An “umbrella antenna” is basically just a vertical monopole antenna that is top-loaded with a large “end-hat”. The hat consists of a number of radial wires that slope away from the top of the antenna. These spoke-wires resemble the ribs of an umbrella, hence the name. The beneficial effect is significantly increased (2x) by connecting the bottom of the sloping wires with a perimeter “skirt” wire. A properly dimensioned hat will make the current distribution along the vertical radiator more uniform. That is, it will taper off much less towards the tip of the radiator. Ref. 6. This increases the radiation resistance of the antenna, and also the bandwidth – both highly desirable. ….more…