Antenna sampling system

Each of a plurality of antenna sectors is sequentially coupled to a receiver via an antenna switch. The switch is operated by control logic circuitry which monitors the peak signal levels of the signals received by each sector. Unique circuitry is implemented which prevents the control logic from responding to spurious transient signals processed by the receiver's tuned circuits following each instance of antenna switching.

BACKGROUND OF THE INVENTION 
The present invention pertains to the radio communication art and, more 
particularly, to a multisectored antenna receiving system. 
Several types of sectored antenna receiving systems have been developed for 
application in the radio communication art. Sectored antenna systems are 
commonly used in applications wherein the remote transmitter whose signal 
is to be received may be positioned in any one of multiple locations. 
Thus, such systems are normally designed in moving vehicle type 
applications. By using a sectored antenna array, as opposed to, for 
example, an omnidirectional antenna, signal to noise performance can be 
significantly enhanced thus producing a superior communication system. 
While numerous sectored antenna scanning systems are known in the prior 
art, a particularly effective one is that described in U.S. Pat. No. 
4,101,836, issued July 18, 1978, entitled "Sectored Antenna Receiving 
System", invented by Timothy Craig and James Stimple, and assigned to the 
same assignee as the instant application. 
In the above referenced antenna scanning system control logic causes an 
antenna switch to sequentially couple each of a plurality of antenna 
sectors to a receiver. The signal level produced by each antenna is 
detected in appropriate circuitry and, if the detected signal levels for 
the one or more sectors exceeds a threshold level the control logic causes 
the antenna switch to scan the sectors in various predetermined modes. 
A problem with such prior art scanning systems, and notably of the system 
referenced above, is that at the instance of antenna switching energy from 
off channel signals is translated to the desired channel and processed by 
the receiver's tuned circuits, thereby creating a transient, or spurious 
response. If allowed to pass to the control logic, this spurious response 
will be treated as a sector received signal. Thus, the system is subject 
to falsing. 
One solution to the tuned stage generated transient signal problem is to 
reduce antenna switching times to the point whereby the generated 
transients are of insignificant magnitude. This solution has proven 
undesirable in applications wherein rapid antenna scanning is required. 
SUMMARY OF THE INVENTION 
It is an object of this invention, therefore, to provide a rapid scan 
antenna system wherein the system is immune to transients generated by the 
switching operation. 
Briefly, according to the invention, the radio frequency communication 
system comprises an antenna having a plurality of sectors, each sector 
being predeterminedly oriented with respect to the remaining sectors. An 
antenna switch has a plurality of inputs, each of which is coupled to a 
corresponding antenna sector, a control terminal and an output terminal. 
The antenna switch responds to signals at its control terminal to couple a 
selected input terminal to its output terminal. The output from the 
antenna switch couples to a radio frequency receiver. A detector processes 
the output from the receiver to detect the level of signals thereat. 
Control logic circuitry responds to the detector by generating and 
applying predetermined control signals to the antenna switch thereby 
causing predetermined switching of the antenna sectors in response 
thereto. An inhibit means inhibits operation of the detector for a 
predetermined time interval following each instant of antenna switching 
whereby the control logic does not respond to spurious responses caused by 
the antenna switching.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 illustrates a preferred application of the invention used as a 
communication system for land mobile operation. Here, a sectored antenna 
array 10 is shown mounted atop a building 12. Communication from the array 
is to be established with a remotely located transmitter, which is 
illustrated as being carried by a vehicle 14. 
The array 10 includes eight sectors, one of which is indicated at 16, which 
are nestled between 45.degree. angle reflectors, two of which are 
indicated at 20 and 22. Each sector is horizontally directive and is 
predeterminedly oriented with respect to the remaining sectors such that 
all sectors in the array are capable of covering a total horizontal angle 
of 360.degree.. It should be understood that, depending upon the 
particular application, any number of sectors may be employed and the 
sectors may be arranged to cover any desired receiving angle. 
Horizontally directive antenna sectors are well known in the antenna art 
and are commercially available from many manufacturers such that a 
detailed description thereof is unnecessary. 
FIG. 2 is a top view of the antenna array 10 showing the eight antenna 
elements, one of which is indicated at 16, and the 45.degree. reflectors, 
two being shown at 20 and 22. As a result of the design of the antenna 
sector and its corresponding reflectors, each antenna sector is capable of 
receiving transmitted signals which are generated from within a defined 
horizontal angle. It should be understood that since each sector of the 
antenna array is horizontally directive, the array is capable of superior 
signal to noise performance over an omnidirectional antenna. 
FIG. 3 is a generalized block diagram which illustrates the fundamental 
building blocks of the scanning system. Here, the array of sectored 
antenna elements 30 are mounted, as for example, in FIG. 1, in 
predetermined orientation on top of a suitable structure. Each antenna 
sector feeds to a filter and preamplifier circuit 40 which provides gain 
to the sector received signals at those frequencies of interest. The 
amplified sectored received signals pass over coaxial cables 50 to an 
antenna switch 60. The antenna switch 60 comprises a plurality of input 
terminals 60a, 60b, 60c, 60d, 60e, 60f, 60g, and 60h, an output terminal 
60j and a control input terminal 60i. In response to control signals 
received at its control input terminal 60f, the antenna switch 60 couples 
a selected one of the sectors 30 to the antenna switch output terminals 
60e. 
The output 60e from the antenna switch 60 is coupled via a suitable 
conductor 65 to the input 70a of a radio frequency receiver 70. Radio 
frequency receiver 70 is of conventional design in that it mixes the input 
radio frequency signals to an intermediate frequency, thereafter filtering 
the signal via an intermediate frequency stage, detecting the signals and 
reproducing the signals as audio or data outputs. The prelimited output 
from the intermediate frequency stage is available at a receiver output 
terminal 70b. The receiver includes conventional squelch circuitry such 
that when an appropriate signal is applied at a squelch input terminal 70c 
the output from the receiver may be disabled. In addition, the receiver 
includes decoding circuitry capable of detecting the presence of a coded 
signal on an incoming sector receiver signal. Such coding systems are very 
common in land mobile communication systems and normally would include 
information as to the identity of the transmitting station as well as 
other pertinent information of use to the system. Commonly, such coded 
signals are predetermined subaudible or audible tones which may be decoded 
by the presence of suitable bandpass filters, vibrating reeds or the like. 
A second output 70d from the receiver is activated in response to the 
receiver decoding the presence of a coded signal on the sector receive 
signal. 
The intermediate frequency output 70b from the receiver is coupled to a 
signal strength detector 80 which, as is shown in FIG. 3, may be a 
separate unit, or it may be incorporated within the receiver 70. The 
signal strength detector 80 produces a signal at its output 80a, which 
signal is indicative of the level of signals received at its input 80b. 
The signal strength detector 80 is provided with a second input 80c which 
responds to inhibit signals applied thereat to inhibit an output from the 
signal strength detector. The signal strength signals are in turn passed 
to the first input 90a of a scan control logic circuitry 90. The second 
input 90b of the scan control logic 90 couples to the coded signal output 
70d of the receiver 70. The scan control logic circuitry 90 processes its 
input signals and produces appropriate control signals on its first output 
90c which are in turn coupled to the control input terminal 60i of the 
antenna switch 60. Also, appropriate signals are created at the scan 
control logic second output 90 d which are coupled to the squelch input 
70c of the receiver 70. Detailed operation of the scan control logic 90 is 
fully described in the copending application of Craig and Stimple, cited 
supra. Briefly, the scan control logic 90 responds to its received input 
signals to operate the sectored receiving system in one of a number of 
modes. For the condition wherein none of the sectors receives an input 
signal above a predetermined level, a control signal from the scan control 
logic applied to the control input terminal 60i of the antenna switch 60 
causes the receiver to be sequentially coupled to each of the antenna 
sectors. Once a sector receives a signal whose amplitude is such that the 
output from the signal strength detector is above the given threshold 
level, the scan control logic operates the system in a second, or 
continuing scan mode. In this mode each antenna sector is again scanned 
for a predetermined continuing scan time, with the relative signal 
strength from each sector being stored in a first sample and hold array 
within the scan control logic 90. At the end of the continued sampling 
period that sector which recorded the largest signal during both scanning 
intervals is coupled to the receiver. If the system is not operating in 
the coded signal mode, the receiver will remain coupled to that sector 
until such time as the signal therefrom drops to a predetermined level for 
a predetermined time period. 
If, however, the system is operating in a mode whereby it only responds to 
the input signals which contain a coded signal, at the end of the 
continuing sampling period the receiver will be coupled to that sector 
which receives the strongest signal and which contains the coded signal. 
In this mode if, by the conclusion of the continuing signal mode, none of 
the sectors has received the coded signal, thus indicating that an 
interferer is present, the scan control logic operates the system in an 
alternate scan mode wherein a general scan of the sectors is made as in 
the first mode but the output from each sector is applied to a second 
sample hold array. Now, each input from a sector to the second sample hold 
array is compared with the stored peak value for the sector and the first 
sample hold array caused by the interferer. If The signal strength from 
any sector in the second sample hold array is a predetermined voltage 
.DELTA.V above its stored value in the first sample hold array the scan 
control logic once more activates the system to the continuing scan mode. 
As before, in the continuing scan mode all sectors are scanned for a 
predetermined time interval, with the peak value signal level from each 
sector being stored now in the second sample hold array. At the end of the 
continuing scan interval the scan control logic couples the receiver to 
that sector which received the largest signal and has the greatest 
.DELTA.V. As before, the receiver then checks to determine whether or not 
the signal received by the selected sector contains the proper coded 
signal. If it does, the receiver continues to be coupled to that sector 
until the signal from that sector diminishes to a given level for a 
predetermined time period. If that signal does not contain the coded 
signal the scan control logic causes the system to revert to the first, or 
general scan mode. 
Since the scan control logic 90 causes activation of the antenna switch 60 
and responds to the peak signal level from the signal strength detector 80 
corresponding to each antenna sector, it is imperative that the signal 
strength levels from the detector 80 properly correspond to actual signal 
levels received by the sectors, and not be due to any spurious responses 
or transients generated within the antenna switching system. It has been 
found that following the instance of antenna switching, a spurious 
response is received at and processed by the tuned stages, such as the 
intermediate frequency filter and amplifier, of the receiver 70 thereby 
producing an undesirable transient response. This transient response is 
illustrated in FIG. 5 wherein the time t.sub.o indicates the instance of 
antenna switching. The transient occurs for a total time period of 
t.sub.f. If this transient is allowed to pass to the signal strength 
detector 80 and then to the sample hold array at the input 90a of the scan 
control logic 90, the signal stored in the array may correspond to the 
amplitude of the transient and not to the actual received signal levels. 
Thus, the control logic might respond to erroneous input information and 
activate to any one of its previously described modes. 
It should be noted that in some applications undesired transients arise 
solely from switching transients induced in high Q stages such as the IF 
filter and amplifier. The instant invention would likewise correct for 
these potential error signals. 
To prevent system falsing, an inhibit circuit 100 couples to the output 90c 
of the scan control logic 90 and senses the instance of antenna switching 
time. Included within the inhibit circuit 100 is appropriate circuitry 
which responds to the instance of antenna switching to produce an inhibit 
signal at the inhibit input 80c of the signal strength detector 80. This 
inhibit signal is of appropriate duration, preferably of length (t.sub.f 
-t.sub.o) or greater, whereby the spurious responses from the receiver due 
to antenna switching do not reach the control logic 90. 
FIG. 4 is a more complete diagram illustrating operation of the inhibit 
circuit 100. Here the antenna sector array 30 is coupled via an antenna 
switch 60 to the receiver 70 at its input 70a. The receiver includes an RF 
amplifier 200, a conventional mixer stage 210 and an intermediate 
frequency filter and amplifier 220. The output from the intermediate 
frequency stage 220 is limited and detected in the typical manner via a 
limiter and detector 230 and passed via an audio amplifier 240 to a 
speaker 250. The output from the intermediate frequency stage 220 is also 
routed via an output 70b to the input 80b of the signal strength detector 
80. In series with the signal strength detector 80 is a shunt switch 260 
which couples the detector outputs 80a to the input 90a of the scan 
control logic 90. Provided in the scan control logic 90 is a low pass 
filter 270 which couples the signal strength level signals from the 
detector 80 to a sample and hold array 280. As described above, also 
provided within the scan control logic 90 is switching logic 290 which 
responds to a condition of peak signal levels in the sample and hold array 
to produce appropriate control signals at the scan control logic output 
90c thereby used to activate the antenna switch 60 in a desired mode. Also 
coupled to the output 90c of the switching logic 290 is the input 300a of 
a monostable multivibrator 300. The monostable responds to transitions at 
its input 300a to produce an output pulse of predetermined time duration 
at its output 300b. This output pulse from the monostable 300 couples to 
the control input 260a of the shunt switch 260. Thus, for the interval of 
the monostable produced pulse, the shunt switch shunts the output 80a from 
the detector 80 such that these signals are prevented from passing to the 
scan control logic 90. 
FIG. 5 illustrates the spurious, or transient noise burst created by the 
tuning circuits in the receiver, especially the intermediate frequency 
stage 220 upon the instance of antenna switching. The time t.sub.o 
represents exact instance of antenna switching, and the total time 
duration (t.sub.f -t.sub.o) is the approximate length of the produced 
transient response. It is important that these transients be prevented 
from entering the scan control logic 90 since, otherwise, the scan control 
logic will act upon these signals as if they were received on channel 
signals from the antenna sectors. Thus, the monostable multivibrator 300 
is designed to produce an output pulse of approximate length equal to 
(t.sub.f -t.sub.o) whereby at the instance of antenna switching the 
monostable created timed pulse activates the shunt switch 60 thereby 
preventing the output from the detector from entering the scan control 
logic 90. 
It should be understood that the time duration of the pulse created by the 
monostable 300 may be adjusted to eliminate any transients caused by the 
system during the instance of antenna switching. By utilizing the inhibit 
circuit 100, the instant invention is capable of flawless scan control 
operation, while permitting very high antenna sampling rates. 
While a preferred embodiment of the invention has been described in detail 
it should be understood that many modifications and variations thereto are 
possible, all of which fall within the true spirit and scope of the 
invention.