What is the minimum required length for a waveguide to fully propagate the dominant?

Hi all,

What is the minimum required length, for a waveguide to fully propagate the dominant mode.

Thanks.

Bouchy

To put it briefly ,the wide side of the wavelength must be longer than the cutoff wavelength of the dominant mode.

Hi bluuice,

can you please explain you comment.
Thanks,

Bouchy

usually, in a rect. wg, only TE10 mode is wanted.
Higher modes may be exited (but unwanted) and if exited they propagate itself.
Also Evanescent modes may be exited, but in any case they are strogly attenuated by the path.
How long shoud be the path to destroy Evanescents modes?
The answer require to set a level of accepted value (i.e. -40 dB) and a lot of complicated calculations.
As a rule of thumb, a path long a "a" side is enough.

Note that the evanescents modes (as every mode) attenuation is exponentially. So doubling the path, make a doubling of the attenuation in dB.

hi sergio mariotti,

Thanks for the useful information. I have another question please, let say I design a RWG with cutoff frequency (freq1), width (a) and height (b=a/2). What will happen to my cutoff frequency if I change the height (b) to be equal (a/4).

Thanks for your support.

Bouchy

Assuming you exite TE10 mode only, changing b dimension don't change cut-off frequency. So what change? Only Zo change.

I have to agree with you, by reducing the height of the waveguide we reduce Z0.
beacuse Z0= (b/a)*the rest of the formula.

Thanks

bouchy