Topband: GAP Vertical Question

[Tom W8JI]

> length. For want of a better word its image has to be a perfect conductor 
> for the antenna system as a whole to be 100% efficient.

The invocation of "image" into efficiency or ground losses shows a 
misunderstanding of antenna basics.

We were taught how to use antenna "images' in EM theory back in the 1970's, 
because computers were uncommon and there weren't any modeling programs. The 
antenna "image" is nothing but a tool to aid in pattern calculation. It 
isn't applicable to radial systems, ground losses in verticals, or anything 
similar.

The "image", as applied to antenna systems, represents the overall effect of 
re-radiation from the ground around an antenna. With a vertical, this area 
extends out for a considerable distance from the antenna. The amplitude and 
phase of a completely fictitious "image" is use to calculate field intensity 
at different points in space, just as if two antennas were being phased in a 
certain phase and current amplitude relationship.

A problem occurred, because people started to think the image was an actual 
"thing" that existed at one point or area in the earth, and that allowed 
them to create false ideas in their heads of what an image is. The image 
antenna is a fictitious tool for estimating patterns, nothing else.

73 Tom