Difference drive diversity antenna structure and method

Details Difference drive diversity antenna structure and method Difference drive diversity antenna structure and method

1999-04-06

2001-01-16

Ho, Tan (Department: 2821)

Communications: radio wave antennas

Antennas

With radio cabinet

C455S273000, C455S086000, C455S090300

Reexamination Certificate

active

06175334

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to antenna structures, and more particularly to producing a sufficiently high decorrelation between two antennas that are in close proximity such that the diversity reception performance is maintained.
BACKGROUND OF THE INVENTION
Portable wireless communication devices such as radiotelephones sometimes use one or more antennas to transmit and receive radio frequency signals. In a radiotelephone using two antennas, the second antenna should have comparable performance with respect to the first, or main, antenna and should also have sufficient decorrelation with respect to the first antenna so that the performance of the two antennas is not degraded when both antennas are operating. Antenna performance is a combination of many parameters. A sufficient operating frequency bandwidth, a high radiation efficiency, and a desirable radiation pattern characteristic, and a low correlation, are all desired components of antenna performance. Correlation is computed as the normalized covariance of the radiation patterns of the two antennas. Due to the dimensions and generally-accepted placement of a main antenna along the major axis of a device such as a hand-held radiotelephone, however, efficiency and decorrelation goals are extremely difficult to achieve.
FIG. 1
shows a prior art two-antenna structure implemented in a hand-held radiotelephone
130
. A first antenna
140
is a retractable linear antenna. When the first antenna is fully-extended, as shown, the length of the first antenna is a quarter wavelength of the frequency of interest. Note that the first antenna
140
is aligned parallel to the major axis
145
of the radiotelephone
130
and has a vertical polarization with respect to the ground
190
.
The radiotelephone
130
also has a microstrip patch antenna as a second antenna
150
attached to a printed circuit board inside the radiotelephone
130
and aligned parallel to a minor axis
155
of the radiotelephone
130
to send or receive signals having a horizontal polarization with respect to the ground
190
. In isolation, the second antenna
150
may well produce horizontally polarized signals, but when the second antenna
150
is attached to the printed circuit board and in the proximity of the first antenna
140
, the polarization of the second antenna
150
reorients along the major axis
145
of the radiotelephone
130
. As the polarization of the second antenna reorients, the first antenna
140
and second antenna
150
become highly correlated and many of the advantages of the two-antenna structure are lost. Commonly, a prior art two-antenna structure implemented in a radiotelephone has a correlation factor of over 0.8 between the two antennas. Effective diversity operation requires a correlation factor of less than 0.6 between the two antennas.
The reorientation of the polarization of the signals from the second antenna
150
is due to various factors, including the fact that hand-held radiotelephones typically has major axis
145
and the minor axis
155
dimensions with an aspect ratio greater than 2:1 and that the major dimension of the radiotelephone is significant with respect to the wavelength of operation while the other dimensions of the radiotelephone are small with respect to this wavelength. Additionally, because the minor dimension of the radiotelephone is small with respect to the wavelength of interest, the second antenna
150
is easily perturbed and detuned, which creates susceptibility to effects of the hand or head of a user
110
on antenna efficiency.
Thus there is a need for a two-antenna structure that maintains decorrelation and efficiency between a first antenna aligned along a major axis of a portable wireless communication device and a second antenna.


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patent: 4613868 (1986-09-01), Weiss
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patent: 0 749 216 A1 (1996-12-01), None
patent: WO 85/02719 (1985-06-01), None
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Diversity Antennas For Base and Mobile Stations in Land Mobile Communication Systems, by Yoshihide Yamada, Kenichi Kagoshima, and Kouichi Tsunikawa, IEICE Transactions, vol. E 74, No. 10, Oct. 1991.

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