Degraded radiation pattern at high frequency

Hello
Usually when we measure the radiation pattern, it start degrading at higher pattern. Is it becasue of introduction of higher order modes. Please help

i presume that when you say 'degraded' you mean more frequent
and sharper far-field nulls.

because frequency increases, wavelength becomes smaller.
this means that far-field cancellations (i.e.nulls) become more
pronounced compared to lower frequencies. you can get get (possibly)
nulls more frequently as you 'walk' around the far-field sphere,
than in lower frequencies.

also, the finite size of the ground plane is the cause of such nulls
due to the diffraction effects on the edges. your backlobes
suffer significantly more from these far-field cancellations, compared
with your main (front) lobe, perhaps.

also, your actual anechoic chamber can be the cause of nulls. you
get reflections from the walls within the chamber and depending on
the case, they can sometime result in constructive or distractive field
pattern!

good to measure