Keith wrote:
"I suppose one might phrase it as "There is no such thing as a power
reflection coefficient" when it is not physically meaningful to separate
the total power as the sum of the incident and reflected power so the
equatiomn:
[rho] = sq. rt. (Pref / Pfwd) has no meaning."

We don`t have a choice of options on a menu to select or reject from.
Reality is whatever it is and we accept it and describe it as best we
can.

Terman says on page 97 of his 1955 edition:
"{rho} = (SWR-1) / SWR + 1."

Power varies as the equare of the voltage, because when you increase the
volts you also automatically increase the amps (Ohm`s law). Thus, Terman
has a subscript at the bottom of page 97 which is relevant:
"The definition of standing-wave ratio is sometimes called voltage
standing-wave ratio (VSWR) to distinguish it from the standing-wave
ratio expressed as a power ratio which is (Emax / Emin) squared."

In my long rxperience, I`ve found it`s never profitable to argue with
Terman. He is as close to infallible as any wrirter I`ve ever read.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Keith wrote:
"I suppose one might phrase it as "There is no such thing as a power
reflection coefficient" when it is not physically meaningful to separate
the total power as the sum of the incident and reflected power so the
equatiomn:
[rho] = sq. rt. (Pref / Pfwd) has no meaning."

We don`t have a choice of options on a menu to select or reject from.
Reality is whatever it is and we accept it and describe it as best we
can.

Terman says on page 97 of his 1955 edition:
"{rho} = (SWR-1) / SWR + 1."

Power varies as the equare of the voltage, because when you increase the
volts you also automatically increase the amps (Ohm`s law). Thus, Terman
has a subscript at the bottom of page 97 which is relevant:
"The definition of standing-wave ratio is sometimes called voltage
standing-wave ratio (VSWR) to distinguish it from the standing-wave
ratio expressed as a power ratio which is (Emax / Emin) squared."

In my long rxperience, I`ve found it`s never profitable to argue with
Terman. He is as close to infallible as any wrirter I`ve ever read.

Terman may be infallible, but I often find it unwise to trust his
interpreters.

The mention of SWR strongly implies lossless lines since VSWR varies
along a lossy line. Perhaps in prose previous to the equation above
he has limited his discussion to the lossless case. Quotes out of
context must be interpreted with great care.

....Keith

Richard Harrison wrote:


In my long rxperience, I`ve found it`s never profitable to argue with
Terman. He is as close to infallible as any wrirter I`ve ever read.


All of the handy-dandy transmission line formulas
that we have been using for many years apply
specifically to lossless lines.

A line with loss has a complex value of Z0. If the
imaginary part of Z0 is more than a few percent of
the real part we should use different methods.

One famous example:

Pload = Pforward - Preflected

is one that has to be treated with suspicion if
the line has appreciable loss (complex Z0).

Another is :

SWR = [1+|rho|]/[1-|rho|]

At high values of rho close to 1.0, SWR becomes a
totally useless concept. This is true regardless
of which formula for rho that we use.

We use the Smith chart outer circle to plot
lengths of transmission line, for example stubs
and matching transformers. We assume these lines
taken by themselves are lossless and have infinite
SWR (the outer circle of the Smith chart is the
"locus" of infinite SWR).

If we know the matched loss of a particular coax
(dB per 100 ft) it is far better to use a math
program and calculate everything, if the matched
loss is not negligible. The computer is much more
revealing than the Smith chart when line loss is
significant.

Bill W0IYH