Up to now, no one fully understood why a piece of dielectric material would radiate EM waves when a radio frequency signal was applied at one end. Now researchers have found a way to reduce the size of GHz antennas by modifying an existing technique, the use of antennas made from a dielectric or insulating material instead of a conductor.
Maxwell equations explain how high-frequency flows of electrons in conductors generate EM waves, but they do not explain how an insulating material, where there is no flow of electrons, would also act as an antenna.
Image: U.S. Patent Office Marconi’s 1900 patent gave 21st century engineers an idea.
Old Patent Holds Clue
Marconi’s 1900 patent gave 21st century engineers an idea.
The researchers noted that Marconi’s famous British patent application from 1900 had a little noticed detail.
It depicted a transmitter linked to an antenna connected to a coil, which had one end dangling while the RF signal was fed to the middle of the coil.
Although counter intuitive, this asymmetric coupling between the spark generator and the antenna allowed the transformation of the RF electric signal into EM radiation.
The researchers realized that it was this asymmetry, that explained the generation of EM waves in Marconi’s transmitter.
You can see such symmetry breaking in a once-common technology: the two-wire ribbons used during television’s first few decades to send RF signals from rooftop VHF antennas to television sets without any loss.
The electric RF current in the two conductors flow in opposite directions and have opposite phase.
Because of the two conductors are parallel the radiation fields cancel each other out, so there is no net radiation into space.
But if you would flare the ends of the two conductors at one end of the ribbon, they aren’t parallel anymore and you break the translational symmetry.
The two electric fields are no longer aligned and don’t cancel each other out, causing the RF signal to be converted into EM radiation.
The researchers say symmetry breaking explains why dielectric matter can transmits EM waves.
“Until now this was not well understood,”
Piezoelectric Materials
Dielectric antennas are already in use, but they are too bulky for on-chip use.
Instead of the dielectric materials in use today, researchers chose a piezoelectric film.
“Normal dielectric antennas are limited by fabrication technologies because we cannot get dielectric materials in thin-film form. Piezoelectric materials can be in thin film form and their thickness can be in the order of 100 to 20 micrometers,”
For their experiment the researchers used piezoelectric filters that consist of two interdigitated contacts deposited on a piezoelectric film, devices similar to crystal frequency filters now used in cell phones.
When excited in a symmetric mode, they act as a simple L-C circuit.
When excited in an asymmetric mode, whereby one of the two interdigital contacts was excited while the other interdigital contact was left floating, the piezoelectric filter acted as a monopole antenna.
In fact, in a way comparable to the antenna described by Marconi in 1900.
Experiments near one of the GPS frequencies (1575.42 MHz) showed that the new antennas had an efficiency (the percentage of the power of the RF signal converted into electromagnetic radiation) of up to 60 percent.
Their antenna was able to radiate one watt of power, which about what’s needed by most portable devices.
For their next experiments they will try to create dielectric antennas for longer wavelengths.
“We are thinking of bands between 200 and 600 MHz, they are interesting, we could, for example, replace large television antennas by smaller ones,”
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