from Wireless Systems Design ©1999
Stubby Antennas Take New Approach
THE ADVANTAGE OF THE COMCYL APPROACH IS THAT IT DOES NOT SUFFER
FROM THE INEFFICIENCIES OF CONVENTIONAL STUBBY DESIGNS.
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
to see enlarged image
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
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.