Antenna switch

The antenna of a transceiver couples through a low pass filter to the input port of a high pass filter network. The high pass network is designed to reflect a very high impedance to its input port when its output port is at ground potential. The transmitter portion of the transceiver couples to the input port of the network with the receiver coupling to the network output port. A semiconductor diode couples from the output port to ground potential. In the receive mode the combination low pass and high pass filters couple received signals in the desired frequency band to the input of the receiver. Upon activation to the transmit mode the diode is forward biased whereby the network output port is taken to ground potential. In this mode the filter network effectively isolates the receiver from the transmitter.

BACKGROUND OF THE INVENTION 
The present invention pertains to the communication art and, in particular, 
to a means for switching an antenna to either a transmitter or a receiver. 
Antenna switching systems are well known in the communication art. Such 
systems are commonly used in transceivers wherein a transmitter and 
receiver share a common antenna. The antenna switch must not only provide 
a means for routing the antenna to either the transmitter or the receiver, 
but it must also isolate the transmitter from the receiver such that, for 
example, in the transmit mode the receiver is not overloaded by the 
transmitted signal. 
One prior art approach to antenna switching is the use of an 
electromechanical switch or relay. Such relays are not only expensive, but 
are also bulky and, thus, not ideally suited for limited space 
applications such as mobile transceivers. A further approach has been the 
use of PIN diodes in a biasing scheme whereby the diodes either connect or 
disconnect the receiver or transmitter to the antenna. The PIN diode 
switching schemes are expensive due to the cost of the diodes, and are 
generally complex to implement due to the sophisticated biasing scheme 
needed to properly control the diodes. 
SUMMARY OF THE INVENTION 
It is an object of this invention, therefore, to provide a means for 
effectively switching an antenna between a receiver and transmitter, which 
means also provides a simple yet effective way to isolate the receiver 
from the transmitter when the system is in the transmit mode. 
Briefly, according to the invention, the antenna switch is operable in 
either a transmit or a receive mode, for switching an antenna to either a 
transmitter or a receiver, respectively. Both the transmitter and receiver 
are operable in a selected frequency band. The switch comprises a filter 
having an input and an output. The filter input couples to the antenna, 
with the filter output coupling to the receiver. The filter is designed to 
normally pass signals from the antenna to the receiver. Preferably, the 
filter has a high pass characteristic whereby signals below the desired 
frequency band are filtered out. However, when the output of the filter is 
taken near ground potential, the filter forms an antiresonant circuit 
having a very high impedance, in the frequency band, at its input. The 
transmitter couples to the filter input and a switching means is provided 
which grounds the filter output when the antenna switch is in the transmit 
mode. Thus, the receive mode signals are normally passed through the 
antenna to the receiver. However, upon activation to the transmit mode, 
the output of the filter is taken near ground whereby the input impedance 
to the filter becomes extremely high. This effectively decouples the 
receiver from the transmitter such that the transmitter energy is routed 
out over the antenna.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
Referring to the drawing, an antenna 12 couples to a low pass filter 14. 
The low pass filter 14 is comprised of a sequence of shunt capacitors 18, 
20, 22 in a pi circuit configuration with corresponding series inductors 
24 and 26. The values of the capacitors 18, 20 and 22, and the inductors 
24, 26, are selected in a well known manner to create a low pass 
characteristic for passing signals within and below a desired frequency 
band. 
The output of the low pass filter 14 couples to the input port 30 of a high 
pass filter network 32. High pass filter network 32 is a "T" type filter 
having a pair of series capacitors 34, 36 with a shunt inductor 38 
therebetween. The capacitors 34, 36 and the inductor 38 are of selected 
values to pass signals in the desired frequency band, and those signals 
thereabove from the input port 30 to an output port 40. In addition, when 
the output port 40 is taken near ground potential, the capacitor 36 and 
inductor 38 form an antiresonant tuned circuit, in the desired frequency 
band, which presents a high impedance through capacitor 34 to the input 
port 30 of the filter network 32. 
The output port 40 of high pass filter network 32 couples to the input 
terminal 42 of a receiver 44. Also coupled to the output port 40 of filter 
network 32 is a switching means 46 which is comprised of a series 
connected diode 48 whose anode connects to output port 40 and whose 
cathode connects through an RF bypass capacitor 50 to ground potential. A 
bias is normally applied to diode 48 via a resistive divider consisting of 
a pair of resistors 52, 54 connected between a source of DC bias B+ and 
ground potential. An RF isolating inductor 56 connects between the common 
point of the resistors 52, and 54 and the anode of diode 48. 
Coupling from the cathode of diode 48 is a series resistor divider 
comprised of a first resistor 56 and a second resistor 58 which couples to 
the aforementioned source of DC bias B+. The common point of the resistor 
56, 58 connects to a manually operable switch 60 which also connects to 
ground potential. Switch 60 is, in the preferred construction of the 
invention, a push-to-talk switch normally found in combination with a 
hand-held microphone. 
A transmitter 62 drives an output power stage comprised of transistor 64, 
whose collector connects both through an inductor load 66 to a bias 
potential 68 and through acoupling capacitor 70 to the input port 30 of 
the high pass filter network 32. 
Circuit operation may be understood as follows. In the receive mode, the 
push-to-talk switch 60 is open (as shown) such that semiconductor diode 48 
is biased to an off state, i.e. to a high impedance state. Thus signals 
received by antenna 12 are first low pass filtered by filter 14, high pass 
filtered by filter 32, and thereafter applied to the input terminal 42 of 
the receiver 44. The low pass filter 14 and high pass filter 32 provide 
bandpass protection against spurious signals located outside of the 
desired frequency band. Also, the component values of the low pass filter 
14 and high pass filter 32 are selected whereby the impedance of the 
antenna is transformed to a proper value at the receiver input. 
On activation to the transmit mode, push-to-talk switch 60 is suppressed 
whereby semiconductor diode 48 is forward biased to a low impedance state. 
Thus, the output port 40 of high pass filter 32 is coupled through 
capacitor 50 at or near the low impedance ground potential. This causes 
the capacitor 36 to resonate with inductor 38 whereby a very high 
impedance is reflected to the input port 30 of high pass filter 32. Now 
the transmitter, driving the output device 64, feeds directly through the 
pass filter 14 and out over the antenna 12. Due to the very high input 
impedance seen by the transmitter at input port 30 of high pass filter 32, 
very little power is transmitted through filter 32 and to the receiver 44. 
Thus, the receiver is seen to be effectively isolated from the transmitter 
62 when the system operates in the transmit mode. 
In summary, a simple yet effective switching system has been disclosed, 
which system effectively isolates the receiver from the transmitter when 
operating in the transmit mode. 
While the preferred embodiment of the invention has been described in 
detail, it should become apparent to anyone of ordinary skill in the art, 
that many modifications and variations thereto are possible, all of which 
fall within the true spirit and scope of the invention.