Tuesday, July 19, 2016

When a beam antenna such as a Yagi or quad produces ...

Q George, VE3LTU, asks, “When a beam antenna such as a Yagi or quad produces a major radiation lobe at, say, 35º, would the radiation angle be improved by tilting the boom 20º toward the Earth? Would the radiation angle with respect to the Earth now be 15º, resulting in improved DX performance?”

A The short answer is “no.” An array as you describe has its major lobe aimed directly along the boom (0º elevation) when it is in free space. When ground is considered, it increases the elevation of the lobe. Therefore, it is the relation of the antenna to ground (height)
that determines take-off angle.

We can look at it another way. The radiation pattern of an antenna is a summation of the radiation each of many antenna segments as they interact with each other and the ground below
the antenna.

We can visualize the result by imagining a mirror image of the antenna below the ground surface by a depth equal to the antenna’s height above the ground. Thus, as we tip the antenna boom downward, the imaginary boom tips upward to counter it—we achieve nothing.

From QST June 2001