Shawn D. Rogers

Reconfigurable antennas for wireless devices

New technologies in communications electronics, such as software-defined radio (SDR) and RF switches implemented using micro-electromechanical systems (MEMS), present new challenges and opportunities for antenna design. In sharp contrast to digital technology where Moores law reigns, a fundamental law of physics constrains the ability to realize electrically small antennas that are both efficient and broadband. As a result, covering several frequency bands concurrently with a single antenna having enough efficiency and bandwidth is a major challenge. One possible solution to this problem is to use reconfigurable antennas that tune to different frequency bands. Such an antenna would not cover all bands simultaneously, but provides narrower instantaneous bandwidths that are dynamically selectable at higher efficiency than conventional antennas. Such tunable-antenna technology is an enabler for software-definable radios, the RF front ends of which must be reprogrammable on the fly. This paper discusses the practical implementation issues, limitations, and measured results of small, narrowband, tunable antennas within portable handsets. Many of the concepts discussed in this paper will likely become practical and cost effective in the near future because of recent advances in RF MEMS switches.

AMCs comprised of interdigital capacitor FSS layers enable lower cost applications

The focus of this work was to reduce manufacturing costs of printed artificial magnetic conductors (AMCs) by employing interdigital capacitors to enhance the effective sheet capacitance of the FSS without resorting to a second layer of overlapping patches. A 2.8 GHz AMC was fabricated and tested to demonstrate the concept. This AMC was then used to demonstrate a 2.9 GHz low profile antenna. Cost savings for this alternative manufacturing technique are estimated to be at least 50%.

Two-port model of an antenna for use in characterizing wireless communications systems, obtained using efficiency measurements

The agreement in these efficiency measurements validates the accuracy of the Satimo range, while highlighting the simplicity of the Wheeler cap. When one needs pattern data or the efficiency of an antenna in the presence of a surrogate hand and head, then a spherical near-field range, such as the Satimo system, is necessary. However, it is seen that a network analyzer and a Wheeler cap are sufficient for accurately measuring total antenna efficiency versus frequency for a small, solitary antenna. This total efficiency is the transfer function of the antenna, and may be used by system designers to evaluate the antennas bandpass and band-reject properties.

A multi-band artificial magnetic conductor comprised of multiple FSS layers

A multi-band artificial magnetic conductor (AMC) is described wherein N capacitive frequency selective surfaces (FSS) are employed to realize a zero degree reflection phase at N non-harmonically related resonant frequencies. A simple Cauer Type I LC network is proposed to model the plane wave reflection coefficient for this type of multi-band AMC. As an example, a dual band AMC (N=2) is fabricated and tested to demonstrate the concept. Both a reflection phase resonance and a surface wave bandgap are demonstrated in the GSM 850 MHz band and the PCS 1900 MHz band.

An AMC-based 802.11a/b antenna for laptop computers

A dual band 802.11a/b antenna system fabricated on two separate artificial magnetic conductor (AMC) substrates is described. This low profile antenna, which is 3.4 mm in thickness and 12 mm in width, is mounted onto the metal housing above a laptop screen. In each band, the antenna system has a peak gain at the horizon of at least 2 dBi and has -5 dBi or greater gain over 80% of the horizon azimuth angles.

AMC edge treatments enable high isolation between 802.11b and Bluetooth/spl trade/ antennas on laptop computers

Although Bluetooth and IEEE 802.11b radios each utilize spread spectrum modulation techniques in the 2.4 GHz ISM band, these two wireless standards still interfere with each other when located on the same platform such as a laptop computer. In order to diminish interference, it is advantageous to use two antennas with low mutual coupling. This may be achieved by locating an artificial magnetic conductor (AMC) that suppresses surface currents over a band of frequencies between the antennas. We have demonstrated that an AMC can be used as a reactive edge treatment to isolate two 2.4 GHz antennas by 45 dB on a surrogate laptop.

An embedded quad-band WLAN antenna for laptop computers and equivalent circuit model

An embedded antenna for WLAN applications covering the frequency bands 2.4-2.48 GHz and 4.9-5.9 GHz is described. The stamped metal forming the antenna has liquid-crystalline polymer (LCP) material insert molded around it for additional mechanical support. This antenna can be mounted at various locations around a laptop screen. An equivalent circuit model is developed to explain the antennas electrical performance.

Experimental results of an AMC antenna fabricated with a magnetically-loaded elastomeric substrate

Experimental results are shown for low profile bent-wire monopole antennas fabricated on artificial magnetic conductors (AMCs) that contain homogeneous magnetically-loaded elastomeric substrates. The elastomeric substrate is a lower cost and lighter weight alternative to the use of ferrite tiles. A 900 MHz antenna with a 20% return loss bandwidth and 40% antenna efficiency is described.