# Determining the correct sidelobe envelope

I need to determine the correct sidelobe envelope for various VSAT antennas. Given the radiation pattern (this one, for example http://www.gdsatcom.com/vsat/technical_docs/patterns/6000-023.pdf), how can I determine whether it is 29-25 log theta or 32-25 log theta?

It is my understanding that it depends on the off-axis angle (theta), but I don't know how it can be calculated. I think I've also seen somewhere that it depends on the azimuth and elevation, but I don't see how exactly they influence the off-axis angle.

Looking forward to your answers!

## 1 Answer

I haven't seen a description like that before, but it looks to me like it's just giving you a piecewise function for the envelope — that is, first you find the range in which the off-axis angle is in, then you compute the value using the formula for that range.

For example, you would use the formula $29 - 25 \log \theta$ whenever $\theta$ is between 26.3 and 48 degrees.

In these plots, they would not be using “azimuth” and “elevation” in the sense of the absolute orientation of the antenna, but rather to mean the off-axis angle in a specific direction (horizontal or vertical, respectively).

• Can you please elaborate on how to "find the range in which the off-axis angle is" ? Please note that I do not have a physical station per se, I only have the technical details. – gsimonen Sep 24 '15 at 6:28
• @gsimonen It gives you some non-overlapping numeric ranges. The off-axis angle, being a single number, will fall in one of those ranges. That's all. Do you mean to ask what is the off-axis angle? – Kevin Reid AG6YO Sep 24 '15 at 13:55