Hi, I would like to understand how to select the distance D in the array factor formula for two antennas, given by AF=2⋅cos⁡(k⋅D⋅cos⁡(a)). Here, D represents the distance between the two antennas.

In cases where we are dealing with large antennas, whose sizes cannot be ignored (unlike simple dipole antennas), how should we define D for the array factor?

For instance, if the sizes of the two antennas are approximately ten times λ0, should we consider D as the distance from the center of one antenna to the center of the other (for example, 40⋅λ0), or should it be measured from the edges of each antenna?

If I consider D as the distance from the center of one antenna to the center of the other, they are approximately isolated from each other and their patterns are seperate. However, the plane wave could be a combination of the two patterns, making the array factor somewhat meaningless. Am I correct in this assessment?

As you can see, the array factor for the distance between the two antennas is completely different.