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Sentient City Survival Kit

artifacts for survival in the near-future sentient city

Antennaware technical notes

Antenna design is part art, part science – an alchemy of sorts.

Virtually any conductive loop will pick up some turbulence in the electromagnetic fields surrounding us. Yet there are specific engineering specifications that detail design criteria and parameters for tuning antenna designs for specific frequencies (such as UHF RFID). But what criteria and parameters are relevant if you want to sew RFID antennas into clothing?

In absence of available information sources, one invariably resorts to hacking. In this case, I picked an antenna design from Texas Instruments that matched the specs of the UHF RFID reader I’m using for testing. The specs for the reader promised a read range of around one meter (enough for viable near-field data skimming). Working from this Instructable, I was able to mock-up a simple prototype that would generate enough of a voltage when the reader was on to detect with a simple circuit incorporating an 18 bit ADC and an Arduino microcontroller. (The ADC is required because of the low voltage fluctuations – around 0.1 mV – I’m trying to detect.)

I then cut out the chip from the TI RFID tag and bridged the antenna contacts with a 82pF capacitor. With this antenna, I was able to get a consistent reading indicating when the reader was on or off.

The next step was to see if I could reproduce the inductive properties of the RFID inlay with conductive thread sewn into fabric. Working with Jamie O’Shea at Eyebeam and using a sewing machine with digital embroidery capabilities, a prototype was produced to test the viability of this approach.

First I created a bitmap image of the antenna design that is used to produce a “stitch file” for the sewing machine tool path.

The colors in the image above represent discrete patterns the embroidery software uses to create a series of instructions for the machine to follow. With this machine, one could control stitch direction and density, as well as the sequence in which the pattern is stitched.

It took a number of attempts to produce one workable prototype for testing. The process was one of trail and error, where the tension of the conductive thread in the machine needed to be continually re-adjusted in order for it not to break. If nothing else, I developed an appreciation for the centuries of thought that went into the sewing machine: how to optimize speed, precision and variety in the process of stitching fabric with thread?

Upon arriving at V2_ in Rotterdam work continued on the project with Piem Wirzt, who suggested trying a commercial embroidery service (the kind that embroiders shirts and baseball caps with names and logos, for instance) to have the pattern stitched.

These are the machines from this shop:

After some persuasion (and one quick test), they agreed to give it a go. Here is the result:

Piem gave an introduction to a machine at V2_ that produced a nice edge for the patch which would be simple to tack on to the garments.

The antenna patch was then tested and compared to the original TI antenna design:

While the voltage read by the ADC when the RFID reader was on was lower than when using the original TI antenna, the antenna patch did return workable results (~ 0.2 mV) – enough to work with.

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