Waveguides and 900 MHz

Thanks Joe, I understand it a little better now. I know a little about light guides and how changing the refraction of the guide helps the light travel the length of the guide. Fiber optics and GRIN lenses work this way. There are layers in the guide so different diameters refract light differently. But a metal tube is akin to a single type of glass. How does the material the tube is made of affect the radio waves. Light must pass through a material to be refracted differently and so it is bent at different angles as it travels the length of the guide. But if it reflects the only thing that material changes is how well it reflects and how much light is absorbed. The angle is the same for all materials as long as it is reflected. I thought that wave guides also reflected the radio waves. Does the material absorbing the radio waves make a big difference the way it does with light? And since copper is a better conductor than steel wouldn't it absorb more radio energy? Just shows how little I know about this. Thanks again, Eric

If I have all this right,

The longest wavelength mode is TE11, 3.41 * r = 0.259 m

Your wavelength is 0.33 m. So it's a waveguide beyond cutoff, and losses will be extreme.

I think.

John

Hi,

A circular guide of only 6in. in diameter (15.2cm) isn't a good choice for propagation at the wavelength corresponding to 900MHz (33.3cm), which is above the guide cut-off wavelength. This, in the dominant TE11 mode, is roughly equal to - sqrt(3) x guide diameter = 26.4cm and so you must expect some attenuation; the graphs also show that this increases very rapidly above cut-off. You will have to factor in material losses as well because you are expecting to use steel pipe. The only formula I could find assumes a copper guide which wouldn't be too accurate in this case. Cheers - Joe

If I remember right, it's the other way round - because it's a better conductor, it's better at carrying the induced currents inside the inner surface of the guide that mirror the EM field back into the space inside the guide, without dissipating energy through resistive loss.

Hi,

There is a continuous reaction between the E/M field and the guide walls such that currents flow in them which then re-radiate the wave. As a result any resistive losses there will absorb energy from them increasing guide attenuation. Copper, being a better conductor than steel, will thus afford lower attenuation. A further consideration is the surface roughness of the guide as any of that will make matters worse and I would expect commercial tubing to be only fair in this regard. Coupling into the TE11 mode is usually by a probe fitted into the wall of the pipe with a similar one parallel to it at the receiving end. For the impedances and tuning thereof, you will have to look in the books. Cheers - Joe

The waves bounce off the walls of a waveguide, so stay inside the pipe. Think of copper as a shiny mirror, and steel as a dirty one.

Radar absorbers are usually carbon or ferrite-loaded plastics, very bad conductors.

John

You can take conductivity to be an imaginary component added to the permitivity. Then, assuming monochromatic signal, you can put that complex permitivity into Snell's law and compute reflected and transmitted waves. You'll probably need to look at an E&M text book to understand the interpretation of the results, but the upshot is that waves travel very slowly in a good conductor so that in some sense, good conductors have very large indexes of refraction.

Rectangular wave guides are easier to compute and visualize than circular ones. Replace your 6 inch pipe with a 6 inch square wave guide. You won't get exactly the same numerical answers, but you get close and the qualitative behavior is similar.

Around here, they coat roofs with it.

John

An old book says that "Pitch" is good at bending microwaves. The stuff seemed to figure in lots of electrical experiments late 1800's yet is not to be seen nowadays. regards john

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

what effect would the tube being grounded have?

Charles

None. The waves are inside, and don't care about the outside. A mirror doesn't need to be grounded.

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

Actually I was thinking the grounding could reduce the bounced energy.

Charles