On 17 jun, 18:45, Roy Lewallen wrote:
Owen Duffy wrote:

While Zo of transmission lines might not be purely real, if the sampler
element is calibrated for V/I being real, then the power is given by
'forward power' - 'reflected power'. This is true even if the
calibration impedance is different to the transmission line in which the
measurements are made, though significant departure will impact
measurement uncertainty.

For example, if I insert a 50 ohm Bird 43 in a 75 ohm line at 1.8MHz,
and measure Pf=150W and Pr=50W, then the power is 150-50=100W. *The
insertion VSWR due to the Bird 43 is trivial in this case, so it hardly
disturbs the thing being measured. Each of the power measurements is of
little value alone, no inference can be made (in this case) of the
actual line VSWR, but the difference of the Pf and Pr readings does give
the power at that point.

I discuss this in my article entitled "http://vk1od.net/blog/?p=1004" at
http://vk1od.net/blog/?p=1004.

Owen

Adding a directional wattmeter to the mix raises an interesting issue.

Recalling my earlier example of a 100 watt transmitter connected to a
half wavelength 200 ohm transmission line and then to a 50 ohm resistive
load, the "forward power" on the line was 156.25 watts and the "reverse
power" or "reflected power" 56.25 watts. The subject of this topic asks
where the "reflected energy" goes. Does it go back into the transmitter?
Well, let's put a 50 ohm directional wattmeter between the transmitter
and the line. It measures 100 watts forward and 0 watts reverse. So what
happened to the "reverse power" on the transmission line? If it was
going back into the transmitter, we've now fixed the problem just by
adding the wattmeter. In fact, we could replace the wattmeter with a
piece of 50 ohm transmission line of any length, as short or long as we
want, and the "forward power" on that line will be 100 watts and
"reverse power" zero. So we've protected the transmitter from this
horrible, damaging "reverse power" just by adding a couple of inches of
50 ohm line -- or a directional wattmeter. Is this cool or what?

Or we can put the wattmeter at the far end of the line and read 100
watts forward and zero watts reverse, eliminating "reverse power" at the
load -- although we've lost 56.25 watts of "forward power". Or put it at
the center of the line, where we'd read a whopping 451.6 watts forward
and 351.6 reverse(*).

These are the "forward power" and "reverse power" on the short 50 ohm
transmission line (or lumped equivalent) inside the wattmeter. So we can
create "forward power" and "reverse power" just by moving the wattmeter
around. And most importantly, we can use it to isolate the transmitter
from that bad "reverse power". What a powerful tool!

I should point out that the example doesn't even mention, or need to
mention, the transmitter output impedance. The entire analysis holds for
any transmitter impedance, whether "dissipative", "non-dissipative",
linear, or nonlinear. Wherever the "forward power" and "reverse power"
come from and do to, it doesn't depend on any particular value or kind
of transmitter impedance.

(*) This brings up a great idea. Drop down to Radio Shack
or HRO and pick up one of those circulator things that separates forward
and reverse power. Put it into the middle of the line where you have
451.6 watts of "forward power" and 351.6 watts of "reverse power". Take
the 351.6 watts of "reverse power", rectify it, send it to an inverter,
and use it to run the transmitter -- you'll have plenty, even with poor
efficiency, and you can unplug the transmitter from the mains. There
should even be enough power left over to run your cooler and keep a
six-pack cold. Then work DX with your 451.6 watts of forward power while
you enjoy a cool one. Goodbye to electric bills! Hello to DX, free
power, and a tall cool one!

Roy Lewallen, W7EL- Ocultar texto de la cita -

- Mostrar texto de la cita -

Dear Roy:
This is a brilliant piece of work, it clarify so much misconceptions
(or extension of certain concepts beyond its scope). However the
question of "why" persist floating somehow.
Without leaving conventional approachs I think in the article cited in
the "Where does it go?" thread there are some helpful and valuable
hints: http://www.ittc.ku.edu/~jstiles/622/...es_package.pdf

Page 25 = "The precise values of Vo+, Io+ Vo+ and Io- are therefore
determined by satisfying the boundary conditions applied at each end
ofthe transmission line.
Page 28 = Although Vo+- and Io+- are determined by boundary
conditions
(i.e., what’s connected to either end of the transmission line),
the ratio V+- / V+- is determined by the parameters of the
transmission line only ( R, L, G, C).

What it is your opinion about?

73 - Miguel Ghezzi - LU6ETJ