antennaelectromagnetismoptical-fibretransmission linewaveguide
In a rectangular waveguide, we know we can find cut off frequency as a function of mode and geometry.
Is there a way to design the waveguide so to ensure that we can limit the frequency of transmission to a single frequency?
If not, can we design the waveguide so that the frequency capable of being propagated is between $$f_a < f < f_b$$
Thanks
The reflection is extremely relevant, old boy! An open ended rectangular waveguide has a return loss of around 10dB, so a considerable amount of the field is reflected at the interface. The impedance of you wave guide will typically be around 450 ohms (dependant on dimensions) - free space is 377 - Hence the mismatch.
Read for yourself and see fibre to the hub is explained with TDSL for the last 100m or so. Judging by covered numbers in illustration, patent was just approved. Maybe in 5~10 yrs (IMHO)
Lots of unknowns, not easy to achieve cost goals.
Read the presentation here
Best Answer
Waveguides have a "low" cut-off frequency at which EM waves will no longer pass down the waveguide but they will pass a decent band of frequencies above this cut-off point and the theory is well known.
Cut-off frequency is: -
Where c is speed of light in the waveguide and \$\mu\$ and \$\epsilon\$ are permeability and permittivity of the air dielectric inside the waveguide. Also "a" is the longer of the two internal dimensions of the waveguide: -
Basically, what this equation tells you is that the lowest frequency is determined by dimension "a" and that frequency has a wavelength of 2a. Half of "a" being a quarter wavelength implies an open circuit at that frequency therefore top and bottom "plates" of the waveguide are, in effect, not connected.
The maximum frequency that can be passed is a little trickier to explain and relies on understanding the modes of operation but it's fairly easy to explain that a decent band of frequencies can be passed without serious attenuation within a waveguide.
The cut-off point should be easily understood but to understand that a waveguide can pass a bunch of frequencies needs a little leap of faith (without going into the math): -
The picture above is a drawing of a waveguide and shows two "busbars" - these can be regarded as the electrical connections in or out. If the busbar is thin, dimension a/2 is half the physical width of the waveguide.
This defines the lower frequency limit of the waveguide.
Now, imagine that for higher frequencies the busbar got fatter. Yes, I know there isn't a busbar inside the waveguide because it's one solid piece of metal shaped into a tube but, the busbar analogy is to help realize the concept that higher frequencies will work...
If the busbar gets fatter then a/2 reduces. Whatever value a/2 is, it will be a quarter wave at some frequency and this will be an open circuit to the electrical signal applied.
Thus, waveguides can pass a range of frequencies and are not limited to exactly one spot frequency.