### Miniature Antennas

October 2, 2017

One of my first electronic circuits, built when I was a 7th grade student, was a VLF (Very Low Frequency) radio receiver. It was constructed from a few transistors to detect lightning strikes, and also the natural radio waves known as atmospherics at those frequencies. It would also register signals when a large electric motor was activated in the neighborhood. The signals were connected to an amplifier and monitored on a loudspeaker. The sounds, greatly enhanced in the morning and called the dawn chorus, were a lot like the communications from extraterrestrials heard in "B" movies.

The circuit was described in an article in an electronics hobby magazine, likely Popular Electronics. Only simple and inexpensive circuits could be presented in such a magazine, and the simplicity of this circuit was a consequence of the VLF radio signals being in the audio frequency band, accessible by any home hi-fi equipment. The inexpensive feature in my case was a consequence of an electronic technician neighbor who donated many of the electronic components.

The antenna used in this radio receiver was a long length of wire. I later learned that this was a very inefficient antenna for VLF signals, since simple antennas like this should be as long as about a quarter wavelength of the signal. There's a very useful equation for conversion of frequency f to wavelength λ using the speed of light c (3.00 x 108 meter/second),
Putting an audio frequency of 10,000 Hz into this equation, knowing that a hertz has dimensions of 1/second, gives a wavelength of 30 kilometers (18.64 miles)! My antenna was obviously a very inefficient collector of radio energy at VLF frequencies.

As I explained in an earlier article (Robert A. Helliwell, June 16, 2011), VLF pioneer and discoverer of the atmospheric radio signals known as whistlers, Robert A. Helliwell, actually built a thirteen mile long antenna. Not only that, but it was built at Antarctica in a location called Siple Station that was active from 1971-1988.[1] The antenna was used to transmit VLF radio signals into Earth's magnetosphere, to be detected half a world away in Canada.

Antenna size has always been a problem for designers of mobile devices. Fortunately, the demand for greater data rates has forced the migration of cellphone communication to higher and higher frequencies. Cellular frequencies started at 450 MHz and 800-900 MHz, and have climbed to 1,800-1,900 MHz, and future devices might operate at 2,500 MHz. The following table shows the wavelengths for these frequencies. The lower wavelengths allow efficient antennas to be included in today's cellphones.

Frequency (MHz) Wavelength (Meters)
450 0.67
800 0.38
900 0.33
1,800 0.17
1,900 0.16
2,500 0.12

Semiconductor technology can be used to fabricate extremely small devices, and these could be used in many applications, including in vivo medical sensing. However, getting these devices to communicate their data wirelessly is a problem, since the allowed antenna dimensions are much smaller than a radio wavelength.

That's the problem addressed in an open access article by a large research team from Northeastern University (Boston, Massachusetts), Westford Academy (Westford, Massachusetts), Andover High School (Andover, Massachusetts), and the Air Force Research Laboratory (Wright-Patterson Air Force Base, Dayton, Ohio).[2-4]

The minimum practical size for an antenna is about a tenth of the wavelength.[2] As Nian Sun, a professor of Electrical and Computer Engineering at Northeastern and corresponding author of the study,
"A lot of people have tried hard to reduce the size of antennas. This has been an open challenge for the whole society... We looked into this problem and thought, why don't we use a new mechanism?"[4]
The new mechanism involves the use of a resonance medium with a wave propagation speed much smaller than the speed of light. Acoustic waves in solids propagate at a speed of just a few thousand meters per second, so their wavelength is smaller by five orders of magnitude at a given frequency.[2-3] They were able to transition from the electromagnetic realm to the acoustical realm by developing acoustically-actuated, nanomechanical, magnetoelectric (ME) antennas.[2]

Their ME antennas are fabricated as suspended ferromagnetic-piezoelectric thin-film heterostructures that transmit and receive radio waves through the ME effect at their acoustic resonance frequencies.[2] In transmission, the bulk acoustic waves in the ME antennas stimulate magnetic oscillations in the magnetic thin film, and these oscillations produce electromagnetic radiation. Conversely, in radio reception, the ME antennas convert the magnetic fields of electromagnetic waves to acoustic waves using the piezoelectric effect.[2]

The concept for this type of antenna was theoretically proposed in 2015.[5] When the excitation was via surface acoustic waves, such devices only worked at a few kilohertz. The antennas of the present study are based on bulk acoustic waves, and they perform as well as conventional antennas, but at a hundred times smaller size.[2] The concept was demonstrated at at VHF and UHF frequencies.[2]

An acoustically-actuated, nanomechanical, magnetoelectric antenna operating at the VHF frequency of 60 MHz.

The frequency of operation is low enough that analysis could be done by resonance assisted by a high frequency lock-in amplifier (LIA) feeding an induction coil.

The antennas are fabricated as thin sheets of a piezomagnetic material, the magnetic analog to a piezoelectric material that expands and contracts in response to a magnetic fields to convert an incident electromagnetic wave to acoustic waves via the magnetic component of the electromagnetic wave.[2-3] The piezomagnetic material in the devices is an alloy of iron, gallium, and boron.[3] A piezoelectric material then converts these acoustical waves to an oscillating voltage. That handles the radio reception. For transmission, a voltage produces vibrations in the piezoelectric, and these are coupled to the piezomagnetic to produce electromagnetic waves.[2-3]

The research team demonstrated the concept with two kinds of acoustic antennas, one having a rectangular membrane for reception of VHF signals, and the other having a circular membrane for frequencies in the gigahertz range. A comparison of the disc antenna with a conventional loop antenna of the same size showed that 2.5 gigahertz signals were transmitted and received about 100,000 times more efficiently.[3]

An acoustically-actuated, nanomechanical, magnetoelectric antenna operating at the UHF frequency of 2.5 GHz.

In this case, excitation was with a horn antenna, and analysis was done with a parametric network analyzer (PNA).