Roy Lewallen wrote:
There are several
examples at Food for thought.txt available at
http://eznec.com/misc/food_for_thought/.

Regarding errors in the first food_for_thought:

A 100w source equipped with a circulator and load while
looking into an open line, will generate 100w and dissipate
100w in the circulator load. That 100w is definitely not free
power. It can be demonstrated to have made a round trip to
the open end of the feedline and then back to the circulator
load.

The error in your thinking is that the source would see an open
circuit when it is equipped with a circulator and load. It won't.
It will *always* see the Z0 of the feedline as its load (assuming
the circulator load equals Z0). That's the purpose of using
the circulator and load - to allow the source to see a fixed
load equal to Z0.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Richard Clark wrote:

Cecil Moore wrote:
Optics engineers figured it out a long time ago.

And you have consistently failed in its demonstration - so what?

I can lead you to water but I can't make you drink.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Wed, 29 Jun 2005 22:39:10 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
There lies our misperceptions; I was not referring to using an HF SWR
meter designed for coax and plugging it into 450 ohm ladder line.

But I specifically stated above the Z0 environment was different
from 50 ohms. The same type of error happens when one uses a
50 ohm SWR meter in a 75 ohm coaxial line.


If that were true then the mere existence of standing waves could
render any measurements worthless. Regardless, I did the experiment a
long time ago -- take a 50 ohm SWR meter and plug it into a 75 ohm
line -- it gives you almost the same measurement (in fact, I didn't
see -any- difference at all). Any small error you might see is, as I
said before, insignificant, especially considering the reason you are
measuring SWR in the first place. The objective is simply to get the
reading as low as practially possible. If you feel the need to quibble
about a couple tenths of a point on a ratio then maybe you're spending
a little too much time playing with the calculator instead of the
antenna.





----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Wed, 29 Jun 2005 23:10:25 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:

Wes Stewart wrote:
In a word, baloney. The error is independent of length. A zero length
bridge calibrated at 75 ohm is in error when measuring in a 50 ohm
system. Period.

Prove it.

A 75 ohm bridge is expecting the ratio of voltage to current
to be 75 for a matched system. In a 50 ohm matched system, the
ratio of voltage to current will be 50. Therefore, the 75 ohm
bridge won't be balanced. A 50 ohm bridge would be balanced.


The bridge is calibrated to the impedance of the directional coupler
(which is usually built to match the expected line impedance, but
cannot be "zero length" in the present state of reality). If the
impedance of the signal is different than what is expected by the
bridge then your power measurements will probably be wrong (to what
extent they are wrong may or may not be important). But if that's the
case then any error will be the same by percentage and sign for both
forward =AND= reflected power because the impedance of the signal is
the same for both forward and reflected power. IOW, the ratio is the
same -despite- the impedance.

If you don't believe me, try it yourself.







----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

On Wed, 29 Jun 2005 22:53:24 -0700, Frank Gilliland
wrote:


cannot be "zero length" in the present state of reality). If the
impedance of the signal is different than what is expected by the
bridge then your power measurements will probably be wrong (to what
extent they are wrong may or may not be important). But if that's the
case then any error will be the same by percentage and sign for both
forward =AND= reflected power because the impedance of the signal is
the same for both forward and reflected power. IOW, the ratio is the
same -despite- the impedance.

Lets make an assumption that we are talking about lossless lines. (If
you are not, then the reflectometer does not provide an accurate
indication of forward and reverse power.)

If you use an ideal 50 ohm reflectometer (that means it is a
negligibly short 50 ohm through line and it is nulled to show zero
reflected power when connected to a 50+j0 load) to measure conditions
in a line, the power flow at that point is the indicated Pf-Pr. If you
had placed an ideal 75 ohm instrument in that spot, the readings are
not necessarily in the same ratio (they are unlikely to be so), but
the difference between Pf and Pr will be the same.

The only other inference you can make from one instrument with regard
to the other will be if one of the instruments shows zero reflected
power, then you know the VSWR that the other instrument will indicate.

Real instruments aren't of zero length, but some types of design are
so close to it at low HF frequencies, you will not detect the error
that is introduced.

Owen
--

Updated:

On Wed, 29 Jun 2005 22:53:24 -0700, Frank Gilliland
wrote:


cannot be "zero length" in the present state of reality). If the
impedance of the signal is different than what is expected by the
bridge then your power measurements will probably be wrong (to what
extent they are wrong may or may not be important). But if that's the
case then any error will be the same by percentage and sign for both
forward =AND= reflected power because the impedance of the signal is
the same for both forward and reflected power. IOW, the ratio is the
same -despite- the impedance.

Lets make an assumption that we are talking about distortionless
lines. (If you are not, then the reflectometer does not provide an
accurate indication of forward and reverse power.)

If you use an ideal 50 ohm reflectometer (that means it is a
negligibly short 50 ohm through line and it is nulled to show zero
reflected power when connected to a 50+j0 load) to measure conditions
in a line, the power flow at that point is the indicated Pf-Pr. If you
had placed an ideal 75 ohm instrument in that spot, the readings are
not necessarily in the same ratio (they are unlikely to be so), but
the difference between Pf and Pr will be the same.

The only other inference you can make from one instrument with regard
to the other will be if one of the instruments shows zero reflected
power, then you know the VSWR that the other instrument will indicate.

Real instruments aren't of zero length, but some types of design are
so close to it at low HF frequencies, you will not detect the error
that is introduced.

Owen
--

I'm not quite sure what you are trying to say Frank.

Frank Gilliland wrote:
On Wed, 29 Jun 2005 23:10:25 -0500, Cecil Moore
wrote in :

The bridge is calibrated to the impedance of the directional coupler
(which is usually built to match the expected line impedance, but
cannot be "zero length" in the present state of reality).

The direction coupler samples voltage across and current through a
given point. There is always a current transformer of some type and a
voltage sample through some type of divider. The "voltages"
representing E and I are summed before detection (conversion to dc).

The "directivity" comes because the current phase sample is reversed
180 degrees from the summing phase, causing voltages to subtract.

This means the directional coupler is calibrated for a certain ratio of
voltage and current, so when they exist you have twice the voltage in
the direction where E and I add, and zero voltage where they subtract.


If the
impedance of the signal is different than what is expected by the
bridge then your power measurements will probably be wrong (to what
extent they are wrong may or may not be important). But if that's the
case then any error will be the same by percentage and sign for both
forward =AND= reflected power because the impedance of the signal is
the same for both forward and reflected power. IOW, the ratio is the
same -despite- the impedance.

?What does that mean?

If the directional coupler is calibrated at 50 ohms and you use it in a
75 ohm system you won't get a total reflected null even if the 75 ohm
line has a 1:1 SWR. But if you subtract reflected power from forward
power readings you will get the correct power, within linearity and
calibration limits of the "meter system". This has nothing to do with
standing waves. It has only to do with the relationship between current
and voltage at the point where the directional coupler is inserted.

I'm not sure if you are saying that or not.

73 Tom

Frank Gilliland wrote:

Cecil Moore wrote:
But I specifically stated above the Z0 environment was different
from 50 ohms. The same type of error happens when one uses a
50 ohm SWR meter in a 75 ohm coaxial line.

If that were true then the mere existence of standing waves could
render any measurements worthless. Regardless, I did the experiment a
long time ago -- take a 50 ohm SWR meter and plug it into a 75 ohm
line -- it gives you almost the same measurement (in fact, I didn't
see -any- difference at all).

Please run it again in the following configuration:

Xmtr--1/4WL 75 ohm line--SWR meter--1/4WL 75 ohm line--50 ohm load

The SWR meter will read 2.25:1 when the actual SWR is 1.5:1

Xmtr--1/2WL 75 ohm line--SWR meter--1/2WL 75 ohm line--50 ohm load

The SWR meter will read 1:1 when the actual SWR is 1.5:1

Any small error you might see is, as I
said before, insignificant, especially considering the reason you are
measuring SWR in the first place.

A 50% error in SWR reading is NOT insignificant.

The objective is simply to get the
reading as low as practially possible. If you feel the need to quibble
about a couple tenths of a point on a ratio then maybe you're spending
a little too much time playing with the calculator instead of the
antenna.

A 50% error in SWR is NOT a couple tenths of a point.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Frank Gilliland wrote:

Cecil Moore wrote:
A 75 ohm bridge is expecting the ratio of voltage to current
to be 75 for a matched system. In a 50 ohm matched system, the
ratio of voltage to current will be 50. Therefore, the 75 ohm
bridge won't be balanced. A 50 ohm bridge would be balanced.

The bridge is calibrated to the impedance of the directional coupler
(which is usually built to match the expected line impedance, but
cannot be "zero length" in the present state of reality). If the
impedance of the signal is different than what is expected by the
bridge then your power measurements will probably be wrong (to what
extent they are wrong may or may not be important). But if that's the
case then any error will be the same by percentage and sign for both
forward =AND= reflected power because the impedance of the signal is
the same for both forward and reflected power. IOW, the ratio is the
same -despite- the impedance.

The error is NOT the same percentage. In a matched 50 ohm system,
the 75 ohm bridge reflected power reading will be off by an
infinite percentage, i.e. division by zero.

If you don't believe me, try it yourself.

I have tried it and you are wrong. Maybe you should try it.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Owen wrote:
Real instruments aren't of zero length, but some types of design are
so close to it at low HF frequencies, you will not detect the error
that is introduced.

The error that we are talking about has nothing to do with
the length of the directional coupler.

The error that we are talking about has everything to do
with an infinite error in the measurement of reflected power.
Infinite errors are hard to sweep under the rug.
--
73, Cecil http://www.qsl.net/w5dxp

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----