On Mar 31, 8:51 am, Cecil Moore wrote:
K7ITM wrote:
On Mar 31, 6:23 am, Cecil Moore wrote:
With as little effort as possible, determine Pfor2 and Pref2.

Given: Pfor1 = 100 watts, Pref1 = zero watts
[(Z02-Z01)/(Z02+Z01)]^2 = 0.5

100w---Z01 ohm line---+---Z02 ohm line---unknown load
Pfor1-- Pfor2--
--Pref1 --Pref2

If the lines are lossless, Pfor2=175W, and Pref2 is obvious.

Yes, assume the lines are lossless, but I don't think
that is the correct answer.

And it's a REALLY weird way to state the impedances.

Yes, to get people to think outside the box. Since the
Z0s are unknown, the voltages and currents are also
unknown, yet there exists a solution to the problem.
This is the typical kind of problem that my EE professors
used to give to the classes at Texas A&M to try to get
us to use our gray matter.

How do we KNOW that Pfor1 is 100 watts and Pref1 is zero?

Given.

How do we KNOW that line 1 and line 2 each have UNIFORM impedance?

Given.

Just because someone
measured 100W forward and 0W reflected on line 1 doesn't make it so,

Just because 100 watts forward and zero watts reflected
were measured by an ideal directional wattmeter calibrated
for a characteristic impedance of Z01, doesn't make it so???
Now that *is* an interesting diversion.
--
73, Cecil http://www.w5dxp.com

In general, in real-world ham problems, it is far from given that we
know that the impedance of a line is really uniform. We don't even
know the impedance for certain. There have been any number of
postings here in the past where someone can't figure out what the
impedance of some unknown line is, and I'm sure there are far more
instances of people ASSUMING that RG-8-type line is 50.00 ohms. It
practically never is. (But it also practically never MATTERS for
typical ham applications.) WHY should I believe you know how to
measure the impedance of a line, and that you know how to calibrate a
bridge to that impedance? I'm NOT saying that you DON'T know how,
only that I need convincing. I've seen plenty of evidence of those
who haven't a clue about either around here, so I prefer to start from
the point of view that any reported measurements of "forward and
reverse power" are likely to be flawed.

Since you "know" that the meter is calibrated for the impedance of the
line, you must know the impedance of the line. Why didn't you just
give the line impedance? Is it possible to determine the forward and
reverse powers by measurement of a real TEM line of unknown impedance
without measuring everything you need to know to find the line
impedance (and in fact to do so nearly trivially)?

Yeah, I shouldn't try to work these things in my head before
breakfast. Pfor2=200.

Cheers,
Tom