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TX Array design, tuning and testing

1. array design: very flexible, really use any number of antennas to achieve optimum beam and null, no single answer: single antena, dual antenna lambda/4, sqaure 4-antenna array, rectangle 4-anntenna array, all explained in AARL book chapter 8.
2. tune antennas independently
3. check TX Array tuning

Transmit from RX antenna closest to TX site, and put 4-channel scope on TX antennas with 50 Ohm input

1. Amplitude

Amplitude of the 4 antennas must be equal for proper phasing to occur

1. insert small power attenuators in 0.5 and 1 dB increments (to computer pwer of attenuator, compute power loss between input and output of attenuator given the power from the PA)
2. design splitter board with pads for power resistors, and compute exact resistors needed to achieve required attenuation for each antenna using on-line calculator.
3. can also adjust height of helical vertical if no attenuator available

1. Phase
   1. measure phase difference and convert to cable length using VOP or velocity factor (.67 for solid PE, 0.695 for solid PTFE, and .85 for foam cable)
   2. insert correction phasing cables for each antenna after the 4-way splitter.

Future: could design a large board, with multiple pads (sometimes make one resistance value from a few in parallel or series)) and SMA connectors to plug in phasing cables made from thin RG316).

1. Verify TX performance and direct path rejection.
   1. Using mobile field strength meter

Transmit from TX antennas all connected the final way, and walk a circle centered on TX with a magnetic antenna and a spectrum analyzer. At each point of the circle (every 5 deg), measure with loop perpendicular to direction of TX and find minimum. then rotate 90deg and find maximum. This is the measurement. Plot in polar coordinates, overlaid to Google Earth satellite image, verify null towards RX array.

1. Use mobile signal generator.

Connect TX cable with antennas all connected the final way, to spectrum analyzer inside the container, or to a receiver channel of the radar, and measure amplitude of the received signal). This is the measurement. Plot in polar coordinates, overlaid to Google Earth satellite image, verify null towards RX array.

1. Compare single to combined antennas.

Connect RX antenna 1 to spectrum analyzer (if passive) or to receiver channel, and measure amplitude of the received signal in the following configuration

This shows you the efficiency of using 4 antenna TX beamforming to reduce direct path.

If front to back rejection is of concern, you could do same measurement with a transmitter on land 100m in the back of the TX array.