A portable 24-GHz FMCW radar that can steer its receive beam anywhere within ±45°. It was the first short-range localization radar in K-band to do this beamforming entirely on a printed circuit board. Instead of phase-shifter chips or bulky modules, each antenna element's phase and amplitude are set by a vector controller made from simple microwave structures and PIN diodes.
Building a K-band system on a PCB brings errors from component variation, mismatch, soldering, and fabrication, so I also developed a calibration method and showed in experiments that it works. Because the vector controllers set amplitude as well as phase, the same hardware can form more complex beam patterns too.
- Frequency sweep
- 23.5–24 GHz FMCW
- Receive array
- 4 elements, each a series-fed patch array
- Beam steering
- Continuous, ±45° in the H-plane
- Size and weight
- 11.5 × 3.9 cm, 69 g
- Power
- A single 9-V battery
System Design
Beamforming with PIN diodes and microstrip
Transmitter
A VCO feeds a single patch antenna. A sawtooth voltage from an op-amp circuit tunes the VCO, sweeping its output from 23.5 to 24 GHz.
Receiver
The receive side is a four-element linear array, where each element is itself a series-fed microstrip patch array. Each element feeds a vector controller, which works by adding signal vectors to set the phase and amplitude of that element. Steering the beam then comes down to choosing the four settings, and the beam can point anywhere within ±45° in the H-plane.
Earlier vector-sum designs relied on integrated circuits or bulky modules. Here the whole controller is built from microwave structures and PIN diodes on the PCB. A six-port circuit detects the FMCW beat signal, and a laptop sound card samples it.
Hardware
Bill of materials
The whole radar is built on PCBs (Fig. 1): the RF board on Rogers RO3006 and the baseband board on FR4. The main parts are listed below.
| Block | Function | Part |
|---|---|---|
| Beamforming array | LNA | MACOM MAAL-011111 |
| PIN diode | Skyworks SMP1302 | |
| FMCW transceiver | VCO | Analog Devices HMC739LP4 |
| Gain block | MACOM MAAL-011111 | |
| Schottky diode | Skyworks SMS7621-040LF | |
| Baseband | Op amp | Analog Devices ADA4851 |
| DAC | Analog Devices AD5668 |
Experiments
Steering, mapping, finding people
Beam patterns in the anechoic chamber
I measured the far-field pattern of the array in an anechoic chamber (Fig. 3) while the vector controllers stepped the beam from −45° to 45° in 15° steps. Fig. 4 shows the measured patterns at each angle.
Mapping a scene outdoors
In the field test (Fig. 5), a car, a lamppost, and a person stood in front of the radar, which ran from a single 9-V battery. The radar swept its beam from −45° to 45° in 2.5° steps, taking an FMCW measurement at each angle.
The resulting map (Fig. 6) clearly shows all three targets. The person's signature, however, flickers from scan to scan, because breathing and small body movements keep changing the echo. This is the vital-Doppler effect.
Telling people apart from objects
That flicker is useful. I ran 10 scans in a row and took the standard deviation of the range spectrum at each point. The car and the lamppost do not change, so they drop out. The person changes all the time, so they remain (Fig. 7).
The result is human-aware localization: the radar can tell where people are, even when they stand still among other objects.
Publication
Where this work appeared
- A κ-band portable FMCW radar with beamforming array for short-range localization and vital-Doppler targets discriminationZ. Peng, L. Ran, and C. LiJournalIEEE Transactions on Microwave Theory and Techniques, vol. 65, no. 9, pp. 3443–3452, Sep. 2017