Hi Sujith,

Assuming you are radiating from the TE10 (dominant) mode, then the EM fields incident upon the aperture of the open-ended waveguide do not change. If this were all that mattered, the radiation pattern would narrow uniformly in both directions as the frequency increases.

However, the TOTAL EM fields, including both incident and reflected waves, which is what determines the radiation pattern, do change. This is because an open-ended waveguide is not a perfect termination. Their is therefore some reflection at the aperture, and the incident and reflected fields add.

An open-ended waveguide I believe will be better matched at the high frequency than low, because it will radiate more effectively at higher frequencies. My guess is this will be a high impedance, so the reflection coefficient will be positive and decreasing with frequency. The E-field of the incident and reflected waves will therefore add, while the H-field of the two waves will subtract. The E-field has only transverse components, whereas the H-field has longitudinal components. The changing beam pattern is essentially the superposition of the radiation patterns of these two fields which are trending in opposite directions as a function of frequency.

Matt

Thank you Matt

The open ended waveguide maintains constant its physical dimensions while increasing frecuency. Given the fact that physical dimensions are related to effective area, and the effective area is related to gain and wavelength by:

Aeff=Gλ2/4π

the more the effective area and the less the wavelength, the higher the gain will be. The gain is related to directivity, so when gain increases, directivity also increases and this causes the beamwidth will be narrower.

However, direction of main beam should not be changed when increasing frequency when you are feeding waveguide in the fundamental TE10 mode. Perhaps you should consider some effects of diffraction at the edge of the apperture or in the near environment. These effects can change the field distribution.

The open ended wave guide is not perfect radiating element and hence for a different frequency, its behavior at sudden open end will differ. Also the edge effect (dispersion) as well as reflection will differ from frequency to frequency. The net result will be the radiation pattern in the space will differ.

Hi all,

Thank you for your comments

 Aeff=Gλ2/4π.

For a particular waveguide, Aeff  is constant. So when λ decreases, gain increases and therefore beam width decreases.

Matt mentioned about the dependency of reflection coefficient and radiation pattern. It is true reflection coefficient is coming down as frequency increases.

But if we consider rectangular waveguides with flange and without flange (a rectangular pipe of same dimension as waveguide), radiation pattern differs, but the S11 plots in bothcases have very small difference (around 1dB difference near to upper cut-off frequency).

For the rectangular pipe, radiation pattern gets narrower in both horizontal and vertical planes and gain increases.

Whereas for the waveguide with flange, beam gets wider (beam width increases) in vertical plane but gets narrower in horizontal plane. (Only TE10 mode within the frequency band of operation of the waveguide)

If this effect is due to the fact that the wave guide is not a perfect termination, then it would have happened for both cases. In both the cases field distribution is not uniform. So is field from the edges the main reason for non-uniform field distribution ?

The radiation pattern from an open-ended rectangular waveguide, excited by the fundamental TE10 mode, will depend not only on the aperture fields of this mode, but also on the radiation from currents induced at the edges of the waveguide external to the aperture. These will be particularly important if the waveguide has no flange present, and can affect the E-plane radiation pattern significantly, resulting in the kind of bifurcation in the radiation pattern that you have observed.

The other thing to note is that due to the abrupt termination at the wavegude aperture, reflected waves consisting of higher-order rectangular waveguide modes can be excited in the vicinity of the aperture plane, as well as a dominant TE10 reflected wave. Whilst these higher-order modes do not, in general, propagate back down the waveguide (they're evanescent if the waveguide is operating within its normal frequency band), they do alter the aperture field distribution which can then influence the radiation pattern. 

Thanks Ray.

Hi Sujith. I'm based in Auckland too by the way.

Regards,

Ray

Hi Ray,

Happy to hear that. :-)

Kind regards

Flanges produce diffraction effects. Here I'm sharing a simulation analysis by ANSYS. It a pyramidal horn antenna, but the effect is similar to the open-ended waveguide. E field is more sensible to diffraction effects.

Thanks Luis

This would be worth considering in the calculation of gain of an open ended wave guide.

T. S. Bird, V. Lingasamy, K. T. Selvan, and H. Sun, “Improved finite-range gain formula for open-ended rectangular waveguides and pyramidal horns,” IET Microwaves Antennas Propag., vol. 11, no. 14, pp. 2054–2058, 2017.