Antennas That You Install With A Spray-Can

With the explosion in cell phones, WiFi, Bluetooth, and other radio technologies, the demand for antennas is increasing. Everything is getting smaller and even wearable, so traditional antennas are less practical than ever. You’ve probably seen PCB antennas on things like ESP8266s, but Drexel University researchers are now studying using titanium carbide — known as MXene — to build thin, light, and even transparent antennas that outperform copper antennas. Bucking the trend for 3D printing, these antennas are sprayed like ink or paint onto a surface.

A traditional antenna that uses metal carries most of the current at the skin (something we’ve discussed before). For example, at WiFi frequencies, a copper antenna’s skin depth is about 1.33 micrometers. That means that antennas have to be at least thick enough to carry current at that depth from all surfaces –practically 5 micrometers is about the thinnest you can reasonably go. That doesn’t sound like a lot, but when you are trying to make something thin and flexible, it is pretty thick. Using MXene, the researchers made antennas as thin as 100 nanometers thick — that’s 10% of a micrometer and only 2% of a conventional antenna.

There are other materials that wind up in thin antennas, but they all have challenges either because they are not very conductive or are difficult to fabricate. MXene is a fairly new family of materials developed at Drexel University. To produce it you start with MAX which is a combination of titanium, aluminum, and carbon. The aluminum is removed in a process that requires acid and stirring for 24 hours, lithium chloride, and a centrifuge. The hydrofluoric acid is nasty to work with, but not beyond the reach of a careful home lab. You can see a Drexel video about making MXene, below. The researchers sprayed the antennas on a thin plastic substrate.

The only thing that looked tricky to us, was that thin flakes of the specific MXene used degrade in the air due to oxidation. That means production needs argon gas and the final product has to be laminated with something to protect it from the air, so that’s going to add thickness in a practical device.

Of course, PCB antennas are nothing new. But if you read the paper, you’ll see these antennas can readily outperform conventional thin antennas.

27 thoughts on “Antennas That You Install With A Spray-Can

    1. I just read the Wikipedia page for HF, that stuff is really scary – the delay from exposure to symptoms is frightening and it penetrates tissue. So skin, eyes, or inhaled are are all entry points and it is also powerful poison. But it does have a really cure nickname “The Bone Seeker”, because it dissolves bone, so you think that you have not screwed up and your skeleton is dissolving away.

    2. We had a tank of the stuff at work for breaking down investment after going gold casting. The dink in the polishing room was being used to took the shot from the tumbler in a bucket filled with Ammonia, Water and Soap around to the sink near the hydrofluoric tank (bucket). Turned around and there was this really cool mist about 1.5m off the ground following me around.
      Turns out that the Ammonia and hydrofluoric vapour’s were reacting in the air. The place was evacuated until it had cleared.

      And lets not discuss the week after when I split the contents of the Cyanide bath all over myself. :(
      Or the day dumped the bracelets I were making directly in to the Sulfuric bath straight after having them in the Cyanide bath and watched them fizz

    3. Agreed. I had to do a training on it for my job (IDK why, it was programming) and any sort of contact requires immediate medical attention. If you can smell it, you’ve already been exposed to too much, and need to go to the hospital. As far as I understand, it isn’t particularly acidic, but it is incredibly poisonous.

  1. A sheet of metal doesn’t need to be thicker than the skin depth to carry current; it just doesn’t reduce resistive losses further if you make it thicker than a few times the skin depth. Higher conductivity would reduce skin depth, but the materials they describe are much, much less conductive than copper. Their only virtue over copper seems to be aqueous deposition processing.

    I checked the Science Advances article to see how such a silly misinterpretation of the significance of skin depth got into Hackaday’s writeup, and it turns out it came right from the paper. Said paper also constantly misuses standard RF terminology, pretends that S11 measurements are somehow an indication of how well a material is suited to making antennas, and doesn’t even report antenna gain in valid units (they keep reporting in just “dB,” by which I’m pretty sure they mean dBi peak gain, not average gain, which would be the only sensible way to use gain as a metric for judging materials for antennas). Maybe their chemistry is okay, but goddamn is their RF knowledge trash.

    Is Science Advances some kind of open-access, no-peer-review dumping ground with the Science name stapled to it, or have their reviewers just completely given up pushing authors to write decent papers?

    1. Oh good, it’s not just me. That whole second paragraph was just one “huh???” after another. But thank you for pointing out that 100 nm is 10% of a micrometer. Although where I come from, we call those “microns”.

      The gist of the article, I’m pretty much guessing, was intended to be that this fancy material, which has a conductivity less than copper (thus thicker skin depth), makes its use for thin antennas ideal. I guess, because you can spread it really, really thin, where it will underperform copper of the same thickness. Next they’ll be suggesting that you apply a copper plating over the MXene. You know, to make it 90% thinner, or something.

    2. Ambitious researcher talks to PR dept. about something cool. PR department writes a press release translating what they thought he said into average person language. “______ University discovers possible breakthrough that could revolutionize ______ soon.”

      Oddly, even technical magazines publish such crap without picking up the phone and talking to the people who did the work to see what the details are. And some supposed engineering magazines don’t have engineers write articles.

  2. Readers should be extremely skeptical of the paper.

    Anyone else catch how the paper makes absurdly non-representitive comparisons? The comparisons made in their transmission line section share almost nothing in common with their reference design, and I suspect the antennas suffer from the same problem. Their IEEE Microwave Components and Letters paper making a structure out of Gold (which they claim to outperform) used a transmission line dramatically smaller (1.7mm vs 0.081mm core conductor width for a start), and implemented on a dramatically different substrate? https://ieeexplore.ieee.org/document/798027/

    Their comparison materials made of Aluminum and Copper aren’t even made in a similar manner. Large flakes are sprayed onto a substrate but the VSWR isn’t measured robustly. VSWR as a measurement of antenna performance is kind of like using the hum of an idle engine as an indicator of how well your car runs. It’s a good start, but it’s hardly descriptive and varies for many reasons. Furthermore, if you drive a car off the lot and the engine sounds like trash, you don’t tend to assume that it’s a reasonable indicator of the other cars you might find. The supplementary materials section (http://advances.sciencemag.org/content/advances/suppl/2018/09/17/4.9.eaau0920.DC1/aau0920_SM.pdf) suggests that the thick film antennas aren’t even properly matched. Silver nano-powder antennas can easily approach the thickness that they’re working with, so they can’t even claim that a comparison to their thin structures couldn’t be fabricated.

    It kills me because this paper is full of babble and hype but otherwise ignores many of the performance details of what could be a really cool material by drawing all sorts of apples-to-oranges comparisons and neglecting basic design practice. What could sell this material is not it’s RF performance, but questions of cost and manufacturing speed. How long does it need to cure, what kind of heat treating does it need, and how clean does the fabrication environment need to be?

    /wimper

    1. Agreed.

      They probably know a lot about chemistry, but nothing about antennas.

      If the return loss systematically gets better when you make the antenna thinner then my first thought would be that the.it is just resistive losses.

      I also see no sign or discussion of a balun. Thats critical for a dipole fes by coax.

    2. “VSWR as a measurement of antenna performance is kind of like using the hum of an idle engine as an indicator of how well your car runs. It’s a good start, but it’s hardly descriptive and varies for many reasons. ”
      wow! tu fer stating it that way! very well said!

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