Waveguide 2

[extassi]

Hello
I want to have a circular waveguide for 2.45 GHz microwaves. Can anyone tell me the required diameter for it. Are there special coatings available for waveguides. Thanks in advance

 

[GraviMan]

Wowsers! Shameful to admit my brayne now aches too. I have read about the use of Schrodinger wave eqn to predict higher than normal ocean solitons. But, this is not quite what I was getting at...

All I am interested in is using the full copper thickness of the waveguide. If the waveguide is circular section it could be thought of as a large hollow wire. If a voltage signal is passed through the waveguide/wire then the skin effect says the current will be concentrated at the outside. If an em wave is passed inside the waveguide/wire then the current is concentrated at the inside. If the two are 180' out of phase, and have the correct levels, the two effects should cancel. The practical upshot is that the current will be evenly distributed through the entire wire/waveguide thickness. Is this wrong?

This is also the crux of my query about how the back-emf current is distributed in a plain copper sheet, subject to the same em wave from both sides simultaneously. Again since there is no actual voltage field across the copper, I imagine that the skin effect is cancelled so the current flows evenly across the entire depth. Ok in this case ther will be a practical upper thickness limit, but i'm really attempting to understand the skin effect mechanism for a flat sheet.

I will be studying Maxwells eqn's (vect calc - how hard can it be) with the Open university. In the interim i am really trying to gain an intuitive understanding of the cause of the skin effect...

Mart

 

[biff44]

Read up on skin effect. I think you are confusing low frequency electronics with microwaves. They are quite different. A circular waveguide is in no way like a "wire".

 

[GraviMan]

Sorry Biff44, perhaps I'm just being thick here...

I spent 2 hours on the web looking for an explanation of why a plain sheet of copper should suffer the skin effect. Nothing. There were plenty of examples explaining how the induced current in a (solid) conductor forces the main current to the surface. I also found a very neat applet which i thought i would share:

http://www.falstad.com/emwave2/directions.html

I really just don't understand why a plane em-wave should induce secondary currents that force the main current to the side from which the em-wave comes. In free space i can understand that the spacial divergence would cause the field to be stronger on the wavefront side, hence skin effect. I just don't see why it affects waveguides, when the wavefront is planar - isn't it?

Mart

 

[GOTWW]

Look into a mirror. There is a reflection. This is due to opposite currents, induced into the metal, that re-reflect the EM field right back at you. Well, however small a time these currents exist in the metal, most stick near the surface, yet the finite Er (permittivity) of the metal requires maxwellian-wise to drop off with an exponentially decaying field strength. Find a perfect metal based mirror, then we will talk., Solitons for all dude!

 

[GraviMan]

Thanks GOTWW. So it's an exponential decay of the opposing currents. Just took me a while to gain intuitive understanding...

Actually there are some nice demos in that applet, that nicely demonstrate the skin effect in lossy conductors.

Thanks all for your patience. I have only limited practical experience in this particular field to fall back on. Can't wait to get started on Maxwell's equations!

Mart