circular polarization ans radiation pattern

hi,
if anyone have worked on circular polarized antenna then plz guide me that is it possible that H plane radiation patterns tilts as antenna is displaced a little bit from centre? i have cp antenna and when i move it half wave length distant from centre where it was previously placed its H plane tilts a little bi and all others parameters such as gain,s parameters remain same.is it due to cp beahvior?but how?

The effect you see is ellipticity. Not many CP antennas are exactly CIRCULARLY polarised but elliptically.

hi, jir,
can u plz explian your answer?actually my antenna is cp and when i plot axial ratio v/s frequency i found it is cp graphs i attached below as well as Ex/Ey ratio.now i have made an array of 1*3 elements and mount that array on deielctric lens now my central element of array has same without tilting patterns but other displced elements have tilted patterns,can you plz explain how radiation patterns tilts when it eleptical polarzed antenna?plz i will be thank ful

I think one element may be fine, but once you make an array, it makes the system frequency dependent. Also adding the lens brings frequency dependence.

Elliptical polarization response is typical for any CP antenna, and none is perfect. Do you measure your system on a good test range? Do you account the ground? What is your reference antenna- is it perfect?

my antenna is is quite small(as designed for 300 GHz) so its circular polarization measurement was very difficult i got all the data by simulation on differnent simulators.and it seems it have circular polarized property,but now when i go for array this tilting of H plane field is quite starnge.even if it is cp or ep will this matters for radiation pattern?/?i have no idea about polarization effect on field patterns.can you plz explain it how polarization can tilt it?thanks in advance

I do not know about simulation but I know about real antennas. Your term "tilt" refers to linear polarization, not circular. Ellipticity in CP is the phase problem between the linear components. I would try to build a model that can be measured say at 50 GHz, then scale it to 300 GHz . At 300 GHz expect many problems as the tiny structures are difficult to make and adjust.

thanks for your answer i am attaching the graph of radiation patterns of antenna that are placed half wavelength distant from centred element and they have almost identical pattern but a bit tilt from 0 degree.i am thinking may be its due to its cp behaviour or some other reason?

NOt knowing your antenna details I cannot find the cause of the problem. As I wrote, most CP antennas have ellipticity, they are not perfectly circularly polarized. At 300 GHz,1 mm wavelength, minimal deviations in manufacture can cause similar "errors". If your model shows such result,I would wait how a real antenna will work To measure the CP response, you need a good standard CP antenna and a good antenna test range. Antenna specialists are happy if their real antenna differs from a model in a way you show.

thanks for answering,hope it will show better result when tested.