V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
an interesting diversion until everyone was convinced that you were off the
wall. now its time to re-open a dead thread???.

No, an interesting diversion that has been resolved. Time to return
to the real issue.

as far as cheap swr meters, the daiwa, swan, and mfj manuals all require 50
ohm coax 'for accurate readings'... a joke by any standard of measurement
for that type of instrument. but just to put this one to rest quickly... i
set up my tdr and ran some quick measurements. this tdr will resolve a 6"
75 ohm jumper in the 25' or so of 50 ohm test cable that i used. i measured
an mfj-815b and a daiwa ns-660pa and they are indistiguishable from the 50
ohm line. so the answer is yes, they do internally look like a 50 ohm line
section.

Try it with 75 ohm coax. I suspect they will be equally indistinguishable
from 75 ohm line.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Dave wrote:
don't bother, its a figment of cecil's imagination and creative vocabulary.

Actually, the question logically follows from the reflection model.
Under what boundary conditions does a piece of transmission line
force Vfor/Ifor = Vref/Iref = Z0 to a specified accuracy?
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
however, once you change the
reference Zo to 50 ohms inside the meter there is no reflected power to
measure since the load presented at the end of the 75 ohm line is 50 ohms.

The question remains: Does the MFJ's physical design ensure the
*physical* reference Z0 is 50 ohms or would it perform just as
well in a 75 ohm enviornment simply by recalibrating it for
75 ohms?

In other words, it is sampling a voltage and current completely
divorced from any *physical* reference Z0. The actual reference
depends upon an arbitrary setting of a calibration cap which
has no effect at all on the *physical* reference Z0.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 17:52:47 GMT, Cecil Moore wrote:
I cannot remember for sure.
Let's see, you know the news group, you know the context, you know the
time, you know the correspondents, you may have even "quoted" the
material (now truly suspect) - but sadly you forget the details that
are somehow unretrievable from a simple google search with such a
wealth of key terms and specific constraints.

And from this you've been sowing an itinerant thought as proof....

How is this distinguished from trolling?

V/I ratio is forced to Z0:was Mythbusters
 

On Sat, 15 Oct 2005 18:25:02 GMT, Cecil Moore wrote:
a calibration cap which has no effect at all on the *physical* reference Z0.

If it had an effect, the Z would change, wouldn't it? The cap has a
Z, that much is sure. The purpose of the cap is not what you ascribe
to it - but that is par for the course as this inclusion quoted above
has no meaning either in or out of context.

V/I ratio is forced to Z0:was Mythbusters
 

"Cecil Moore" wrote in message
m...
Dave wrote:
an interesting diversion until everyone was convinced that you were off
the wall. now its time to re-open a dead thread???.

No, an interesting diversion that has been resolved. Time to return
to the real issue.

as far as cheap swr meters, the daiwa, swan, and mfj manuals all require
50 ohm coax 'for accurate readings'... a joke by any standard of
measurement for that type of instrument. but just to put this one to
rest quickly... i set up my tdr and ran some quick measurements. this
tdr will resolve a 6" 75 ohm jumper in the 25' or so of 50 ohm test cable
that i used. i measured an mfj-815b and a daiwa ns-660pa and they are
indistiguishable from the 50 ohm line. so the answer is yes, they do
internally look like a 50 ohm line section.

Try it with 75 ohm coax. I suspect they will be equally indistinguishable
from 75 ohm line.

ok, just for you i swapped all the cables around. and you are wrong, it is
not indistinguishable from 75 ohms, it is easily picked out as a 50 ohm
section.

What is a 50-ohm environment. ???
 

"Cecil Moore" wrote in message
m...
Dave wrote:
don't bother, its a figment of cecil's imagination and creative
vocabulary.

Actually, the question logically follows from the reflection model.
Under what boundary conditions does a piece of transmission line
force Vfor/Ifor = Vref/Iref = Z0 to a specified accuracy?

then look up article 3.19 in 'fields and waves in communications
electronics' which is where they derive the fringe effects for a step change
in spacing between two plates. this is where they refer you for calculating
the effects of evanescent modes from a more complex problem stating that the
results are identical with the static case in 3.19... in the derivation of
the complex case of a step change in a cable they show that a single lumped
capacitance added at the step is an adequate representation of the
discontinuity when calculating the evanescent modes below cutoff. in 3.19
if you disregard the fields along the length of the step it ends up in an
equation: Z=h/pi(exp(pi*W/V0)-1-pi*W/V0_j*pi) the important part is
obvioulsy the decay factor in the exponential which goes as exp(-x/h) where
h is the separation in the planes... or you get one 1/e reduction for each
distance equal to the spacing which in rg-58 or rg-8x that i used is
something like 1mm or less.. so how far down do you want to be? in 4.5mm
you are down to 1% which is well under the accuracy of these cheap meters,
and much less than the length of even the connectors on the meters.

Mythbusters: V/I ratio is forced to Z0
 

On Sat, 15 Oct 2005 17:57:35 GMT, Cecil Moore wrote:


Try it with 75 ohm coax. I suspect they will be equally indistinguishable
from 75 ohm line.

Cecil, you have got everyone else running around based on your
apparent misconceptions.

Is it not time you put in some time on the experimental side to
support / validate your conceptual contribution?

Enough of the "I suspect", "I seriously doubt". Where does that fit in
scientific method... you are not the eminent professor running a bunch
of PHD students around.

Owen
--

V/I ratio is forced to Z0:was Mythbusters
 

Richard Clark wrote:
On Sat, 15 Oct 2005 18:25:02 GMT, Cecil Moore wrote:
a calibration cap which has no effect at all on the *physical* reference Z0.

If it had an effect, the Z would change, wouldn't it? The cap has a
Z, that much is sure. The purpose of the cap is not what you ascribe
to it - but that is par for the course as this inclusion quoted above
has no meaning either in or out of context.

Here's a schematic of an MFJ SWR meter at the bottom:

http://www.mfjenterprises.com/man/pdf/MFJ-816.pdf

What do you think is the purpose of the 10pf variable cap
if not to vary the voltage in the voltage divider?

The question is: Between the "transmitter" terminal and
the "antenna" terminal, what determines the physical
characteristic impedance of the sampling circuit?
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
ok, just for you i swapped all the cables around. and you are wrong, it is
not indistinguishable from 75 ohms, it is easily picked out as a 50 ohm
section.

What causes that?
--
73, Cecil http://www.qsl.net/w5dxp