Hi, my self prasanth , am designing elliptical patch antenna using Ansoft HFSS using microstrip feed of driven modal solution type. So I have a doubt about the distance of radiation radiation boundary .If I take radiation from boundaries of substrate i am getting less gain,but if i extend from boundary of substrate getting good gain. So please tell me which procedure is correct
The radiation box must be between lam/4 to lam/2..... It can be more than lam/2, but should not be less than lam/4.....
1. HFSS needs airbox to model free space radiation. A radiation boundary is used to emulate free space by truncating infinite free space to finite calculation domain. This minimize reflections from outer surfaces and ensures maximum absorption. This is very similar to an anechoic chamber.
2. For this, the distance from the radiator must be a minimum of λ/4 (or λ/2) for strong radiating surfaces. Here λ represents the wavelength for the minimum frequency used.
3. A distance of λ/4 from the radiating surface is taken for the reason that similar impedance mismatches can be cancelled at quarter wavelengths. This ensures that there are no reflected waves and thus the radiation boundary absorbs all the radiation.
4. Maximum absorption of energy occurs when the fields are perpendicular to the boundary.
This is a usual question where antenna designers are puzzled mostly.. actually radiation boundary should be equal or more than quarter wavelength from the each boundary of the structure. where free space wavelength can be evaluated by the center frequency ; center frequency is the average of both lower and upper frequency in which you want to plot the results.. further to validate the results you can check the radiation efficiency at frequency of interest; if it is less than 100% you can consider that you designed the far field well..
in general radiation boundary must be at least at quarter wavelength spacing from the antenna. in some designs the gain varies if you vary the height of the radiation boundary. so it is better to keep the distance between antenna radiating surface and radiation boundary greater than quarter wavelength of the operating frequency and at the same time you should not simulate the antenna over a wide frequency range at a time, rather break your simulation.
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