COMPACT ULTRA-WIDEBAND CARDIOID VIVALDI RADIATOR WITH RECTANGULAR IMPEDANCE INSERTS

Abstract

The paper presents the design of the cardioid-shaped Vivaldi radiator with rectangular impedance
inserts along the edges of its metallization. The influence of impedance inserts, their location, and parameters
on the characteristics of the radiator is investigated. The frequency characteristics of the voltage
standing wave ratio (VSWR), realized gain, efficiency, and cross-polarization level of the radiator without
and with impedance inserts are given. The developed radiator is electrically compact (electrical height at
the upper operating frequency is equal to 0.740 λ, and at the lower operating frequency is equal to
0.127 λ) and ultra-wideband with an overlap ratio (OR) of 5.809:1 in the operating frequency band
127.3–739.5 MHz. The width of the impedance inserts varied from 5.0 mm to 25.5 mm towards the tapered
slot. At the same time, an increase width leads to a slight expansion of the operating frequency band and
an increase of OR. But the realized gain practically does not change since the radiator is weakly directional
and its realized gain depends mainly on the size of the aperture. The numerical values of efficiency
and cross-polarization characteristics also remained virtually unchanged with increasing insert width.
The optimal width of the impedance inserts is equal to 25.5 mm. The height of the impedance inserts was
measured from the top of the radiator. The influence of impedance inserts with a height of 60, 100, 140,
145, and 160 mm is considered. It has been determined that as their height increases, the width of the
operating band increases, but the average VSWR level in the frequency band 180–280 MHz gradually
increases. The realized gain, efficiency, and cross-polarization level also remain virtually unchanged with
the increasing height of the inserts. The optimal height of the impedance inserts is equal to 25.5 mm. Thus,
the introduction of impedance inserts makes it possible to expand the operating frequency band of the
radiator.