Wi-Fi Backscatter

I have heard over the years, of radio experimenters, hams, and hackers that converted radio waves into electrical power. The concept is not complicated. You receive radio waves and instead of consuming power to modulate that wave and extract a signal, just convert it directly back into DC power. Tesla actually described this in principle over a century ago.

The problem from the get go is that this is a hugely lossy process. In the whole system you have loss to heat conversion, wave dissipation, AC to DC loss, etc etc. This is not a net neutral exchange. But in a broadcast most of the power is wasted. If you imagine a room mostly filled with radio waves, imagine how much of that wave if actually being received by your wifi devices: the laptop, tablet, printer, cell phone, maybe your Roku?  Btu the signals fill whole rooms and serve no devices. That's all loss. If you recover some of that energy, (without losing more in the process) then that's gravy.

A rectenna is a rectifying antenna. It;s most common application is to convert microwave energy into direct current. the designs are usually simple: a dipole antenna (or array of dipole antennas) with a diode connected across the dipole elements. The diode rectifies the AC current induced in the antenna by the microwaves, to produce DC power, which powers a load connected across the diode. Schottky diodes are usually used because they have the lowest voltage drop and highest speed and therefore have the lowest power loss.

It was patented by William C. Brown at Raytheon back in 1964, though development had been ongoing since at least 1958.  Back in the 1970s the development of rectennas inspired scifi writers to imagine solar satellites beaming energy down to earth. The utopian solar power dreams never came to fruition, though the technology ended up in RFID systems. So here we see the same idea being re-purposed with wifi backscatter. Wi-Fi backscatter uses radio frequency signals as a power source and reuses existing Wi-Fi infrastructure to provide Internet connectivity to battery-free devices. It has some advantages over solar satellites in being small and cheap and also very narrow band which makes the antenna much more efficient. More here and here.

Wi-Fi Potatos

Broadcasting a radio signal inside any structure is a challenge. I've covered one corner of this topic previously in a 2008 post about radio in tunnels. There is a similar situation encountered today by radio engineers trying to offer WiFi inside airplanes. It's a long tunnel shaped space, filled with human beings who would like to use the Internet. Unfortunately each one of those people acts as a couple hundred dielectric pounds of joy. Physics is a bummer.

Generally speaking, a human being is a big bag of saltwater. I've explained this before as well in a 2010 post on hand capacitance "A dielectric is just an insulator... Chemically the human body is about 75% water, that's H20, Oxygen and Hydrogen. Water is not a good conductor of electricity. That's why we are a good dielectric."  In an enclosed space like your apartment, a human body has the effect of attenuating radio signals, like WiFi . More here.

The problem is even worse on an airplane. An airplane stuffs in human mass like sardines in a tin making for less airspace than there is dielectric mass. Both the segments of the 700MHz-3.5GHz range used for mobile telephony and both the 2.4 GHz and 5.0 GHz ranges used for WiFi are particularly sensitive to this effect. This is a serious engineering obstacle. Nonetheless Delta, American Airlines, United, Virgin and even Jet Blue tout their WiFi service on flights.

It was Boeing that first solved the problem of modeling a long metal tube filled with manimals. It turns out that for the purposes of science we are very similar to a sack of potatoes. We and the humble spud have a couple things in common in terms of water content. Boeing described the experiment  on their website.

"Using a de-commissioned airplane, the team from Boeing Test & Evaluation laboratories conducted a series of tests. The team determined that sacks of potatoes were ideal stand-ins for passengers, given their similar physical interactions with electronic signal properties. Much of the testing was conducted on the grounded airplane with the seats filled with 20,000 pounds of potatoes in sacks. The test data was then validated with human stand-in "passengers."

This is why science is awesome.

Hedy Lamarr Invented Wifi

The story is strange, and not particularly credible on the face of it.. but as it so happens, true. Sometimes unexpectedly someone we know for abilities in one arena works furtively in another. Proficiency in multiple disciplines was the foundation of being a Renaissance man, an idea sort of lost in our modern pre-fab world where most people don't know how to much of anything. but Hedy Lamarr comes from a different era entirely. She was and is famous as an actress of the 1930s and 40s. On the side she was an inventor and a surprisingly capable one.

Hedy Lamarr was born in 1913 as Hedwig Kiesler. She was raised in Vienna her father was a banker, her mother a pianist.Her father was said to have given her an interest in technology. She invented scores of inconsequential wonks: a cola-flavor tablet similar to Fizzies, an attachment to tissue boxes to hold used tissue, and then the  not-so-wonky  US Patent 2,292,387. More here and here.

Back in 1933, Ms. Lamarr was married to Fritz Mandal, the first of her six husbands. Mr. Mandal worked in the manufacture of military aircraft in Austria. Lamarr clearly learned a few things before they divorced in 1937. By 1940 she was married to Gene Markey, and met  George Antheil at a party. Antheil had previously experimented with automated control of musical instruments and was a successful composer. She bounced her latest idea off him, radio control for torpedoes that employed frequency hopping to avoid detection and jamming. After consulting with an electrical-engineering professor at the California Institute of Technology,they filed a patent for a "Secret Communication System", on June 10, 1941. Radio control was not new, Tesla had invented that more than to decades earlier. But frequency hopping, that was a completely new idea. 

Their system used a synchronized punched rolls of paper, like piano rolls to scan across a set of 88 frequencies. Note that there are 88 keys on a piano so the design stayed solidly within Antheil's experience automating player pianos. The Navy scoffed, but eventually tried out the idea in 1962 during a blockade of Cuba.It was a few decades late for her dreams of foiling the Nazis. Other groups were equally dismissive usually recommending she just go out and raise money for war bonds. It probably didn't help that she wasn't yet a naturalized citizen, but some of it was certainly sexism as well.

I've read the patent. It would probably even sort of work, but it's reliance on a Rube Goldberg mechanical timing would likely lose synch in a moving torpedo. But in the U.S. we patent not just devices but also ideas. Lamarr called her idea "frequency hopping" a term we still use today. It was the forerunner of the spread-spectrum technology used in Wi-Fi, CDMA (Code Division Multiple Access) technology in cell phones, cordless phones and of course... certain secret communication systems. Antheil died in 1959 and never saw his work used, but Hedy certainly did. One can only hope she felt vindicated... even if the patents had expired.

Lamarr continued her film career and retreated from public life in the 1980s and retired in Miami Beach, FL. Clearly growing eccentric, she was arrested in 1991 for shoplifting laxatives. She died in January of 2000 at the age of 86.

Delaware Valley Radio Ephemera

This is an odd one. This is some kind of business card for C.H. Davis, Inc a stereo and television store in Ardmore, PA. Inexplicably the store still exists today, just a few blocks away from the junk shop where I bought the book that contained the card; presumably as a book mark. But this is an old card. They were still selling portable radios and phonographs when this card was made.I do notice there are no Am stations listed. That indicates that it's after 1960 —during the ascension of FM.

The call changes can narrow the date of this scrap of paper. Some isn't helpful.  WJBR is still on 99.5 unchanged since 1956. WPEN became WMGK only in 1975. WFIL only became WIOQ in 1971.We know it's older than that.  92.5 FM was WIFI back then and is WXTU now. They changed format from pop to country before 1984. WIP-FM hasn't been on 93.3 since 1965, after which it has been WMMR. On 93.7 these days we have WSTW, it hasn't been WDEL-FM since abotu 1966. (WDEL-AM remains on 1150.) WFLN-FM is now WBEN, it only dropped the heritage FLN calls in 2006. WHAT-FM also changed calls in the late 1960s when it dropped it's jazz format. 98.1 FM is now WOGL as it has been since 1987. Back in the 1960s WCAU-FM  was simulcasting MOR with WCAU-AM.   

WSNJ-FM 98.1 helps narrow the date a bit. That station signed on in 1946 as WSNJ. (FYI: Wikipedia incorrectly dates that to 1961) What happened in 1961 was that WSNJ swapped calls with 107.7 WPBS. they had already been on air for 5 years.  They changed calls to WUSL in 1976. WSNJ remained on 107.7 until about 8 years ago. So, it had to be printed before 1961 the date of that call swap and after 1956 the date WJBR signed on. (Math corrected by Fybush)

WXPN, WHYY and WRTI are about all where they were even if the programming has changed a lot. WHYY has a typoed frequency. But WPWT is more of a mystery. they were a share-time with WKDU on 91.7 from the1950s into the 1980s. I hear they were a crazy psychedelic hippie station. I'd love to hear an aircheck.  I'll have to write a post about them sometime.

Water Bottle Antenna

In developing nations there's are big problems wiring up your laptop to the network. There are two fundamental causes:

1. The power grid is unstable and often unavailable
2. Many areas lack a terrestrial data connection

But even basic Wi-Fi is a real problem due to the scarcity of essentially all involved hardware. As with all engineers, scarcity is the step-mother of invention. In this case, a plastic water bottle is used to make a dielectric resonator antennas (DRAs.) These liquid dielectrics can be used effectively at frequencies between 1-4 GHz.

What they're doing at BottleNet is finding bottles with the correct dimensions to use as a waveguide. The diameter of the cylinder largely determines what frequencies the antenna can transmit or receive. This is a mathematical relation to the cylinder's length so calculations must be made to cut it to the correct legnth. More here.

But plastic can't reflect microwaves right? Well yeah, that's a bit of a problem. But appropriately size-cans are in short supply. So these little geniuses just wrap the puppy in a wire mesh called fly-screen so it has a reflection profile. The mesh has a 1-millimeter weave which in comparison to the wireless frequencies is effectively the same as a solid reflective metal surface.

Mali must be a nation of a million MacGyvers. Here's what you'll need to do this at home for materials and tools. The final product actually functions on par with a typical low-end factory made wireless antenna. Full instructions here.

MATERIALS
(1) 1.5 litre bottle of Diago mineral water
(1) Piece of metal woven flyscreen 300 mm x 220 mm
(1) Piece of metal woven flyscreen 100 mm x 100 mm
(1) 31 mm length of 14 or 16AWG wire
(1) Appropriate connector (N-type typically)

TOOLS
Pliers
Awl or pointy object
A soldering iron and solder
Scissors for cutting mesh
Leather gloves for handling sharp edges of flyscreen