Owen Duffy wrote:
Richard Clark wrote in
:
On Fri, 02 Mar 2007 06:16:15 GMT, Owen Duffy wrote:
that does not imply matching in
the Jacobi Maximum Power Transfer Theoram sense.
-um, OK-
I find negative propositions a bit oblique, what DOES it imply?
I was responding to your words:
"I will at this point re quote Chipman to roughly this scenario (being
more general, he didn't specify the reflection).
"At the signal source end of the line ... none of the power
reflected by the terminal load impedance is re-reflected on
returning to the input end of the line."
The ellipsis reveals that the source Z matches the line Z."
The confusion arises out of Richard's misleading quotation, which is out
of context and includes an inaccurate final sentence ("The ellipsis
reveals...") added by Richard himself.
The context in Chipman's book is specifically about the scale of
"reflection loss" as found alongside a Smith chart. Reflection loss is
the power delivered into a mismatched (reflecting) load impedance,
relative to the power that would have been delivered into a matched
(non-reflecting) load.
Chipman points out that "The concept is directly applicable only to
transmission-line circuits of the form shown in Fig 9-26, in which the
source impedance is equal to the characteristic impedance of the line."
In more detailed analysis, he explains why the definition of reflection
loss only holds good if there is no further re-reflection at the source,
so the "reflection loss" scale on the Smith chart can only be used in
cases where the source impedance Zs is equal to Zo of the line.
Richard's comment that "The ellipsis reveals that the source Z matches
the line Z" is misleading, for that isn't at all what Chipman was
saying. Chipman makes it perfectly clear that this section is dealing
with a special case, which only applies if Zs has deliberately been made
equal to Zo. Most of the rest of the book deals with the more general
case where Zs is NOT necessarily equal to Zo.
However, that special case often does apply to signal generators and
similar test equipment. If the RF output comes through an attenuator
that has a design impedance of Zo, and if the attenuation is large
enough, this creates a good approximation to a source of impedance Zo.
Then the reflection loss concept becomes valid.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek