On Wed, 29 Jun 2005 19:06:14 -0700, Wes Stewart
wrote in :

On Wed, 29 Jun 2005 14:07:26 -0700, Frank Gilliland
wrote:

On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore
wrote in :

Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most
SWR meters used for HF have a directional coupler that is much shorter
than the operating wavelength.

Point is that they are usually calibrated for Z0=50 ohms
and are in error when used in Z0 environments differing
from Z0=50 ohms, e.g. Z0=75 ohms.


The point is that the error is insignificant when the directional
coupler is much shorter than the wavelength.

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.


The error is even more
insignificant when there are a host of variables and confounds between
the SWR meter and the transmitted field that can (and frequently do)
affect the objective -- field strength.

Often, field strength is of zero importance. What do you do when the
device under test isn't supposed to radiate?


That device probably wouldn't make a very good radio, would it?


The simplest example of
this would be a CATV system, yet VSWR is *extremely* important in
cascaded networks.


Thank you for making my point.


It's much simpler (and just
plain logical) to measure the field strength directly instead of
measuring an abstract value halfway towards the objective and relying
on nothing more than speculation that the rest is working according as
expected.

More baloney and it isn't even sliced.


The word is "blarney". And although the syntax of my statement was
somewhat 'convoluted', the logic is sound -- you can dyno your engine
all day, but the only way to know for sure how fast you can get down
the quarter mile is to run the race.





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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.
--
73, Cecil http://www.qsl.net/w5dxp

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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.







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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:
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

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On Wed, 29 Jun 2005 19:42:53 -0700, Frank Gilliland
wrote:

[snipped in the interest of brevity]

The error is even more
insignificant when there are a host of variables and confounds between
the SWR meter and the transmitted field that can (and frequently do)
affect the objective -- field strength.

Often, field strength is of zero importance. What do you do when the
device under test isn't supposed to radiate?


That device probably wouldn't make a very good radio, would it?

My "SWR Meter" is one of these:

http://users.adelphia.net/~n2pk/VNA/VNAarch.html

I have 1 mW to radiate. What kind of FSM should I use?



The simplest example of
this would be a CATV system, yet VSWR is *extremely* important in
cascaded networks.


Thank you for making my point.

Not even you have made your point.



It's much simpler (and just
plain logical) to measure the field strength directly instead of
measuring an abstract value halfway towards the objective and relying
on nothing more than speculation that the rest is working according as
expected.

More baloney and it isn't even sliced.


The word is "blarney".

My Webster's says:

baloney n (bologna) : pretentious nonsense : BUNKUM --- often used as
a generalized expression of disagreement....

I could not be more accurate.


And although the syntax of my statement was
somewhat 'convoluted',

A ray of hope

the logic is sound

smashed

-- you can dyno your engine
all day, but the only way to know for sure how fast you can get down
the quarter mile is to run the race.

Uh huh. By this convoluted "logic" I guess you would avoid any dyno
testing at all and just go do hit-and-miss tuning at the drag strip.

Frank wrote, "Prove it."

OK, here I am at the track (the bench). I have an SWR meter that I've
verified with my HP8653 to behave like a short section of 50 ohm line
at the frequency of interest. I put a load on its output that I've
also verified to be 50 ohms at the frequency of interest. I've applied
power to the load through the SWR meter. The indicated SWR is 1.23:1.

I took the SWR meter apart, and located a particular resistor. I
changed its value slightly. I re-verified that the meter still looks
like a short section of 50 ohm line. I re-ran the experiment of
applying power through the meter to the load. The indicated SWR is now
1.05:1.

Yes, I really have done that!

This particular meter is built, as very many of them are, to sample
current and voltage at a point of essentially zero length on the line.
The current sample (through a current transformer: line center passes
through a toroid; secondary is several turns, loaded by that
calibration resistor) is converted to a voltage by dropping it through
a resistance, and by changing that resistance, I can change the
relative amount the current contributes to the measurement. In other
words, if the voltage sample is v(samp)=k*v(line), I want to adjust the
current sampling so v(i(samp)) = k*Zo*i(line), where Zo is the
impedance to which the meter is calibrated to measure SWR. In some
meters, there is a means to adjust the voltage sampling ratio easily
with a variable trimmer capacitor. Either way works. The adjustment
DOES have a TINY effect on the impedance the meter presents to the line
it's in, but that is very minor, compared with the range of adjustment
of the impedance calibration value.

Yes, I really have adjusted a meter which uses the variable capacitor,
too.

Cheers,
Tom

K7ITM wrote:
Yes, I really have adjusted a meter which uses the variable capacitor,
too.

In the old Heathkit SWR meter were instructions to
install either two 50 ohm resistors for a 50 ohm SWR
meter or two 75 ohm resistors for a 75 ohm SWR meter.
We used a lot of RG-11 back in those days.
--
73, Cecil, http://www.qsl.net/w5dxp

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Earlier today, I wrote, "...HP8653..." Ooops. Belay that. It's
HP8753E.