RadioBanter - View Single Post - Re-Normalizing the Smith Chart (Changing the SWR into thesame...

August 29th 03, 03:54 PM

wrote:
But power can only be computed using the resultant voltage and current
(or, if you prefer, NET voltage and current).

That's true for two batteries. That's not true for transmission lines
where Z0 forces the ratio of the traveling waves to be V/I=Z0. If you
could use circuit math on transmission lines, there would be no need
for transmission line math, Smith Charts, etc. You are allowing your
shortcut to try to dictate reality. That won't work. Assume a constant
V/I ratio is forced on each battery in your circuit and see what you get.

Let me make it simpler then: the lamp cord from the wall to the table
lamp; lamp off and lamp on. Most lamp cord looks like twin-lead; some
have said it is approximately 100 Ohms but for the purposes of
discussion feel free to assume any convenient Z0 for the cord.

I don't think you understand the problem. That lamp cord is about
0.0000005 wavelength at 60 Hz. Any reflected wave effects are completely
negligible. It's like expecting reflections due to resistor leads at
HF RF. They are theoretically there but usually too small to measure.

And if it is not an RF transmission line?
If it is a DC circuit? Say your phone line when on hook?
If it is a lamp cord?

The transmission line has to be a non-negligible percentage of a
wavelength for one to have to switch from circuit math to
transmission line math. What's the wavelength of DC? What's the
wavelength of voice audio? What is the wavelength of 60 Hz?

If the phone line is an appreciable percentage of a wavelength,
there will be reflections (echoes) as anyone doing a transcontinental
phone call can attest. Echo cancellation is big business with the
telephone companies. Your theory would make telephone line echoes
impossible for long unterminated lines. It is known that unterminated
telephone lines result in the largest magnitude of echoes.

It just occurred to me that you can revolutionize the telephone
industry if you are right. You could patent your idea of no
reflected energy on a long unterminated telephone line and sell
it to the telephone companies for millions of dollars. Just have
every intercontinental telephone line unterminated by using
high impedance receivers on each end. Since no reflected energy
could cross the voltage minimum point, echoes would be eliminated
without the expensive echo cancellation equipment.

Do we really need directional wattmeters to understand what is
happening?

No, one can learn the principles of EM waves and transmission lines
and alleviate one's ignorance. I am capable of estimating the
forward power, reflected power, and SWR on my 450 ohm window line
without actually measuring it.

Please, replace wall outlet with lamp cord to table lamp.

Are you capable of measuring the 0.0001 degrees of phase shift
between the forward wave and reflected wave? Your measuring
instrument needs to be about plus or minus 0.0000001 degrees
to get an accurate measurement.
--
73, Cecil, W5DXP