Spreading-Element Doublet (SED) is an antenna based on a simple doublet but having other elements accompanying the main radiator. The accompanying elements are at some distance from the main radiator, and this distance is not constant along the doublet arms but grows larger when we approach the ends of the arms.
This antenna, apart for the main radiator, has 2 accompanying elements. It can be installed either in vertical plane or horizontal plane, the latter being more resistant to wind. The difference in gain due to antenna orientation is negligible.
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One element more and the SWR goes down and gain improves.
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Adding one more element does not improve antenna performance but makes the antenna narrower.
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SED does not have to be a flat antenna. See below the 5-element SED Mark 2 with accompanying wires arranged around the main central element. In case of this antenna, the cross-shaped structures at the ends of the antenna arms (shown in gray ink) can be either conductive or non-conductive with minimal impact on antenna performance.
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