[Jeri] Builds a Magnetic Loop Antenna

Most new hams quickly learn that the high-frequency bands are where the action is, and getting on the air somewhere between 40- and 160-meters is the way to make those coveted globe-hopping contacts. Trouble is, the easiest antennas to build — horizontal center-fed dipoles — start to claim a lot of real estate at these wavelengths.

So hacker of note and dedicated amateur radio operator [Jeri Ellsworth (AI6TK)] has started a video series devoted to building a magnetic loop antenna for the 160- and 80-meter bands. The first video, included after the break, is an overview of the rationale behind a magnetic loop. It’s not just the length of the dipole that makes them difficult to deploy for these bands; as [Jeri] explains, propagation has a lot to do with dipole height too. [Jeri] covers most of the mechanical aspects of the antenna in the first installment; consuming a 50-foot coil of 3/4″ copper tubing means it won’t be a cheap build, but we’re really looking forward to seeing how it turns out.

We were sorry to hear that castAR, the augmented reality company that [Jeri] co-founded, shut its doors back in June. But if that means we get more great projects like this and guided tours of cool museums to boot, maybe [Jeri]’s loss is our gain?

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Jeri Ellsworth Tours A Radio And Tech Museum

[Jeri Ellsworth] has done some YouTubing again (yes, that’s a word, just like YouTuber) after a four-year hiatus. She’s recently uploaded a very enjoyable four-part series touring the Museum of Radio and Technology in Huntington, West Virginia.

Part one contains radios spanning the ages, starting with a spark gap transmitter, some wonderful crystal sets, pocket radios from the 1940s, commercially available amateur radio transmitters and receivers from the 1930s to the 1950s, and more. There’s even a lovely hack of a transmitter built into an old refrigerator. Part two contains educational toys, three covers television sets and cameras, and four is about all types of record players and hi-fi. Each contains equipment as old as the spark gap transmitters in part one.

You may know of [Jeri] as co-founder of castAR, an augmented reality startup that recently shut its doors, but before that she was famous among hackers for her numerous projects ranging from a flexible electroluminescent display,  a centimeter wave scanner using hacked feed horns, to yours truly’s personal favorite, a Commodore 64 bass keytar.

So nuke some popcorn, sit back, and enjoy the tour following the break.

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[Jeri] spills the beans on her AR glasses


In the last year, [Jeri Ellsworth] has been very busy. She was hired by Valve, started development of an augmented reality system, fired by Valve, and started a new company with [Rick Johnson] to bring her augmented reality glasses to the market. On the last Amp Hour podcast she spilled the beans on what went down at Valve, how her glasses work, and what her plans for the future are.

[Jeri] and [Rick]’s castAR glasses aren’t virtual reality glasses like the Oculus Rift or other virtual reality glasses that cut you off from the real world. The castAR glasses preserve your peripheral vision by projecting images and objects onto a gray retro-reflective mat and allows you to interact with a virtual environment with an electronic wand. So far, there are a few demos for the castAR system; a Jenga clone, and a game of battle chess called Team For Chess, a wonderful reference to Valve’s hat simulator.

The electronics inside the castAR glasses are fairly impressive; new frames are drawn on the retro-reflective surface at 100 Hz, positioning accuracy is in the sub-millimeter range, and thanks to [Jeri]’s clever engineering the entire system should be priced at about $200. Not too bad for an awesome device that can be used not only for D&D and Warhammer, but also for some very cool practical applications like visualizing engineering models of 3D prints before they’re printed.

[Jeri] uses light bulbs in an oscillator

Way back when [Ms Ellsworth] was a kid, she kept seeing the same circuit over and over again in her various op-amp books. It was a Wien bridge oscillator, a small circuit that outputs a sine wave with the help of a light bulb. Now that [Jeri] is much wiser, she decided to play around with this strange oscillator and found it’s actually pretty impressive for, you know, a light bulb.

The interesting portion of the Wien bridge is the gain portion of the circuit. It’s just a simple resistor divider, with a light bulb thrown in on one of its legs. When the current increases, this causes the light bulb to warm up (not enough to glow, though). When the temperature increases, the resistance in the light bulb increases, making the oscillator reach an equilibrium.

It’s a clever setup, but what about swapping out a resistor in place of the light bulb? In the video, [Jeri] tries just that, and it’s a mess. Where the light bulb circuit is amazingly stable with very, very low distortion, the resistor circuit looks like a disaster on the scope with harmonics everywhere.

A very cool build that would be perfect for an audio synth, but as [Jeri] says in her YouTube comments, “This doesn’t have enough distortion for indie bands.”

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[Jeri] shows off a delta sigma ADC

[Jeri] has had a bear of a time moving up to Valve Software, but electron microscope is safely in her garage (!) and her electronics lab is slowly taking shape. Since she can’t bring out the real-life gravity gun she’s working on, she decided to show off a one-bit ADC that uses just a flip-flop to sample an analog waveform  into digital data.

By toggling the clock input of a 74xx74 (or any flip-flop, really) and feeding the complimentary output to back into the data input, [Jeri] can get an output that is a 50% duty cycle feeding into the input of the chip. Adding an audio input to this data input with 10k pot to this feedback loop will cause the duty cycle to change in relation to the analog input, making a one-bit ADC.

As with any electronic shortcut, there are a few drawbacks: the clock cycle feeding into the flip-flop has to be pretty fast; at least a few dozen kilohertz if you’re sampling audio. Still, if you don’t have a free ADC pin, or you’d just like to build a bitcrushing guitar pedal, it’s a very simple (and cheap) way to get analog into a digital micro.

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An in depth interview with [Jeri Ellsworth] about everything

Here’s an interesting interview with [Jeri Ellsworth] over at the Jenessee Network. Usually the interviews I see popup with people are fairly short and sweet but this one really delves into many subjects and takes its time to explore. They start off talking about how [Jeri] began with hacking, which was literally smashing toys “with rocks” to see what was inside. They move on to discuss her adventures in building a race car, and then racing it as a teen… as an act of defiance.

In case you didn’t know, [Jeri] has been full time at Valve for about a year. Much of the discussion focuses on this from about 20 minutes in. She doesn’t hold back on information about what her daily life is like at valve as well as her experience during the hiring process. An interesting fact is that she didn’t initially recognize the name “Valve” and ignored them for a while. She does admit that if they had mentioned portal she would have paid a little more attention.

I was unaware that she had a side job putting the overflow of pinball machines she aquires into bars. When she moved to valve, she shut down that business, but she’s been flooding the halls with pinball machines, much to the enjoyment of the older folks.

[Jeri Ellsworth] on making her c64 bass keytar

[Jeri Ellsworth] finally set aside some time to talk about the build process for her Commodore 64 bass keytar. We think what started by taking a band saw to the guitar body ended up as a fantastic new instrument.

When she was showing off the project at Maker Faire we really only got a cursory look at what it could do. Her most recent video covers all that went into pulling off the project. Once the bulk of the guitar body was gone she tore the guts out of a dead c64 in order to mate the case with the guitar neck. Always the craftsman, she altered the computer’s badge to preserve the iconic look, then went to work adding pickups to each string using piezo sensors. This was done with Maker Faire in mind because magnetic pickups would have been unreliable around all of the tesla coils one might find at the event. These were amplified and filtered before being processed via an FPGA which connects to the original c64 SID 6581 chip.

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