Roy Lewallen wrote:
The concepts of "forward" and "reflected" power are sometimes (but not
often) useful, but have to be carefully confined to a very specific set
of conditions and applications. When you start thinking of them as real
packets of power bouncing around inside and outside a cable, you can
easily be led into a number of traps which you can get out of only by
distorting reality and ultimately reaching conclusions which are more
and more wrong.
Consider an earlier example made up of lossless lines:
100W XMTR---50 ohm---+---one second long 291.5 ohm---50 ohm load
The voltage reflection coefficient at the load is 0.707. The power
reflection at the load is 0.5, i.e. half the power is reflected.
After steady-state has been reached, the XMTR has output 300 more
joules than the load has accepted. A smaller real-world experiment
will easily verify that it is a fact that all energy sourced that
has not reached the load must necessarily be confined to circulating
energy or losses in the transmission line.
Question: In the above example, where are those 300 joules of energy
located and what is happening to them?
We know that 300 joules is wave energy and RF waves always move
at the speed of light, i.e. they cannot stand still. So please
determine how much energy is moving and in which of only two
possible directions.
The knife cuts both ways. Ignoring the energy gives one a very
warped view of reality where TV ghosting cannot exist, RADAR
can't possibly work, there's no such thing as non-glare glass,
and RF energy just sorta slowly mushes side-to-side inside a
transmission line. Can you spell M-A-G-I-C?
I strongly suggest forgetting completely about "forward" and "reverse"
power.
Good way to condemn yourself forever to a non-understanding of
energy movement in a transmission line. If that's your choice,
be my guest but please don't try to pass yourself off as an
energy expert by ignoring energy movement.
Many authors of transmission line textbooks disagree with your
stance.
--
73, Cecil
http://www.qsl.net/w5dxp