From Human Waste

What's wrong with kWh? For industrial processes that's a common energy unit.

It may confuse you but it doesn't confuse other people. In fact it makes more sense to most people since they have some frame of reference for how much energy a kWh but they don't have an intuitive frame of reference for Joules.

Comment: Re:even better (Score 5, Informative) 133

by MattskEE (#48639121) Attached to: Tesla About To Start Battery-Swap Pilot Program

Do you know how many solar panels it takes to charge an electric car? You're basically looking at a football field's worth, each.

Ah, to be young and full of made up numbers. Let's do the math.

The large Tesla battery is 85kWh. A solar cell typically has an efficiency of 10-20%, so with about 5kWh/m^2/day of typical solar radiation (check PVWatts for specifics in your town you can produce about 0.5-1kWh/m^2 per day.

If we assume 15% charging losses it will take 100kWh to charge a Tesla battery, which will require 100 to 200 square meters to produce in one day. A football field has about 5300 square meters, so we could expect one football field of tightly packed solar panels to charge around 26 to 53 Tesla's per day.

Comment: Re:wimpy talk (Score 5, Interesting) 187

by MattskEE (#48606605) Attached to: Graphene: Fast, Strong, Cheap, and Impossible To Use

Graphene in addition to the engineering challenges does have some very fundamental scientific challenges as well.

The most important challenge is its lack of a bandgap meaning that graphene transistors cannot be turned off. That drawback means that while it may have a ~500GHz cutoff frequency on par with silicon and below the InP records it will not modulate current in an energy-efficient way, and while it can create some forms of logic the lack of a bandgap limits its power amplifying frequency to a measly 50GHz, well below the competing technologies. Contrast that with Northrop Grumman's recent 1000GHz amplifier, which is admittedly not a great amplifier since it is run very near its cutoff frequency it has 1dB or less gain per stage, but it works which is still quite impressive.

So far the various methods that can give graphene a bandgap also take away the extremely fast electron transport properties that made graphene so interesting for electronics in the first place. Some of us working on competing technologies wonder why hundreds of millions of dollars have been spent on graphene transistor development without solving the fundamental bandgap problem - of course we just want that money directed to our own research, but some of us try to be realistic about the capabilities of what we are developing ;-)

I'm sure graphene will be useful for some things but so far there are still some fundamental problems that need to be solved before using it for high-speed electronics for wireless applications or digital logic. We'll see how it does.

Comment: Re:Usability Nightmare (Score 1) 63

by MattskEE (#48288897) Attached to: LG's 0.7mm Smartphone Bezel Is World's Narrowest

Cynical much? One phone does not need to be all things to all people in order to be successful. Not all of us need cases on our smartphones: in my 4.5 years of smartphone usage and two smartphones I have yet to damage my uncased phones in any way.

Since you need a case on your smartphone you should buy a different one, others of us enjoy the beautiful design of a tiny bezel. But I hope to keep my phone at least another 1-2 years so I'm not in the market currently.

Felson's Law: To steal ideas from one person is plagiarism; to steal from many is research.

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