# Ultra-Wideband Antenna Design

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```					Ultra-Wideband Antenna Design

Stanley Wang
Prof. Robert W. Brodersen
January 8, 2002
Antenna in Communication Systems

• At Transmitter
– Antenna is modeled as a circuit component; real
part in it determines the radiated power (for =)
– Current in the antenna determines Erad
– E-field at the Rx is translated to a voltage source
– By reciprocity theorem, Zant,rx=Zant,tx
Frequency Response: s11 of Monopole
• The smaller the s11, the larger the radiation
• Resonant at f=c/(0.5), which leads to freq. hump
• Two ways to avoid ringing & flatten the freq. response
– Make the conductive wire more resistive
– Shorten the dipole

s11 for 6cm monopole           s11 for 2cm monopole
Far-zone Electric Fields of the Monopole

6cm monopole                    2cm monopole

• When L is much smaller than /2, no ringing happens
• Radiated energy is decreased, but it’s ok
– Undetectable UWB system transmits at noise level
Short Monopole: Input V/I Characteristics

• The input V/I behavior resembles that as driving a capacitor
– Radiation is too small; energy stored in near-field
• Modeling a 2cm monopole by a 0.315pF capacitor
• Given the same Vs & Rs, Is in two cases overlap perfectly
• The radiated field is the time-derivative of the INPUT current
– The dimension is small. Phase difference between I(z) at
each part is ignorableQuasi-static condition
Experiment on Short Antenna
• Planar Square Antenna (2cm x 2cm)
• Amplifier at the receiver to reduce the quantization noise
• Use AVERAGE function in the scope to enhance SNR

Pulse
Digital                                          Generator
Sampling
Oscilloscope

Courtesy to Enyi Lin
Experiment on Short Antenna - Results

• Blue line: no objects located
between Tx & Rx
• Red line: a table put in the
middle between Tx & Rx
– An extra transmission path
• Two lines overlap most of the
time, even after the extra path
has arrived Rx
– The pulse width of the        Extra path arrives
• Short antennas get rid of ringing
Driver Circuit Schematic
• Inverter chain sharpens the edge of the input signal
• Pre-driver NAND/NOR circuits skew the signals so
that the driver NMOS & PMOS won’t be on at the
same time
• Bond-wire inductance might introduce ringing
Antenna as
Input Signal
a capacitor

Driver Enable

Section Enable
Driver Circuit Layout
• STMicroelectronics 0.13um CMOS process
• Chip area: 0.49mm2
• 1.2V Vdd
• 2 drivers with
enables  Can                          Driver
either drive a
monopole or dipole
• Each driver with 16
levels of driving
capabilities

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