Mythbusters: V/I ratio is forced to Z0
 

On Sat, 15 Oct 2005 14:38:15 GMT, Cecil Moore wrote:
long before you entered the thread
More guessing, and certainly poor research. As many may acknowledge,
a glance at the beginning of this thread dominates all speculation
about who entered when.

No doubt a response is in order as to what you "meant" - as is so
often the case.

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
"Cecil Moore" wrote:

Owen Duffy wrote:
This has nothing to do with the stated myth: Measurements with a Bird
43 of the conditions on the Thruline section are invalid unless it has
some minimum length of 50 ohm line on both sides of itself.

Would you be willing to make the same statement about an MFJ wattmeter?

now your are just trying to muddy the waters... i wouldn't trust an mfj to
measure anything!

No, I'm just trying to get back to the original discussion which
was: Do SWR meters need 50 ohm coax surrounding them to establish
the assumed 50 ohm environment? The majority of SWR meters have
no Thruline and MFJ seems to make no attempt to establish a 50
ohm environment like Bird does. What is the Z0 of a meandering wire
surrounded by an aluminum box?

The original discussion (V/I ratio is forced to Z0) had nothing to
do with Bird wattmeters. The original discussion was about SWR meters
in general (which the Bird is not). The mythbusters thing was an
interesting diversion away from the original question which remains
unanswered.
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

Richard Clark wrote:

Cecil Moore wrote:
I guess you will take that delusion to your grave.

Yet more guessing?

It's a guess as to whether you are going to correct
your errors about thin-film coatings before you
expire. What is it about the following that you don't
understand?

www.mellesgriot.com/products/optics/oc_2_1.htm

"Thin-Film Interference"

"Thin-film coatings also rely on the principles of interference. Thin
films are dielectric or metallic materials whose thickness is comparable
to, or less than, the wavelength of light."

Speaking of 1/4WL thin-film coatings:

"Clearly, if the wavelength of the incident light and the thickness
of the film are such that a phase difference exists between reflections
of p, then reflected wavefronts interfere destructively, and overall
reflected intensity is a minimum. If the two reflections are of equal
amplitude, then this amplitude (and hence intensity) minimum will be
zero."

"Amplitude minimum equal zero" means all reflections have been eliminated.
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

Richard Clark wrote:

Cecil Moore wrote:
long before you entered the thread

Canceled it immediately after I wrote it when I realized the
subject line had changed.

More guessing, and certainly poor research. As many may acknowledge,
a glance at the beginning of this thread dominates all speculation
about who entered when.

No doubt a response is in order as to what you "meant" - as is so
often the case.

No doubt an apology is in order. The posting no longer exists
on my news-server. I apologize that my canceled posting made
it off my news-server. Sorry, my subject line font is so small
I cannot read it without a magnifying glass.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

- or even more vague - an assumed 50-ohm environment?

Or an evironment of any other impedance.

An engineering definition please! In plain English.

Then perhaps I can understand what you are all waffling about.

Thank you.
----
Reg, G4FGQ.

V/I ratio is forced to Z0:was Mythbusters
 

"Cecil Moore" wrote in message
om...
Dave wrote:
"Cecil Moore" wrote:

Owen Duffy wrote:
This has nothing to do with the stated myth: Measurements with a Bird
43 of the conditions on the Thruline section are invalid unless it has
some minimum length of 50 ohm line on both sides of itself.

Would you be willing to make the same statement about an MFJ wattmeter?

now your are just trying to muddy the waters... i wouldn't trust an mfj
to measure anything!

No, I'm just trying to get back to the original discussion which
was: Do SWR meters need 50 ohm coax surrounding them to establish
the assumed 50 ohm environment? The majority of SWR meters have
no Thruline and MFJ seems to make no attempt to establish a 50
ohm environment like Bird does. What is the Z0 of a meandering wire
surrounded by an aluminum box?

The original discussion (V/I ratio is forced to Z0) had nothing to
do with Bird wattmeters. The original discussion was about SWR meters
in general (which the Bird is not). The mythbusters thing was an
interesting diversion away from the original question which remains
unanswered.
--

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

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.

What is a 50-ohm environment. ???
 

"Reg Edwards" wrote in message
...
- or even more vague - an assumed 50-ohm environment?

Or an evironment of any other impedance.

An engineering definition please! In plain English.

Then perhaps I can understand what you are all waffling about.

Thank you.

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

Mythbusters: V/I ratio is forced to Z0
 

On Sat, 15 Oct 2005 16:18:22 GMT, Cecil Moore wrote:
Yet more guessing?
It's a guess
and it took 32 lines to say that? I can understand why editors dump
your submissions.

What is a 50-ohm environment. ???
 

Reg Edwards wrote:
- or even more vague - an assumed 50-ohm environment?
Or an evironment of any other impedance.

The transmission line reflection model tells us that the Z0
of a transmission lines forces the following relationship.

Vfor/Ifor = Vref/Iref = Z0

A "50 ohm environment" used in the context of the previous
discussion would be one in which the above relationship
is forced on the system at certain points in the system.

600 ohm transmission line going from a tuner/balun to an
antenna establishes a 600 ohm environment for the signals
on the transmission line. An SWR meter calibrated for 600
ohms will indicate the actual SWR.

About a year ago, based on a discussion that you and I were
having, I asked the sci.physics.electromag newsgroup how
long a piece of RG-213 coax has to be to establish the
above relationship. The answer was that the non-TEM product
terms decrease at about 1/e every two inches for RG-213.

The RG-400 coax leads going to and from my SWR meter are
two feet each. So I asserted to you that my SWR meter
was reading the actual SWR in the middle of that run of
RG-400. I think you disagreed with my assertion but I
cannot remember for sure.
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

"Dave" wrote in message
. ..

"Cecil Moore" wrote in message
om...
Dave wrote:
"Cecil Moore" wrote:

Owen Duffy wrote:
This has nothing to do with the stated myth: Measurements with a Bird
43 of the conditions on the Thruline section are invalid unless it has
some minimum length of 50 ohm line on both sides of itself.

Would you be willing to make the same statement about an MFJ wattmeter?

now your are just trying to muddy the waters... i wouldn't trust an mfj
to measure anything!

No, I'm just trying to get back to the original discussion which
was: Do SWR meters need 50 ohm coax surrounding them to establish
the assumed 50 ohm environment? The majority of SWR meters have
no Thruline and MFJ seems to make no attempt to establish a 50
ohm environment like Bird does. What is the Z0 of a meandering wire
surrounded by an aluminum box?

The original discussion (V/I ratio is forced to Z0) had nothing to
do with Bird wattmeters. The original discussion was about SWR meters
in general (which the Bird is not). The mythbusters thing was an
interesting diversion away from the original question which remains
unanswered.
--

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

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.


ah, good cecil hasn't replied yet... sorry, i should have put this in the
same message, but the sun was trying to come out and i had to take advantage
of it. we haven't seen the sun here for the last 10 days which is the only
reason i was bored enough to bother getting into one of these 'discussions'
again.

now for the other part of the story. cecil states that a 50 ohm
'environment' must be established around these meters to get proper
readings. this 'environment' requires some unknown length of 50 ohm cable
on both sides of the meter. i can disprove this one easily... i again used
the two meters above. first, attach 50 ohm dummy load to the meter, set
transmitter to 100w forward on the meter, read reverse... there is always
some small movement, but not much more than a needle width on either one.
now, get 1/2 wave of 75 ohm cable, attach between the dummy load and the
meter antenna output... again adjust for 100w forward and voila, same needle
wiggle as before. what does this mean? well, if indeed the meter was
relying on the length of 50 ohm coax on the output to 'establish the
environment' as cecil seems to think is necessary then the 75 ohm cable
should have upset this 'environment' and caused an incorrect reading...
since the reading was correct for a 50 ohm meter then it must be that the
required length of 50 ohm cable is zero.

now of course cecil will rant and rave and say the meter is reading
incorrectly since there is obviously some reflected power in the 75 ohm
cable. and he will be correct on that point, 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.
QED.