A grating lobe or grating beam is a beam identical to the main beam but in undesired direction.
The power radiated by the array gets divided between the main beam and the grating lobe. The power efficiency in the direction of the main beam is consequently reduced. A grating lobe should be avoided in the radiation pattern.
A grating lobe appears when
Where
is an integer.
For broadside array the grating lobe appears when , and for the end-fire array it appears when . We can conclude that to avoid grating lobe in the radiation pattern for any array the inter-element spacing should be .
Effect of Antenna radiation pattern on the array
The radiation pattern of the antenna elements used in the array is called the ‘primary pattern'.
If the array consists of non-isotropic but identical elements, the effect of the primary pattern can be accounted for very easily.
Since the radiation due to every element is weighted by the primary pattern, the total radiation pattern of an array is the product of the primary pattern and the AF.
Radiation pattern = Primary pattern X Array Factor
While analyzing the array of non-isotropic but identical elements therefore first find the AF assuming the elements to be isotropic and then multiply the AF with the primary pattern to get the total radiation pattern.
Using this concept the arrays can be analyzed by dividing the array into sub-arrays.
is an integer. 
, and for the end-fire array it appears when 
. We can conclude that to avoid grating lobe in the radiation pattern for any array the inter-element spacing should be 
. 
