Waveguide
Waveguide
(OP)
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
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
RE: Waveguide
RE: Waveguide
What type of coatings? At that low frequency, you could probably coat it with many things and never notice the effect.
kch.
RE: Waveguide
RE: Waveguide
Thank you for your replies. Which text book you refer logbook. I want to heat a reactor using microwaves.
Regards
RE: Waveguide
RE: Waveguide
Thank you bearing01 will buy one right now. The magnetron is fixed to a rectangular waveguide in a specific geometry to avoid back coupling i guess. How should the magnetron be fitted in a circular waveguide to avoid back coupling.
Regards
RE: Waveguide
Is this right, or am I missing something?
Mart
RE: Waveguide
sorry for the delay in answering. I missed your question.
The book referred to was "Microwave Circuit Analysis and Amplifier Design" by Samuel Liao (1987).
GraviMan,
cavities do suffer from the skin effect. The waves are inside the cavity/guide and are constrained by the guide walls. Currents flow in the guide walls and therefore dissipate heat. I seem to recall cavities being plated with silver to increase the conductivity in the inner surface layer, increasing the Q of the cavity. Note it is the inner surface which is in contact with the wave. The skin depth is so small that the wave does not penetrate the conductor and get to the outer surface.
RE: Waveguide
RE: Waveguide
Thanks for helping my mechano brain, Logbook! If there was an identical wave approaching, say a flat sheet of copper from the other side, would the skin effect still apply? Basically the em field would no longer have a gradient across the sheet.
For the cavity I was thinking about it being circular cross section. I take it that if it was actually carrying voltage a signal, that current would be forced to the outside? Is there a case for a weak signal in the waveguide, matching at 180', the em field being carried? This would also avoid any radiation, as well as expensive metals!
Mart
RE: Waveguide
RE: Waveguide
That's about 5um in metrispeak. What's the best way to get this done? Any idea how much this would cost, say per cm^2 (or inch^2)? What sort of field strength can this handle (V/m or teslas)?
Mart
RE: Waveguide
The depth of penetration into the silver coating will be:
D [meters] = 0.0642 / (f)^1/2 = 1.3 micron or 51 microinches, hence the 200 microinch recommendation.
Conductivity of silver is 6.17X10^7 mho/meter.
"Resistance of a conductor with exponential decrease in current density is exactly the same as though current were uniformly distributed over a depth D"
If it is helpful, the surface resistivity in ohms per square is Rs = 2.52 X 10^-7 * (f)^1/2
OF COURSE, if there are any discontinuities in your resonator, like where a lid meets the cavity body, you had better be exciting a resonator mode that does not put a current maximum there.
RE: Waveguide
Beware if the plating wants to first plate with some lossy metal, like nickel!
RE: Waveguide
Mart
RE: Waveguide
RE: Waveguide
The concept is that the voltage would induce a current in the outside circumference, while the em wave induced the current in the inside. By chosing the signal voltage the wave would still be totally reflected along the wave (ie no leakage), but the entire thickness of (say) copper would pass the induced current. This would minimise resistance losses...
Mart
RE: Waveguide
There are "active resonators" being studied for improved Q. They involve an active semiconductor device making up for resonator losses. I am not sure how practical they are, but there are some papers around. There is a term "vacuum port" used sometimes in RF space communication where you can use reactive energy to improve a receiver's noise figure over what should normally be considered to be the theoretical limit.
Give me a better idea what you are interested in doing, maybe I can help.
RE: Waveguide
"The main goal of our research is the achievement of substantial squeezing of the vacuum fluctuations associated with a soliton. If the soliton is injected into the probe port of a Mach-Zehnder interferometer and the squeezed vacuum is injected into the vacuum port, sub-shot noise measurements of the phase difference between the two arms of the interferometer can be performed."
http://www.stormingmedia.us/10/1034/A103473.html
So if you want to squeeze some solitons, don't squeeze too hard? This stuff makes my brain ache. Solitons, BTW, are waves that never decay because they keep reforming themselves. Sounds a little like what you are trying to do? Solitons were discovered in nature, from satellite pictures of the oceans. People realized that every once in a while, a wave forms that goes most of the way around the world. Cool huh? You can make a broadband microwave frequency multiplier with solitons, but that is a different story.
RE: Waveguide
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
RE: Waveguide
RE: Waveguide
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
RE: Waveguide
RE: Waveguide
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