Floyd L. Davidson wrote:
All of the above is equally true for twisted pair transmission
lines. Note however that there will *not* be "a lot of loss"
from mixing 50 and 75 ohm coax. It will actually be a very
small loss. (There are other factors that are much more
significant, but that gets more complicated than I'm going to
get in this article. Certainly if someone wants to discuss it,
we can and there will probably be a number of people who can
describe it in detail.)
I think most of you people are somewhat confused when dealing with the
characteristics of transmission lines of all types. For example, the
mixing of impedances as indicated above results in a "return loss" or
reflection loss. There also ohmic losses in transmission lines as well
as dielectric losses. Impedance is a constant characteristic of a
coaxial transmission, not a variable. It is a fixed quantity and is
calculated in the following manner.
Characteristic Impedance of a co-axial transmission line whose
dielectric material is air..
Example:
d1= the inside diameter of the outer conductor = 3"
d2= the outside diameter of the inner conductor=1.5"
e=1= dielectric constant of "air" (changes according to the
dielectric material used)
Z = 138/sqrt(e)x(log(d1/d2))
Z = 138/sqrt(1)x(log(3/1.5))
=41.54 Ohms
This co-axial transmission line has a characteristic impedance of 41.54 Ohms
Likewise, dielectric losses, ohmic losses and overall attenuation (which
all vary with frequency and line size) may be quantified through
calculation. Transmission lines are not devices that can be properly
evaluated through observation and/or measurement, their characteristics
are too complex.
Their sole purpose in life is to efficiently transfer energy from
generator to load. This is accomplished by having both the generator
and load impedances equal to the characteristic impedance of the
transmission line.
So, enough of the crap such as, "the wires in the shield are longer than
the center conductor. Therefore the outer conductor is longer than the
inner conductor". Keerist, that's even crummy observation.
Read a textbook for cripe's sake.
<snip>
--
Regards,
Gary
I know not with what weapons World War III will be fought,
but World War IV will be fought with sticks and stones.
Albert Einstein