Excerpted from Wireless Systems Design ©1999

Stubby Antennas Take New Approach

BY NATHAN COHEN
AND JOHN CHENOWETH

WIRELESS design has relied on the stubby antenna for more than two decades. These antennas comprise plastic cylinders that encase helical coils or short lengths of wire, some with additional dielectric material for loading. Now there is a new approach to stubby antennas with high efficiency and easily designed multiband capabilities.

These antennas have been called conformal communications cylinders, or ComCyl antennas. They are intricate fractal antennas or other complex patterns etched on thin circuit boards, which are then curled inside a hollow plastic stubby casing. Figure 1 shows this type of small, cylindrically conformable circuit board, a fractal antenna loop on a thin low-loss substrate before insertion in the plastic casing.

Fig 1. This small, cylindrically conformable circuit board shows a fractal antenna loop on a thin low-loss substrate before insertion in the plastic casing.

The advantage of the ComCyl approach is that it does not suffer from the inefficiencies long known in conventional stubby designs. Fractal antennas in this new context are very efficient, even when moderately electrically small. Furthermore, the conformal printed circuit design's fractal and related designs produce a natural multiband loading capability, where, through choice of the design pattern and iteration, true multiband agility and control are attainable.

In conjunction with T&M Antennas, a joint ComCyl product is now offered for dual-band cellular/personal-communications-services (PCS) use. This dual-band ComCyl stubby (fractal stubby) is indistinguishable in outward appearance from any other short cellular phone. Inside, however, its curled fractal circuit board provides an efficient monopole replacement with good to excellent standing-wave-ratio (SWR) characteristics when mounted on small, handheld cellular phones.

The SWR of a dual-band ComCyl stubby antenna is shown. This monopole ComCyl antenna was mounted at the top of a small, cellular-phone-sized circuit board to simulate the transceiver. The good match to 50 at analog Advanced Mobile Phone Service (AMPS) and PCS is evident for the ComCyl antenna--without any matching network.

The field-strength comparisons are particularly revealing. Figure 2 shows anechoic azimuth measurements of ComCyl and sleeve dipole antennas at 849 MHz. At this frequency and size, short stubby antennas usually prove inefficient and cannot be made to work equivalently to a dipole.

Fig 2. Field-strength comparisons show anechoic azimuth measurements of ComCyl and sleeve dipole antennas at 849 MHz.

The field strengths reveal that the ComCyl monopole antenna on the cell phone is virtually indistinguishable from a full-sized sleeve dipole. Average field-strength differences of less than 0.28 dB are seen, with maximum decibel differences for the two patterns being less than 0.8 dB. These do not include minor gain corrections for the slight SWR mismatch. These values imply an attainable efficiency of 95 percent for the ComCyl antenna at AMPS/Global System for Mobile Communications (GSM) frequencies.

Fractal Antenna Systems, Inc. will have additional ComCyl versions available by January 2000. These include AMPS/PCS/GPS and AMPS/PCS/2.4 GHz. GSM and industrial-scientific-medical (ISM) band equivalents will also be available. An extremely low-profile ComCyl antenna for the Bluetooth (2.4-GHz) wireless local-area network (WLAN) is also planned for next year. This new technological approach enables very efficient, multiband use in stubby form factors for telecom and wireless applications.

Contact Information:

Comcyl® Available exclusively from Fractal Antenna Systems, Inc.

NATHAN COHEN, CTO, Fractal Antenna Systems, Inc., 300 Commercial Street, Suite 27, Malden MA 02148 ; (617) 381-9595, FAX: (617) 489-6207. Fractal Antenna Website: http://www.fractenna.com.