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
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