A Compact, Low-Profile Simultaneous Transmit and Receive Antenna With Monopole-Like Radiation Characteristics

A compact, vertically polarized simultaneous transmit and receive (STAR) antenna with a high transmit–receive isolation level and an omnidirectional radiation pattern in the azimuth plane is presented. The proposed STAR antenna consists of transmit (TX) and receive (RX) antennas whose phase centers are collocated. The TX antenna is a monopole with top-hat loading, placed directly below the central point of the crossed loop. The RX antenna consists of a pair of orthogonal half-loops, each capacitively fed at both of its ends with a
$\text{180}^\circ$
phase difference. The four ports of the RX antenna are fed with the same amplitude and relative phase values of
$\text{0}^\circ$
,
$\text{90}^\circ$
,
$\text{180}^\circ$
, and
$\text{270}^\circ$
to produce an omnidirectional radiation pattern and high isolation with the TX antenna. The STAR antenna was designed for operation in the 2.45 GHz ISM band with electrical dimensions of
$D \times H=$

$0.44\lambda \times 0.15\lambda$
(diameter × height). A prototype of this antenna was fabricated and experimentally characterized. Measurement results show that the antenna has a TX/RX isolation level better than 38 dB and maintains monopole-like patterns for both TX and RX modes over a frequency range of 2.4–2.7 GHz.