|Image of the Sleeve monopole antenna.|
The standard monopole is one of the most commonly used antennas. In its simplest form, it consists of a quarter-wavelength wire driven with respect to a large ground plane by a coaxial feed line. This simple topology has the drawback of being fairly frequency-sensitive - something that can be curbed through the addition of a sleeve. Although adding the sleeve increases manufacturing complexity, the cylindrical shape is maintained, instead of changing to a conical or more three dimensional (larger volume) structure typical of most other wide bandwidth monopole variations. This might be useful when wind loading need to be considered. Another advantage of the Sleeve monopole is it's in-band pattern stability.
The image below shows an S11 comparison between the ...
standard- and sleeve monopole antenna. From this graph it is clear that a standard monopole is better matched over a narrow band where the Sleeve monopole is less efficient, but operates over much wider frequency band.
|S11 comparison between the standard- and sleeve monopole antennas.|