Rectangular Waveguide - Page 1
Forum section: electronics / projects

author: mechknoll
author_rank: Contributor
timestamp: March 05, 2015, 02:14:16 am
content: Hi all!I am doing some design work on rectangular waveguides and keep running into a subtle problem with design rules I find in books and online. Every chapter written on rectangular waveguides describes the cutoff frequency for TE or TM modes as a function of the cross sectional dimensions (length and width). Now I have noticed that the length plays a crucial role in what I will call "side-band modes". If I design a guide I will get the same response if I increase the length by an integer amount, but if I just add say 10% the side lobes move out from the center frequency. What I am wondering is if there are some design rules for the length of a guide. Then to apply these rules to power dividers.Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 05, 2015, 03:53:59 am
content: Not a microwave expert, I only know enough about fields to be dangerous,Seems to me, I was never explained the reason why traditional waveguide is rectangular, and the ratio that it is; but I can tell by inspection that it has to do with pushing the cross polarized mode to a much higher frequency, so that the TEM0,0 mode is fairly unperturbed over a usefully wide range. A perfectly square waveguide, of course, would support both vertical and horizontal polarizations in the same frequency range, which would give rise to weird interactions, reflections, "drool" and so on due to nonuniform perturbations between the modes (the square not being perfectly equal or perfectly, well, square; connectors, couplers and so on being not quite centered, and not quite parallel or perpendicular, as the case may be; etc.).The same is true of a circular waveguide, of course.I don't know about propagation outside of the main mode, though. There are higher modes (0,1, 1,0, etc.) but by nature and by design, those start an octave or so higher. I expect the usual time-space dualities apply, but I wouldn't know what to look at, as far as what defines those lobes or whatever.Maybe... you're referring specifically to lobes in a power divider structure? So the periodic openings yield a Fourier transform that looks like, well, sampling or aliasing?Tim

author: skipjackrc4
author_rank: Regular Contributor
timestamp: March 05, 2015, 05:06:57 am
content: It would really help if you could provide details about the actual physical structure. Is it just a straight rectangular waveguide? Dimensions? Frequency span?

author: LaserSteve
author_rank: Super Contributor
timestamp: March 05, 2015, 04:40:38 pm
content: The mother of all microwave books is the WWII "RADLAB" series where Radar was the #2 priority after the Manhattan Project.During and after the war it was decided that too much was learned to keep secret, and thus a series of guides was published.They contain a mix of practical and theoretical. Most modern texts are traceable in some way to the Radiation Laboratory Series..This should help if you can follow the math, at least it will give you access to much of the original work on Waveguide.http://www.febo.com/pages/docs/RadLab/VOL_8_Principles_of_Microwave_Circuits.pdfhttp://www.febo.com/pages/docs/RadLab/VOL_9_Microwave_Transmission_Circuits.pdfhttp://www.febo.com/pages/docs/RadLab/VOL_10_Waveguide_Handbook.pdfhttp://www.febo.com/pages/docs/RadLab/VOL_28_Index.pdfWhat your speaking of is probably what is known as Longitudinal Mode in Laser Cavity Optics, which is related to waveguide theory.They are largely ignored in many microwave texts as anything short enough for them to matter in is a filter, impedance matching, waveguide stub, antenna feed, or resonant cavity.Steve