Apparatus for direction finding of wireless signals
Provided herein is a compact and economical direction finding antenna using a mono-pulse antenna system, where a plurality of antenna elements are disposed in a circular array. The directional antennas may be formed by any type of antenna element, including a patch or reflector. The antenna beams of the directional antenna elements overlap, so that from any azimuthal direction, the point is covered by more than one antenna beam. Signals from each pair of adjacent antenna elements of the circular array are processed in order to determine the angle of arrival of a received signal.
1. A direction finding antenna system, comprising:
a plurality of antenna elements disposed in a circular array, wherein each antenna element has equal angular spacing about the circular array and is equidistance from the center of the array and receives a wireless signal originating from a mobile wireless device;
a plurality of signal splitters, one for each antenna element, wherein each said splitter splits an output signal from its respective antenna into two identical split signals;
a plurality of branch line couplers, wherein each branch line coupler receives first and second split signals from an adjacent pair of said antenna elements and generates a sum of said split signals and a difference of said split signals for said adjacent pair of said antenna elements, respectively;
signal samplers adapted to sample the output of said branch line couplers and generate a signal strength indication; and
a processor adapted to utilize the signal strength indication from said signal samplers to:
calculate a first ratio of the sum of said split signals and the difference of said split signals for a first pair of adjacent antenna elements and identify first and second potential angles of arrival for said wireless signal by comparing said first ratio with stored information for said first pair of adjacent antenna elements;
calculate a second ratio of the sum of said split signals and the difference of said split signals for a second pair of adjacent antenna elements, different that said first pair of adjacent antenna elements, and identify third and fourth potential angles of arrival for said wireless signal by comparing said second ratio with stored information for said second pair of adjacent antenna elements; and
determine an angle of arrival for said wireless signal by correlating one of said first and second potential angles of arrival with one of said third and fourth potential angles of arrival.
2. The antenna of claim 1 , wherein said first pair of adjacent antenna elements and said second pair of adjacent antenna elements have a common antenna element.
3. The antenna of claim 1 , wherein said signal samplers generate an RSSI value.
4. The antenna of claim 3 , wherein said signal samplers receive an RF signal and generate a digital output.
5. The antenna of claim 1 , wherein said stored information comprises at least one of:
a look up table specific to a configuration of said circular array;
a curve or equation specific to a configuration of said circular array.
6. The antenna of claim 1 , further comprising,
a first commutating switch interconnecting a plurality of said branch line couplers that sum said signals to generate sum signals; and
a second commutating switch connecting a plurality of branch line couplers that difference said signals to generate difference signals, wherein a single signal sampler samples all said sum signals for all adjacent antenna pairs and a single signal sampler samples all said difference signals for all adjacent antenna pairs.
7. The antenna of claim 1 , wherein said signal splitters, branch line couplers, signal samplers and processor are fabricated on a single circuit board.