Examine your coordinate systems and where the antenna is pointing. Make sure you know which way the antenna is supposed to radiate. Sometimes the coordinate systems are not what we expect.

Is it an endfire antenna? Mostly, such antennas radiate along the axis of the antenna plane.

"Why is the radiation pattern not oriented to the 0 degree?" in other words it is also referred tilt or squint in the radiation pattern of given antenna. The reason for this, could be fallowing:

1. we might have designed antenna, but while realizing if there is some

phase imbalance due to manufacturing process in that condition it may

happen.

2. coordinate systems in your antenna range and assumed coordinate systems while designing the antenna are not matched. This also known as antenna coordinate systems mismatch or placement error.

3. If there are reflection in the test range and these reflected wave front entering in to antenna aperture.

4. If it is a printed antenna, in this case uniformity of material property over the area of substrate will also give the squint. This known as defect in homogeneity of substrate.

5. Impedance mismatch in interconnecting cables also attribute this problem.

6. Return loss of referred antenna is not up to mark.

Hello,

There are few reasons of getting such radiation pattern such as coordinate system has not defined well, losses, impedance mismatching and it is a endfire array antenna, as you have not mentioned about type of antenna.

Thanks,