Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2000-06-29
2001-12-04
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S803000, C343S815000, C343S818000
Reexamination Certificate
active
06326922
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a Yagi antenna system wherein the active and parasitic elements of the antenna can be co-located on one printed circuit board (PCB) with a low noise amplifier (LNA). Furthermore, surface mount devices (SMDs) can replace the balun that is conventionally used for impedance matching between the symmetrical radiator impedance and the asymmetrical LNA input impedance.
BACKGROUND OF THE INVENTION
Yagi antennas are used in high frequency applications such as satellite radio transmission. There presently exists a population of 4 billion people that are generally dissatisfied and underserved by the poor sound quality of short-wave or terrestrial radio broadcast systems. This population is primarily located in Africa, Central and South America, and Asia.
FIG. 1
shows an overview of a satellite broadcast system
10
comprising various broadcast stations
2
for transmitting multiple audio signals, for example, to a satellite
4
, which in turn transmits these signals to the receivers
9
. The satellite broadcast system
10
is particularly useful for providing high-quality broadcast programming to users in Africa, Central and South America, and Asia. The present invention relates to a low-cost antenna that can be mounted on a portable radio receiver
9
for reception of satellite radio transmissions. This invention is particularly useful for the reception of satellite signals where a receiver antenna gain on the order of 9 dbi together with a noise figure on the order of 1 dB are required due to the low power flux density available at the receiver location.
Yagi antennas generally consist of three types of elements: reflector, radiator, and directors. The radiator (e.g., a folded dipole) is an active element that receives the power concentrated by the parasitic elements. The reflector is a parasitic element with an inductive quality. The directors are also parasitic elements but with a capacitive quality. Yagi antenna systems use the parasitic elements in combination with active elements to control the direction and width of the beam. The Yagi antenna optimizes gain by using specific director lengths and spacing between the directors and the driven element (e.g., the radiator).
In addition, the Yagi antenna typically employs a balun (e.g., a half wavelength coaxial line) to achieve a 180 degree phase shift of the signal. Specifically, as seen in
FIG. 2
, a coaxial cable
32
is physically connected to the driven element (e.g., folded dipole)
40
. The inner sheath
38
is connected to one side of the folded dipole
40
and the feed cable
34
, and the opposite inner sheath
39
is connected to the opposite side of the dipole. The outer sheath
36
is connected to ground. As the signal travels around the inner sheath from
39
to
38
it becomes 180 degrees phase shifted from the original signal. This cable and dipole arrangement is cumbersome and prevents an antenna arrangement from being constructed on a simple printed circuit board. A need exists for a more compact means to drive the components of a Yagi antenna.
A compactly designed Yagi antenna is disclosed in U.S. Pat. No. 5,612,706. However, this antenna merely reduces the distance between two rods and is not well suited for radio receiver portability. It is more convenient to have a Yagi antenna that can be folded for transportation. Further, it would be advantageous to have a less costly implementation than the one disclosed in U.S. Pat. No. 5,612,706.
Removal of the balun is described in U.S. Pat. No. 5,898,410. A log periodic dipole array antenna system achieves impedance matching by adjusting the distance between a focusing element and one of several dipoles or driven elements. The antenna system therefore has plural active elements and, correspondingly, impedance matching requirements for each of these elements. A need exists for a low-cost antenna having a simple active element impedance matching design.
A performance limitation of the Yagi antenna is the signal loss caused by cables and connectors between the antenna feed point and the low noise amplifier input stage. There is currently a requirement to match the antenna feed point to a standard impedance (such as 50 ohms) which can be accommodated by off-the-shelf connectors and cables, and then again match the impedance to the low noise amplifier input stage. This sequential impedance matching requirement incurs line and connector losses, which in turn detrimentally affect the performance of the Yagi antenna.
As shown in
FIG. 3
, some patch and Yagi antenna systems
10
use dual circular polarization outlets which can be costly due to the type and number of components. For example, the system shown requires two outlets, that is, a right-hand circular polarization outlet
18
and a left-hand circular polarization outlet
16
, two low-noise amplifier (LNA) input stages
24
and
26
, an electronic polarization switch
14
, and at least two housing mounts
12
and
13
.
Manufacturing costs are also a contributing factor to the expense of the receivers
9
. It is known in the art to use coaxial cables
20
and
22
to connect the LNA input stages
24
and
26
to the antenna outlets
18
and
16
to achieve impedance matching. However, as mentioned in U.S. Pat. No. 4,518,968, balanced low impedance feeders have been recommended, but have not often been adopted in practice. This is because such feeders, when engineered for dipole and Yagi-Uda array matching impedances, are dimensionally awkward to manufacture and install. Further, since the folded dipole and the director elements are separate from the low noise amplifier (LNA), two fabrication procedures are needed, thereby increasing the likelihood of problems due to manufacturing tolerances. Thus, a need exists for a low cost Yagi antenna design that is easily mass-produced with a low error tolerance.
It is known, for example, from U.S. Pat. No. 5,272,485, to use antennas embedded in substrates in microwave frequency applications where a feedpoint and via are used as an input to a low noise amplifier, thereby obtaining optimum impedance matching. However, these diagonally-fed electric microstrip dipole antennas are patch antennas that are constructed on at least two layers of a dielectric substrate. These types of patch antennas cannot be designed for high gain without using an array of patches, thereby incurring a negative effect on complexity and size.
Accordingly, a need exists for a more simple means of impedance matching of a Yagi antenna with only one driven element. A need also exists for an active antenna system that is low cost and readily mass-produced while providing reasonably high gain, directivity and noise performance. A foldable design is desirable to keep the antenna compact for travel.
SUMMARY OF THE INVENTION
These needs and others are satisfied by the Yagi antenna system of the present invention which, in a preferred embodiment, comprises an LNA, reflector, radiator or driven element, and at least one director all located on the same printed circuit board. Therefore, the present invention can eliminate the need for two separate housings, that is, one containing the LNA and the other containing the radiator and the directors.
An object of the present invention is to provide a low cost antenna that allows for simple and cost-effective mass manufacturing. This is possible because the antenna system of the present invention can be located on one printed circuit board, thus allowing for tighter tolerances during mass production.
Another object of the present invention is to eliminate the need for a balun cable. Since all the elements of the antenna can be located on the same printed circuit board, signal losses caused by coaxial cables and connectors and by the impedance matching between the LNA and the driven element are minimized as well.
Yet another object of the present invention is to provide a simple means of achieving the 180 degree phase shifting requirement for the feed to the opposite dipole side. The present invention eliminates the need f
Nguyen Hoang
Roylance Abrams Berdo & Goodman L.L.P.
Wong Don
WorldSpace Corporation
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