V/I ratio is forced to Z0:was Mythbusters
 

On Sat, 15 Oct 2005 22:36:11 GMT, Cecil Moore wrote:

these voltages match the voltage drops across R1 and R2
in amplitude.

There is NO R1 and/or R2

This is familiar bait and switch methods so common to your trolling.
Changing the schematics to suit the argument can make any answer come
alive.

Like I said, if you were already familiar with this, you would have
said so and skipped all this stageshowing.

In other words, it is the main Z0 calibration setting

An interpretation spanning as many documents as it takes to assemble
all the words xeroxed to this posting.

If the range of C1 and C2 are great enough, the wattmeter
could be calibrated for 75 ohms rather than 50 ohms.

Notably, there are no C1 and/or C2. Even more, if there were, any
competent tech in this group would be aware that it also would take a
similar shift in value of your R1 and/or R2. In fact, this
overstatement by calling out BOTH Cs needing change begs the chuckle
reflex being stifled.

The question is: Between the "transmitter" terminal and
the "antenna" terminal, what determines the physical
characteristic impedance of the sampling circuit?

It is very lightly loading as a series load by
design and as evidenced by Dave's measurements.

Exactly how much effect does that light loading have on
the primary voltage/current amplitude and phase? Enough
to be detectable if the V/I ratio is not 50 ohms?

[clue: this is in the portion of my original post that you omitted.]

I would normally ask my students "What do you think?" but there is a
danger of that here, with so little obtained in return for so much
said.

Rather, I will offer "Think about it. How much power does it take to
swing a 1mA meter even full scale when there is a 500KOhm calibration
resistor (imagine that, do we assign THAT as the Z?) limiting it."

Something about I²R comes to mind for max power (300W) which works out
to half a watt out of 300 supplied. A simple, back-of-the-napkin
computation suggests less than 1%. A similar study of the actual load
of 82 Ohms needs only be cast back through the N² turns ratio. Seeing
that it, too, is probably a quarter Watt resistor that we are still
talking about less than 1% consumption.

So, anyway you slice and dice it without actually counting the turns
ratio, the tech at the bench would easily offer the meter injects only
half an ohm or less series resistance into the line.

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 22:37:18 GMT, Cecil Moore wrote:
Which is better? Third hand or under hand? :-)
If you can't get it across the plate, neither.

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 22:55:14 GMT, Cecil Moore wrote:
The guy from sci.physics.electromag whom I quoted previously
in the "V/I forced to Z0" thread. It appears that he
accidentally replaced the conductor spacing of 0.203" with
2.03" in his calculations.
Nice thing about third hand quotes, you can blame them for your
transcription errors and they can't defend themselves.

What is a 50-ohm environment. ???
 

Owen Duffy wrote:
So you were aware of the apparent defect in the work you were citing
as recently as less than two hours ago, and it took someone else to
notify the defect here?

You should have been able to figure it out from the posting times.
I reported the problem at 5:55pm, after you had reported it but
before I had read your posting which reported it. You've heard of
two ships passing in the night? Ain't usenet great?
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Dave wrote:
ugh! all of this was over a slipped decimal point??? so we are down to .2"
transition, which pretty much agrees with the one i came up with, and which
basically means that by the time you are out of the connector shell you are
back at Z0. and since the meter takes its own 50 ohm 'environment' with it
for sampling it is reading every thing exactly as it should... and exactly
as has been measured... and there is no requirement for some particular
length of 50 ohm coax on either side of a meter... what a waste of a
perfectly good argument, you better apologize big time for this one cecil!

I do apologize for the slip of the decimal point and for not finding it
before today but I think you guys completely missed the context of the
original argument and instead went off on several interesting tangents.

Now back to the original context: As Reg pointed out, you can't have an
SWR on a feedline that doesn't exist so how much 50 ohm feedline must
exist *external to the SWR meter* to be able to report a valid 50 ohm SWR
reading *on that coax*? Turns out to be around an inch, more or less, and
that is a good thing to know. Thanks for answering the question even if
in a time consuming way. If you had simply said that 0.2" of 50 ohm
coax is required to establish a 50 ohm environment *external to the SWR
meter*, we could have started arguing from that point and it would have
saved a lot of time and effort.
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0
 

Richard Clark wrote:

Cecil Moore wrote:
these voltages match the voltage drops across R1 and R2
in amplitude.

There is NO R1 and/or R2

I gave the reference, Richard, and am looking at it right now.
Page 27-9 of the 15th edition of the ARRL Antenna Book. R1 and
R2 are 10 ohms each.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Richard Clark wrote:
Nice thing about third hand quotes, you can blame them for your
transcription errors and they can't defend themselves.

The evidence is always on Google.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

On Sun, 16 Oct 2005 04:20:58 GMT, Cecil Moore wrote:
Nice thing about third hand quotes, you can blame them for your
transcription errors and they can't defend themselves.
The evidence is always on Google.
Three degrees of separation between a poor third hand quote and its
validation.

What is a 50-ohm environment. ???
 

On Sun, 16 Oct 2005 04:16:56 GMT, Cecil Moore wrote:
but I think you guys completely missed the context
What is the validity of context when it is buried in error? This is
this is the pollution of argument that discards proper method and
accuracy to prove absolutist principles.

The point you miss is that when the errors are corrected, they
invalidate your premise - context is quite obviously out the window at
that point.

V/I ratio is forced to Z0
 

On Sun, 16 Oct 2005 04:20:01 GMT, Cecil Moore wrote:
I gave the reference, Richard, and am looking at it right now.
On Sat, 15 Oct 2005 20:37:11 GMT, Cecil Moore wrote:
Here's a schematic of an MFJ SWR meter at the bottom:
http://www.mfjenterprises.com/man/pdf/MFJ-816.pdf
Like I said, your common technique of bait and switch to please your
argument. This technique is so shop-worn that you couldn't even
respond in the same thread - what a wheeze. Please pretend you don't
understand... again. This is probably another reason why the editors
dump your submissions.