View Single Post
  #22   Report Post  
Old January 27th 08, 03:24 AM posted to rec.radio.amateur.antenna
Keith Dysart[_2_] Keith Dysart[_2_] is offline
external usenet poster
 
First recorded activity by RadioBanter: May 2007
Posts: 492
Default Derivation of Reflection Coefficient vs SWR

On Jan 26, 12:15*pm, Roger Sparks wrote:
On Fri, 25 Jan 2008 19:13:31 -0800 (PST)





Keith Dysart wrote:
On Jan 24, 10:33*pm, Roger Sparks wrote:
[snip]
By examining this derivation, the reader can see that power and energy
is reflected when a wave encounters a discontinuity. *The reader can
also see that more power is present on the transmission line than is
delivered to the load.


This is the conventional phraseology for describing the behaviour at
the impedance discontinuity.


Allow me to offer a specific example for which this phraseology is
inappropriate.


Consider a 50 V step function generator with an output impedance of
50 ohms driving a 50 ohm line that is 1 second long terminated in an
open circuit.


Turn on the generator. A 50 V step propagates down the line. The
generator is putting 50 J/s into the line. One second later it
reaches the open end and begins propagating backwards.
After two seconds it reaches the generator. The voltage at the
generator is now 100 V and no current is flowing from the
generator into the line. In the 2 seconds, the generator put
100 joules into the line which is now stored in the line.
The line is at a constant 100 V and the current is zero everywhere.


Computing Pf and Pr will yield 50 W forward and 50 W reflected.
And yet no current is flowing anywhere. The voltage on the line
is completely static.


And yet some will claim that 50 W is flowing forward and 50 W
is flowing backwards.


Does this seem like a reasonable claim for an open circuited
transmission line with constant voltage along its length and
no current anywhere?


I do not find it so.


...Keith


This is a reasonable observation for a static situation where energy is stored on a transmission line. *

If the example contained an ongoing consideration, like "Where does the power move to?", then it would be reasonable to consider that the wave continued to move, simply to avoid the complication of what EXACTLY happens when a wave starts and stops.


So have you thought about "where does the power go?"

When the generator is matched to the line so that
the reflected wave does not encounter an impedance
discontinuity when it arrives back at the generator
(and therefore is not reflected), where does the
reflected power go?
Does it enter the generator?
Is it dissipated somewhere?

Answers to these questions will quickly lead to
doubts about the *reality* of "reflected power".

...Keith