Apple Patents “Micro-Slot” Antennas

On Tuesday the U.S. Patent Office granted Apple, Inc. patent 8373610 for “Microslot antennas for electronic devices.” Most TV transmitter engineers think of slot antennas as those big cylinders 40 or 50 feet high used to transmit UHF, and sometimes high-band VHF, signals.

(I once explained how slot antennas work in my Feb. 1996 RF Column.) The simple explanation is that under Babinet's Principle, a slot in a metal sheet can be considered the electromagnetic opposite of the piece of metal removed to make the slot.

It’s difficult to get RF into or out of tablets with conventional metal cases without making part of the case from some type of RF-transparent material. As the number of frequency bands these devices have to cover increases—I hope that at some point FM and TV bands will be included—the idea of potentially using the entire case for antennas is attractive.

It seems obvious after reading the patent, originally applied for more than five years ago, that slots in the metal case of a tablet or phone could be used as an antenna. The patent includes enough drawings to allow an RF hobbyist to construct his or her own case antenna with the right tools. There’s even an SWR plot showing how multiple slots can be combined to widen the bandwidth. Slots can be made very thin (just as a dipole can be made out of wire) and filled with a dielectric material.

I see one potential problem with using slots in the case of a wireless device as an antenna, and that is the ability for a user’s hands, or metal objects, such as a protective case or mount, to detune the antenna. Of course, that’s likely to be a problem with any antenna in a handheld device.

Doug Lung

Doug Lung is one of America's foremost authorities on broadcast RF technology. As vice president of Broadcast Technology for NBCUniversal Local, H. Douglas Lung leads NBC and Telemundo-owned stations’ RF and transmission affairs, including microwave, radars, satellite uplinks, and FCC technical filings. Beginning his career in 1976 at KSCI in Los Angeles, Lung has nearly 50 years of experience in broadcast television engineering. Beginning in 1985, he led the engineering department for what was to become the Telemundo network and station group, assisting in the design, construction and installation of the company’s broadcast and cable facilities. Other projects include work on the launch of Hawaii’s first UHF TV station, the rollout and testing of the ATSC mobile-handheld standard, and software development related to the incentive auction TV spectrum repack. A longtime columnist for TV Technology, Doug is also a regular contributor to IEEE Broadcast Technology. He is the recipient of the 2023 NAB Television Engineering Award. He also received a Tech Leadership Award from TV Tech publisher Future plc in 2021 and is a member of the IEEE Broadcast Technology Society and the Society of Broadcast Engineers.