Why is it that TE10 is the only mode considered while observing output in case of waveguides or microstrip structures? Why not TE20 or TM21 et al.? Is it because the higher order modes have higher cut off frequencies? Or is it because more probes/SMA connectors are required to excite/observe the characteristics?
Particularly, for rectangular waveguides, the TE10 mode has the lowest cutoff frequency and so, called the dominant mode. All other modes have higher cutoff frequencies (except, in the case of the square cross-section for which TE01 has the same cutoff frequency). This implies that at the frequency-band of operation only the dominant mode is propagating, while all higher-order modes are "cutoff."
In general, an excitation of the guide at its cross-section excites all possible waveguide modes. The modes with cutoff frequencies higher than the frequency of excitation decay away (evanescent) from the source. Only the dominant mode has a sinusoidal dependence upon distance and thus possesses fields that are periodic in space and "dominate" the field pattern far away from the source, at distances larger than the transverse dimensions of the waveguide.
Hope it clarifies!
I understand that TE10 is the dominant mode ... et al. My question is why is TE20 mode or TE11 and other modes not considered while studying/simulating/measuring a waveguide or microstrip line? Why only TE10 mode?
"... at the frequency-band of operation only the dominant mode is propagating, while all higher-order modes are cutoff."
Let the waveguide operate in the X-band (WR90). Here TE10, TE01, TE11 and TM11 modes can propagate (TE10 and TE01 have different cut off frequencies). It is not just the dominant mode that is propagating in the waveguide!
I already explained it above. Propagation could be of two types: oscillatory which is wave type, or evanescent which is exponential damped. Dominant modes have wave nature, others are evanescent. Evanescent waves cannot travel significant distance. You want wave type oscillatory mode of propagation!
All frequencies which are higher than cutoff frequency are capable of propagating. In waveguides it is preferred to work in dominant mode because E field can be predicted from the basis of waveguide theory. If you feed a signal which can excite TE20 or higher mode, predicition from waveguide theory will not be consistent.
Attenuation increases with the increase in mode number value. A waveguide act as an high pass filter and support all possible modes above the cutoff frequency but the attenuation losses are minimum in the lower order mode (dominant mode) due to this reason dominant mode is used for RF transmission. Ideally a waveguide must support the propagating mode to the infinite length. The higher order modes attenuate over a small length, whereas the RF power in dominant mode can be effectively transferred to longer distance.
We also have overmode waveguide in which there exist multiple modes. Say, if the operating frequency is twice the center frequency of a certain waveguide band, then the mode can be TE10 or TE20. In this case, we can have a single TE20 mode operation as long as we can reject the field of TE10 mode and meanwhile keeping the field of TE20 mode less disturbed.
So, we also consider multiple modes in some cases.
Hi Suhas,
The first problem is already in your question... Indeed, most often people only consider the dominant mode and in most cases this is right, but in some cases can also lead to issues.
Let us take a hollow pass-band waveguide filter as an example. Such filter is always used in satellite input sections in order to provide high attenuation of the signals outside the receive (pass-)band of interest.
A simple simulation of the filter shows that attenuation at a few GHz from the pass-band is high, which is then confirmed by measurement... The problem here is that higher modes might not have been excited due to the measurement setup... but might actually be excited in the antenna input section (or come from the high power transmit section, e.g. via antenna coupling and propagate when the filter is integrated onto the S/C, this might then have a non negligible impact on performance. Assuming a filter with a pass-band centred at 28.5 GHz, we shall consider the next modes (TE01, TE20, TE11, TM11) for which the cut-off frequencies are (~35 and 39 GHz) only a few GHz from the dominant mode. If they get excited these modes will not be attenuated as the dominant mode above their cut-off frequencies and might well reach the next amplification stage... Such problem can be avoided by simulating the propagation of the next higher order modes during the design phase of the filter. Carrying out measurements of the filter response for the higher mode could be done but is complicate and expensive (e.g. using mode extractors and/or converters). On the other hand simulation is fast, easy and will provide useful information for the system designer... Now what about even higher modes? Well, in this case the next cut-off frequency is above 50 GHz and we can consider that these modes will be evanescent in the filter and waveguide structure and will not be an issue at next amplification stage... Again this might not be true for the closest higher modes to the dominant one.
Each problem is specific and high modes might be rejected at the iris of such filter...provided there is one... In usual cases, do not only rely on simulation of the dominant mode, not even on measurements (these are of cause necessary to confirm the performance for the dominant mode). Depending on your design, system, application, it might be worth considering the higher propagation modes to avoid unexpected problems. you will also find a link to some publications from a renown colleague of mine, he also wrote a book which is dealing with the topic of microwave antennas feed systems. Regards, Patrick
Article Comparison of surface mounted high quality filters for combi...
Additionally, you might want to check this company, their design / simulation SW is used throughout the world
https://www.mician.com/
Regards,
Patrick
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