On 4 mayo, 14:23, Cecil Moore wrote:
On May 3, 9:06*pm, Wimpie wrote:

Regarding the two-port single-port issue. One can setup a reasoning
based on a two-port setup, but that significantly complicates the
matter without giving any additional insight. *I tried to keep it
simple ...

"Everything should be made as simple as possible, but not simpler."
Albert Einstein

Here's my earlier example:

Source-----Z0=50----x----1/4WL Z0=100----200 ohm load

s11 is measured at point x equal to 0.3333 and also 0.3333 at the
load. Nowhere is s11 equal to 0.0000. Put everything to the right of
point x into a black box and s11 measures to be 0.0000 under exactly
the same conditions??? And you guys want all of us to trust that
measurement enough to predict the disputed source impedance of an RF
amp when it cannot even predict the load impedance in the above very
simple passive circuit?

There are reflected waves at point x (s11*a1) that are equal in
magnitude and 180 degrees out of phase with the reflected waves
transmitted back from the load (s12*a2). The two waves undergo
destructive interference at point x which creates a V/I ratio of 50 at
point x. But the absence of *net* reflected energy at point x does not
mean that there are no reflections at point x. There are actually two
sets of reflections at point x that mask any attempt to determine the
actual value of the load impedance by measuring s11 when the system is
installed inside a black box. It is foolish to presume that there are
no similar interference patterns inside an RF amp. In fact, the only
condition where there is no interference inside a simple voltage
source is when there are no reflections or the reflections are
orthogonal to the source signal.

There is a good discussion of the role of interference in the creation
of virtual impedances in section 4.3 of "Reflections", by Walter
Maxwell. Even though a lot RF engineers scoff at the laws of EM wave
physics from the field of optics, the best explanation of interference
I have ever read is the chapter by the same name in "Optics", by
Hecht. Another good chapter in "Optics" is "The Superposition of
Waves".
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Hello Cecil,

Basically, it doesn't matter what is inside the box. It can be fully
characterized by its impedance versus frequency (small signal
approach).

I am fully aware of that there are (back and forth) reflections in the
quarter wave line inside the black box. However in case of a black
box, you don't know that (black box principle).

Same thing happens in most narrow band antennas. The antenna wire
itself may be subjected to (for example) VSWR = 10, though the
impedance can be 50 Ohm (and therefore doesn’t introduce reflection in
a 50 Ohms feed line). I know that this 50 Ohms is the result of
interfering waves/signals, but an MFJ 259B antenna analyzer, or my
FT7B doesn't (and doesn't need to know it).

One could only guess what is a black box based on its impedance versus
frequency curve, S11 curve or Time domain response (I had to do this
in school).

So if we decide to open the black box and we put the reference plane
inside the box, we get a new thread, something like: "why has a PA
certain output impedance?", or "what is the large signal output
impedance of valves, BJT, FET, etc?".


With kind regards,

Wim
PA3DJS
www.tetech.nl