The invention relates to a compact ultra-
wideband omnidirectional antenna comprising a first 
metal layer, a second 
metal layer and a 
dielectric substrate. The first 
metal layer and the second metal layer are printed on the upper layer and the lower layer of the 
dielectric substrate respectively, and each metal layer is composed of a double-
branch flag-shaped radiator, double parasitic bands, a curved groove, a short open-circuit 
branch knot and a center metal disc. A short 
branch is loaded on a conical feed structure, the 
reactance characteristic of the antenna at 
high frequency is effectively suppressed, a high-frequency 
resonance point is introduced, and when double parasitic bands are added, a new 
resonance point is added between two existing 
resonance points, so that the bandwidth is improved, the 1.8-3.85 GHz 
passband characteristic is formed. A short open-circuit branch knot is added to the 
tail end of the first branch knot, a curve-shaped groove is etched, 
capacitive coupling and a current path are increased, and the low-frequency 
cut-off frequency is reduced. Finally, the 
impedance bandwidth of the antenna can reach 78% (1.69-3.85 GHz), in the 
impedance bandwidth, the 
cross polarization level on the horizontal plane is smaller than-20 dB, and the efficiency in the working 
frequency band is 80%-91%.