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# Parallel plate wave guide problem

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#### amirsde

##### Newbie level 5
Hi all
As you know we have modes in wave guides
For example in parallel plate TM mode we have
h=nΠ/b , fc=n/2b(µε)^0.5
my question is that how do this modes change ?
We have n=1 , fc=1/2b(µε)^1/2
n=2 , fc=1/b(µε)^1/2 and so on
They change with my operation frequency ?
for example 100Mhz-200Mhz --> mode1
200Mhz-300Mhz --> mode 2 ?
Or not ?
Also how can I know the wave guide is on which mode on special frequency ?

Waves of different mode
What do you mean about different mode waves ?
It means waves with different frequencies ?

Waves of different mode propagate independently, generally all modes above the respective cut-off frequency are propagable. You'll usually try to avoid higher modes by the waveguide transition design, also in some cases special mode filter elements.

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I mean different mode waves of same frequency, as said.

I mean does the frequency of wave specify the modes wave includes ?

I mean does the frequency of wave specify the modes wave includes ?

No. The amplitudes of each mode are determined by the excitation method. It is (theoretically) possible to have a waveguide operating at a frequency at which 100 modes could propagate, but only one is excited.

amirsde

### amirsde

Points: 2
A more practical answer could be, waveguide are usually operated in a frequency range where no higher modes can propagate. For standard rectangular waveguides, a band with stable H10 mode is assigned to each waveguide size.

amirsde

### amirsde

Points: 2
No. The amplitudes of each mode are determined by the excitation method. It is (theoretically) possible to have a waveguide operating at a frequency at which 100 modes could propagate, but only one is excited.
I can not get my answer
If I have fc1=100Mhz , fc2=200Mhz , fc3=300Mhz for example (cutoff frequencies)
so my dominant mode n=1 and I have fc1=100Mhz so for f<fc I have no propagation
if I have f=150Mhz I am on n=1 mode
if I have f=250 Mhz I have n=1,2 together right ?
So my frequency specify that ?

but only one is excited
Why ?

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