Greetings, while exciting circular polarized antenna (achieved using geometric perturbations) in CST should we change the number of modes as 2 in waveguide port settings. If possible, explanation along with supporting you-tube lectures on the simulation procedure in CST is awaited. Thanks in advance.
Else, the simulation procedure should it be the same as linear polarized antenna with default settings since I opt using geometric perturbation technique to achieve CP.
This depends mostly on your feeding line.
1. If the circularly polarized antenna is fed with some kind of slow wave structure, where one of the field components shifts in phase by 90 degree upon the end of the feeder, the only one mode is needed, and its direction has to be set with Polarization angle property in waveguide port setup. Besides, you have to excite the correct mode. To check it, tick the field Calculate port modes only in solver settings and see the field distribution.
2. If the feeder does not contain polarizer itself, you have to increase the number of modes for the waveguide port and check them for the desisred field distribution, e.g. TE10 (phase = 0 deg.) + TE01 (phase = 90 deg.), then in solver settings set the Source type to List. And in Select... menu adjust the modes needed along with their correct phases.
Dear Rabin Kanisha,
Incorporating circular polarization (CP) in a circular waveguide system is straightforward since it support TE11 Mode but in case of rectangular waveguide a bit complicated, because standard waveguides (like rectangular waveguides) support only linearly polarized modes by default (e.g., TE₁₀ in rectangular waveguides). To generate circular polarisation from rectangular wave guide fallowing steps may be fallowed.
- First, convert rectangular waveguide (which supports linearly polarized TE₁₀ mode) to a circular waveguide.
- In the circular waveguide, combine TE₁₁ modes in quadrature (±90° phase) to get circular polarization.
- Requires a phase-shifting network or hybrid coupler to feed the two modes correctly.
- Insert two orthogonal probes inside circular waveguide. It should be away from tapper portion of circular waveguide ie from junction of circular and rectangular wave guide. Typically quarter length.
- Feed both with equal amplitude and 90° phase difference.
- It generates circular polarization from the superposition of the TE₁₁ modes.
This configuration will generate RHCP for one position of 90 degree excited probes and if we change its position opposite side of it present position in that situation LHCP will generate. Keeping 0 degree excited probe position at same place.
In CST use wave port for excitation to probe and monitor only single mode. This is because we are observing dominate linear component E vector rotating in either clockwise or anti clockwise.
This method of generating CP usually narrow band in nature. For broad band we to insert a helical twist dielectric slabinside a circular waveguide adjust length and pitch to provide 90° phase difference Output becomes circularly polarized. But it is difficult and more complex to fabricate.
Vladimir D. Burtsev Shrivastav a.k. , Greetings, should i still increase the mode numbers in waveguide port settings albeit CP in a singly fed antenna were to be met using geometric perturbations. How far this change in mode settings disrupts the reliability between simulation and fabrication results of the prototype.
Rabin Kanisha
For a simple feeder (e.g. circular waveguide) you have to calculate two modes TE10 and TE01, then in Solver settings excite them with the proper phase shift.
Answering your question, yes, you have to increase the number of modes by one to calculate both fundamental modes (TE10, TE01).
Back to reality, you'd probably need to use septum polariser, slow-wave structure ot sth like that to obtain pure circular polarization in a waveguide from a single excitation.
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