Monday, November 28, 2022

Will Space-Based Wireless Power Beams Solve the Energy Problem?


A recent article in a good-news website called The Brighter Side claimed that in a few years, we may be getting lots of energy from space-based wireless power stations.  A research group at Airbus, the European aerospace firm, has built a prototype and has high hopes for the technology's future.


Here's how it would work.  A solar panel about 2 km (more than a mile) across would be in geostationary orbit, probably an orbit with a tilt to it so that the satellite would be in continuous sunlight 24/7 rather than going through an Earth-caused eclipse once a day.  The amount of power thus generated—comparable to a standard fossil-fueled or nuclear plant—would then be converted into microwave energy, probably the same type of microwaves that heat your pizza.  The microwaves would be beamed through a large, sophisticated antenna array to suitable locations on Earth equipped with things called "rectennas"—antennas specially designed to receive microwaves and convert them efficiently into DC power.  The power would then be converted into standard AC for transmission on a grid, or conceivably used by mobile devices such as trucks and airplanes.


Sounds great, doesn't it?  Compared to earth-based solar panels, the ground-based technology is cheaper.  A rectenna is a lot easier to make than a solar panel.  As solar energy is more intense in space than on earth, it takes less solar-panel area in space to generate a kilowatt than it does on earth.  And there's the flexibility of beaming the power basically anywhere you want it, just as communications satellites beam signals to different locations.  What's in the way of our putting up lots of these and throwing fossil-fuel plants, and nuclear too for that matter, in the existential trash bin?


As a trained microwave engineer, I can answer that question.  Microwave power from solar-energy satellites is not a new idea.  Raytheon engineer William C. Brown conceived the idea in the early 1960s, and a key component was a crossed-field amplifier that he had invented earlier.  By 1976, Brown worked with NASA to transmit 30 kW of power over a distance of 1.5 km (almost a mile) using a 26-meter dish and a rectenna that was only 7.3 x 3.5 meters—about 10 by 25 feet.  I saw a video of that demonstration in which they used the power to light up an array of spotlights, and the lights gradually came on as the beam was directed at the rectenna.  The Airbus people have so far demonstrated a link only 36 meters long. 


If the idea has been around so long, why hasn't it been deployed commercially yet?  I can think of several reasons.


First, as even the optimistic Airbus researchers admit, the orbiting part of the system has to be really big to produce a useful amount of power.  Currently, it seems that the largest structure in orbit is the International Space Station, which is a little longer than a football field.  The solar array envisioned by the Airbus people would be twenty times longer and wider, if it was square.  At the current rate of space commercialization, however, such huge projects in space may become feasible.  But not yet.


Once you put the orbiting microwave power station in space, you have to be careful where you aim the beam.  The focusing ability of the station's antenna depends on how big the antenna is, and while it will be smaller than the solar array, you could easily imagine an antenna, say, 100 meters in diameter.  Some simple calculations I will spare the reader tell us that the beam on the ground from an antenna of that size in geostationary orbit, using the microwave-oven wavelength, will expose an area on the ground about 24 miles square to a microwave power density of more than a watt per square meter.  This level of energy would definitely be detrimental to human health and not real good for other vertebrates, either.  So a very large area on the ground, probably covering most of a good-size county, would have to be sequestered off and devoted to a giant rectenna farm.


The more speculative statements by the Airbus people of directly conveying microwave energy to mobile platforms such as airplanes are pretty much pipe dreams.  Not all the microwaves would be absorbed by the plane, and so people on the ground underneath would be at risk.  The tighter the beam you want, the larger the antenna has to be, so unless the designers want to make an antenna as big as the solar array, which brings up mechanical difficulties, I don't see how they can direct parts of the beam to highly specific locations on earth, although they could probably manage as many as a dozen or so without too much trouble.


The Airbus people are to be congratulated for dusting off an old idea that was clearly premature at the time it originated in the 1960s.  Back then it was prohibitively expensive to launch large structures into orbit.  We can count on both launch and space-based construction costs to decrease in real terms in the future.  So that is one big factor that makes reconsidering wireless power transfer from space a good thing. 


The drawbacks are substantial, however:  hazards to people and animals on the ground, the possibility of a space-based error that could send the beam skidding across the countryside into a major population center (there's a sci-fi scenario for you), and the difficulties of upkeep and maintenance—I suspect you'd have to have a few people permanently in space simply to keep the station running.  But it eliminates one of the big problems with most types of renewable energy these days: the fact that it's available only when the sun is out or the wind is blowing.  A properly designed geostationary-orbit power satellite would be available 24/7, through clouds, night and day. 


We'll know this technology's time has come when somebody like Elon Musk starts a company to do it.  In the meantime, though, it will remain what it has been for more than fifty years:  only an engineer's dream.


Sources:  I thank my wife for calling my attention to the article in The Brighter Side at  I also referred to the Wikipedia article "Wireless power transfer," and my 1975 ITT Reference Data for Radio Engineers handbook for antenna-beam calculations. 

Monday, November 21, 2022

Electric Vehicles and "Ancient Modulation" Don't Mix


Many of the currently available electric vehicles (EVs) on the market have a wonderful array of bells and whistles you won't get in a gas-powered car, but some new EV owners are surprised by the lack of one feature:  an AM radio (referred to by radio amateurs as "ancient modulation" because it was the first radiotelephony technology to be invented).


Now for most younger readers, an AM radio is not going to be missed.  While radio in general still has its uses for mobile platforms that can't yet conveniently connect to the Internet (although this problem will eventually go away too), AM radio is the oldest and lowest-quality medium in the broadcast hierarchy.  Consequently, much of its programming is devoted to talk shows, sports, and the kind of music that doesn't suffer much from the thunderstorm crackles, power-line buzzes, and night-time fading that AM radio is subject to.  Nevertheless, millions of people listen to AM stations across the U. S. and in other countries, and probably a majority of them do it in their cars.


So why do many EV makers leave out the AM-radio feature?  It has to do with an obscure branch of electrical engineering called "electromagnetic interference" (EMI). 


EMI studies how electric, magnetic, and electromagnetic fields and their related voltages and currents go from one electronic subsystem (quaintly sometimes called the "aggressor") to another subsystem (called the "victim") and mess up the victim's functioning somehow.  As our world becomes increasingly digital, casual experience with EMI is no longer common, as the characteristic of most digital systems is to work flawlessly until the interference reaches a certain threshold.  Then the whole thing collapses and you get nothing. 


Analog systems—the old analog TV that went away about 2009, conventional AM radio that is still with us, and to some extent, FM radio—are different.  As the interference gets stronger, if you were watching an analog TV picture, you'd first see some little random black specks here and there, then solid rows of them, then wider bands and the sound would start to feature a buzz-saw noise, and finally you'd lose everything in a kind of snowstorm (video noise was in fact called "snow".) 


AM radio is the same:  a few pops in the speaker here and there, then a steady buzz, and finally the signal is overwhelmed.


So why don't most EVs have AM radios?  Because the thing that makes EVs go is kilowatts of electric power, hundreds of volts at dozens of amps, being switched on and off thousands of times a second.  And the efficiency of the electronics depends on how fast those switches work.


Unfortunately, switching large amounts of power on and off very fast generates tons (metaphorically speaking) of energy that runs roughshod through the whole AM broadcast spectrum, which ranges from 540 kHz to 1600 kHz.  And a typical EV is just full of such currents, voltages, and magnetic fields, because the currents run through cables by necessity from the battery through the control electronics to the motors. 


I've never tried the experiment, but if you had a Tesla or other EV running on a test stand, and you got a little cheap portable AM radio, tuned it to an empty spot in the band, and moved it close to the car, you would in all likelihood be greeted with a banshee set of howls and growls that would be a good soundtrack for a horror film. 


Most of the energy thus produced is probably in the form of near-field magnetic fields.  These don't radiate very far (that's why I've been passed by many Teslas on the road while listening to an AM station and never noticed a problem), but within a few feet, which is where you are when you try to put everything into one vehicle, they can be quite intense.  And in contrast to electric fields, which are fairly simple to shield against with conductive screens, magnetic fields are very hard to enclose and shield against.  It takes special types of magnetic metals that are (a) expensive, (b) fragile, and (c) hard to shape, as they are usually supplied in the form of thin tapes that have to be hand-wrapped around the thing to be shielded.


Despite what the automakers say, it would be possible to make an EV that wouldn't interfere with an AM radio on board.  Take a standard EV to any big military contractor and tell them what you want.  They'll put a bunch of EMI experts to work, and they'll redesign the whole vehicle.  It'll probably weigh another few hundred pounds when they're finished and cost twice as much as it did before they went to work, but you'll be able to play your AM radio and drive at the same time.


See the problem?  That's one of the reasons the EV makers have just quietly dropped the AM radio, because it would mess up everything else if they put it in and made it work.


I don't see any good outcomes of this problem for standard AM broadcast services.  There's something called HD radio, as well as several other competing digital-radio services.  The basic idea is to use the allocated FCC frequency band granted to a station (plus maybe some parts of the adjacent channels) and stick a sophisticated orthogonal-frequency-division-multiplexed digital signal in there to carry as many as four audio channels.  This is being tried both with AM and FM, but I suspect the digital AM is wrecked just as thoroughly by EV EMI as the conventional AM is. 


So the alternative that at least one article posed, is to hope that in your metropolitan area, your favorite AM signal is also being carried by an HD-radio FM station and you can pick it up that way.  FM signals use much higher frequencies (88-108 MHz), which are much less affected by the electromagnetic trash that EV power electronics puts out. 


But I just went to the HD radio website and checked, and poor little San Marcos, halfway between San Antonio and Austin, doesn't have any HD radio signals.  Dallas-Fort Worth is another matter. 


So it may be that AM radio for cars, at least the old-fashioned kind, may go the way of buggy-whip holders on automobile dashboards.  Nobody missed those then, and maybe nobody much will miss AM radio in the future.


Sources:  Numerous articles are available on the absence of AM radio in EVs, and I referred to this one:  I also referred to the HD radio website and the Wikipedia article on HD radio.

Monday, November 14, 2022

How Old Is Too Old? The Dallas Air Show Crash


On Saturday, Nov. 12, an estimated four thousand or more spectators gathered at the Dallas Executive Airport about ten miles south of downtown to watch a Veterans Day air show put on by the Commemorative Air Force (CAF).  The CAF is a volunteer organization dedicated to keeping older military aircraft flying.  Their motto is "Educate, inspire, and honor."  Most of their inventory of 180 planes worldwide comes from World War II, and prominently featured during the show was a B-17 Flying Fortress, one of only a handful left from WW II service as heavy bombers.  Also featured were P-63 Kingcobra fighter planes. 


Around 1:20 PM, the B-17 had just flown low over the airport where the spectators were gathered.  As shown in a number of videos posted after the event, a P-63 approached it from the rear and appeared to collide with the rear section of the bomber.  Both planes fell out of the sky within seconds, and a fireball and black smoke rose from the site of the crash.


In a news conference later that afternoon, CAF CEO Hank Coates could provide few specifics out of deference to the National Transportation Safety Board (NTSB), which was scheduled to take over the investigation that evening.  He said the bomber was "fully crewed" which normally means a crew of five.  Adding the pilot of the P-63 means that as many as six people probably died in the crash, which occurred over an empty field.  Information from the Allied Pilots Association confirmed that two of its former members had died in the crash. 


In his news conference, CEO Coates emphasized that although all their pilots are volunteers, they spend many hours in training and certification efforts, and often have 20 or 30 years of experience as retired military or airline pilots.  Nevertheless, something went wrong Saturday, and it will take the NTSB some time to figure it out.


Once it does, what then?  Let's try to get some perspective on just how dangerous flying CAF planes is.


Statistics provided by the NTSB in an Associated Press story of the crash indicate that from 1982 to 2019, 23 people died in 21 accidents involving World-War-II-era planes.  Mr. Coates indicated that the CAF flies an average of 6500 hours a year.  If we assume that has been the case for the past 40 years, we can do a little math to come up with the average fatality rate per million hours flown. 


An airline-safety website tells me that for commercial airlines, the current fatality rate is about 0.34 per million hours flown.  General aviation (private planes) is about 50 times worse than that—say 17 per million hours.  If my assumptions are correct, the fatality rate up to 2019 for the CAF is at least 95 per million hours, or about one fatality per 10,000 hours flown—more than five times that of general aviation.


Now, no type of aviation is completely safe.  Any human activity, even getting out of bed, involves some risk.  The question here is whether the good that the CAF does—and there is much to be said for it—is worth the risk of getting pilots killed, and the small chance of a much larger number of fatalities.  If the crash had occurred a few hundred yards away from where it did, hundreds of spectators might have been killed.


Some will say that the risk, however small, is an essential part of the activity.  If it wasn't at least a little dangerous, it wouldn't be nearly as much fun.  I am not a pilot—the most risky thing I do typically is ride my bike two miles on city roads every day.  So far my worst accident happened when I was looking at the gears of an unfamiliar bike I was riding and ran into a trash barrel.  I rolled off the bike and did an unintentional backflip.  My back was sore for a day or so, but there were no other consequences.  I haven't ridden that bike since, however.


I'm sure the FAA has some kind of certification processes for both the hardware the CAF flies and the pilots who fly them.  We will have to wait for the NTSB's investigation to complete before knowing what caused this particular accident: pilot error, mechanical failure, or some combination thereof.  But judging by their fatality rate, it's clear that mostly retired pilots flying seventy-year-old planes is not as safe as flying a 747 to London.


I am sympathetic with CAF members who spend hundreds of volunteer hours doing difficult and sometimes dangerous things to keep their old planes in the air and educate the younger generation about what machines and people flying them did during twentieth-century wars.  I love the feel and look of old hardware, and if the CAF flew antique avionics as well as antique planes I'd be right in there with them (unfortunately ,they have to have modern equipment in that department for safety reasons). 


At the same time, there will come a day when the hazards of flying piles of fatigued aluminum gets to be simply too dangerous.  We are about out of pilots who flew the planes during WW II, so those who fly them now have had to learn from their elders, and you have a small cadre of skills that has to be handed on in order for the whole CAF to keep flying.  It would be sad to see all that come to an end so that the only place you could see a B-17 would be in a museum, not making a horrible racket as it actually takes off from the ground. 


But in the nature of things, that day will come.  Who decides when it comes?  Ideally, the CAF itself, but the other parties involved—the NTSB and the FAA to name two—will have some say in the matter.  I can picture the magnitude of this tragedy leading to public calls for such shows to cease, and that would be a shame.  But it might happen.  The prudent thing is to wait for the NTSB report, and then take stock of the whole situation.  But prudence these days seems to be in short supply.


Sources:  I referred to an AP story on the crash carried at, a CBS news report at, a report at, and data on aviation safety at're%20really%20really,times%20safer%20than%20general%20aviation.

Monday, November 07, 2022

Breathing While Black: Discrimination By Pulse Oximeters


For several years now, you have been able to go to your local drugstore and buy for less than $50 a device called a pulse oximeter.  It's a little thing you clip on your finger, and in a few seconds it displays two numbers.  One is your pulse rate, and the other is supposed to be the percent of maximum capacity of oxygen that your blood is carrying.  Most healthy people show a blood-oxygen percentage of around 98%, but anything considerably less than that means you're not getting enough oxygen to your tissues.


Hospitals and doctors have more sophisticated versions of these devices, but apparently they all share the same flaw these days:  they can give falsely reassuring readings on people whose skin has significant melanin content.  Black people, in other words.  So for decades, anyone in that category whose blood oxygen has been monitored with a pulse oximeter has been in danger of going untreated for low blood oxygen, compared to a person whose skin was lighter. 


This is not news.  The problem has been known for decades, but received added publicity during the COVID-19 pandemic.  Studies have shown that people of color receive less supplemental oxygen than average during medical treatment, and bad pulse-oximeter readings only exacerbate this problem. 


Fortunately, some engineers at Brown University are trying to address the problem.  In a report carried by the health-information site Statnews, Kimani Toussaint, a Black professor of engineering, is reported to be working with students on a patentable idea that will lead to pulse oximeters that give the correct reading no matter who is being tested, and what color their skin is. 


I wish them well, and hope that they can make a significant difference in what has to be one of the most embarrassing deficiencies in healthcare technology to come to light in years. 


It didn't have to turn out this way.  The same article cites a report in Wired on one of the first oximeters to hit the market way back in the 1970s, developed by what was then a medical branch of the instrumentation company Hewlett-Packard.  In their typically thorough way, H-P included 248 people of color in their volunteer pool of testing subjects, and made sure the readings were as good for them as for the other volunteers. 


Of course, the H-P device was a little fancier than the ones you get at Walgreen's.  It examined eight wavelengths of light, not one or two like the current ones do, and was about the size of a small beer cooler.  I'm sure it sold for more than fifty bucks, too.  But it got the oximetry ball rolling, and from that point on it was a question of how cheaply the device could be made, and whether inaccurate readings on a minority of the FDA-required sample population could be disregarded in the approval process, which they apparently were. 


I don't think anybody in the healthcare industry deliberately intended to make devices that discriminated in a purely technological way against people of color.  But beyond a certain point, ignorance was no longer an excuse, as studies were published describing the problem and cautioning clinicians not to trust readings of pulse oximeters with darker-skinned patients. 


But this is not a good solution.  The right fix, as Toussaint and his colleagues recognize, is to make pulse oximeters that work right for everybody, not just for white folks.  Supposing the Brown academics succeed (which seems pretty well guaranteed at some level, as H-P got it right in the 1970s with vastly inferior technology), what happens then? 


Like any industry, the healthcare-technology industry wants to make money and serve its customers as well as it can.  Compared to consumer products, devices sold for healthcare purposes are highly regulated and licensed, and jumping through the regulatory hoops is a cost that makes up a significant fraction of the price.  Unless the FDA insists on changing its rules so that pulse oximeters have to read equally accurately for all colors of patients, the industry doesn't have much of an incentive to adopt a newer technology that does that, whether it's patented by Brown or developed on their own.  For one thing, it means a whole new round of proof-testing and regulatory approval.  And for another thing, the market for pulse oximeters is probably not that big, and making a substantial investment in it for a benefit that will show up in only a minority of patients is a hard marketing sell.


I'm reluctant to use the phrase "systemic racism," but it might well apply in this case.  As I said, I don't think any individual manager or pulse-oximeter company set out to discriminate against people of color in developing devices that don't work quite as well for that group.  But somewhere along the long road of development between H-P's giant 1970s device and the $50 versions of today, somebody compromised some things and created the problem.  It would require a huge effort of investigative journalism and probably subpoenas to find out exactly how it happened, but the outcome is clear.


Sometimes, adverse publicity by itself will make an industry clean up its act.  Maybe if enough people of color ask questions of their clinicians about pulse oximeters, it will have an effect back up the supply chain and the companies will go to the trouble and expense of dealing with the issue.  But it's not going to happen automatically.  In the old days, a letter-writing campaign might have had some effect.  These days, social media is the obvious channel to use in letting people know there's a problem.  It's a pretty blunt instrument, though, a little like putting out a cigarette with a fire hose, and it can backfire on the user as well.


But as the Statnews article quoted Toussaint as saying, this problem is a poster child for increasing diversity in science.  If it's not a problem to you or people you know, you simply tend to ignore it.  Now that we know it's a problem—all of us engineers—I think it's time somebody should do something about it. 


Sources:  Statnews carried the article "‘A poster child’ for diversity in science: Black engineers work to fix long-ignored bias in oxygen readings" at  The Wired article about the 1970s H-P oximeter is at