Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1999-07-08
2001-07-03
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S860000
Reexamination Certificate
active
06255993
ABSTRACT:
TECHNICAL FIELD
This invention pertains to an antenna that separates transmitted and received signals having a common frequency.
BACKGROUND OF THE INVENTION
A prior art system
10
for interrogating one or more radio frequency transponding modules
12
is described with reference to FIG.
1
. The system
10
includes an interrogator
13
operating in response to commands from a controller
14
. Data and commands are exchanged between the interrogator
13
and the controller
14
through interconnections
15
.
In one mode of operation, a transmitter TX
16
contained in the interrogator
14
supplies RF signals through interconnections
17
to a transmit/receive (T/R) antenna system
18
. The T/R antenna system
18
in turn radiates an interrogation signal
20
to one or more of the transponding modules
12
. When the interrogation signal
20
is received by one of the transponding modules
12
, a response signal
24
may be generated and transmitted. The response signal
24
typically includes modulation allowing some property or set of properties of the transponding module
12
to be determined.
The response signal
24
is received by the antenna system
18
and is coupled to a receiver RX
28
. The receiver RX
28
demodulates the received response signal
24
and supplies information determined from the received response signal
24
to the controller
14
via the interconnections
15
. The controller
14
, in turn, may be able to supply information derived from the response signal
24
to an external processor (not illustrated) via a bus or other data link
30
.
Systems such as the system
10
find wide-ranging application in a broad variety of settings. The IPASS toll collection system presently in use in the Chicago area is one example of such a system. In transponder-based toll collection systems, the presence of an object, such as a moving vehicle (not illustrated), is detected by a detector. In a toll collection system, the detection process may rely on reflection of electromagnetic waves, detection of magnetic anomalies or detection of a large mass.
In response to detection of the presence of the vehicle, the controller
14
causes the transmitter TX
16
to transmit interrogation signals
20
having carefully limited range. The transponding module
12
is typically a dash-mounted unit that has been pre-programmed to identify the vehicle and to provide information regarding a pre-existing account associated with that vehicle.
The transponding module
12
transmits a response signal
24
containing information to the interrogator
13
. The information in the response signal
24
identifies the vehicle and the account associated with that transponding module
12
. The receiver RX
28
receives the response signal
24
, demodulates this information and couples the received information to the controller
14
. The controller
14
may pass the received information to one or more computers (not shown) via the data link
30
, causing the account associated with the vehicle to be appropriately decremented. As a result, the vehicle need not slow for the toll collection process, providing smoother traffic flow, reducing fuel consumption and generally facilitating both vehicular transportation and toll collection.
Similarly, an automated fuel retail system (not illustrated) may use the system
10
including the interrogator
13
. The interrogator
13
exchanges signals with the transponder
12
that is attached to vehicle (not illustrated) to determine whom to bill for fuel dispensed to that vehicle when the vehicle is fueled at the fuel dispensing station.
In these kinds of systems
10
, a range of the interrogation signal
20
is carefully limited to avoid interrogation of more than one transponding module
12
at a time. Additionally, it is often desirable to limit the amount of power that is required by the transponding module
12
in order to provide a stand-alone transponding module
12
that is able to operate without requiring an external source of power and that has long battery life.
Further, it is generally desirable to provide systems
10
that are as simple as possible. One approach to realizing these goals is to receive and transmit the interrogation
20
and response
24
signals in a common frequency band. In some systems, the response signals
24
are derived from the interrogation signals
20
and the transponding module
12
may even be one that simply modulates and re-transmits the received interrogation signal
20
to provide the response signal
24
.
One simple technique for modulating the interrogation signal
20
to form the response signal
24
is to either (i) forward bias a diode that is coupled across an antenna in the transponding module
12
, corresponding to a first logical state, or (ii) to leave the diode unbiased or reverse biased, corresponding to a second logical state. The transponding module
12
repeatedly switches between these two logical states according to predetermined patterns while the interrogation signal
20
is present to modulate the response signal
24
with a binary signal. These kinds of systems
10
are known as backscatter systems.
In some backscatter systems, the interrogation signal
20
includes a modulated preamble that carries data identifying which transponding module
12
in a population of such transponding modules
12
is targeted to respond. The preamble of the interrogation signal
20
further may include a request for specific data from the targeted transponding module
12
. In such backscatter systems, the interrogation
20
and reply
24
signals must necessarily use the same frequency band and both the interrogation
20
and response
24
signals are present at the same time.
Systems
10
that simultaneously or contemporaneously transmit interrogation signals
20
and receive response signals
24
having common frequencies using a common antenna or closely spaced transmit and receive antennas for the T/R antenna system
18
often include some sort of component for separating the transmitted interrogation
20
and received response
24
signals. These systems
10
must particularly ensure that the transmitted interrogation signal
20
does not feed directly into the receiver RX
28
.
For example, circulators (not illustrated) are employed in some types of systems where a single antenna is used for both transmission and reception of signals in a common frequency band. Circulators typically have three or more ports, and have the property that each port is strongly coupled to one of the two adjacent ports (referred to as forward coupling) but is not strongly coupled to the other of the two adjacent ports (referred to as reverse isolation).
However, circulators tend to be somewhat bulky, include a large permanent magnet and provide limited reverse isolation. For example, an antenna that is matched to provide a VSWR (voltage standing wave ratio) of 1.5:1 will provide a 20 dB return loss. When portions of transmitted signals are reflected from the antenna back into the circulator, an unacceptably large amount of RF energy may be coupled back into the receiver RX
28
from the transmitter TX
16
.
Another approach for reducing unwanted coupling between the transmitter TX
16
and the receiver RX
28
is to employ separate transmitter and receiver antennas (not illustrated) in the antenna system
18
. The receiver antenna is placed in or near a null in a radiation pattern associated with the transmitter antenna. Typically, the receiver antenna is placed somewhat behind the transmitter antenna.
One problem with this approach is that an object that reflects substantial portions of the interrogation signal
20
may pass through the radiation pattern of the transmitter antenna such that a large reflected interrogation signal
20
impinges on the receiver antenna. When such reflections occur, large undesired signals may be introduced into the receiver RX
28
, causing the received RX
28
to fail to respond to weaker but desired signals.
Similar kinds of systems
10
are presently of great interest for identifying, sorting,
Greeff Roy
Khatri Dirgha
Ovard David K.
Micro)n Technology, Inc.
Tran Chuc D.
Wells, St. John, Roberts Gregory & Matkin P.S.
Wong Don
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