BTW, directional wattmeters for the ham market are often not capable of
reasonable accuracy on loads other than the nominal 50 ohm load. There
are a range of tests that such an instrument should satisfy, but for
hams, it is usually considered sufficient if the "reflected" reading is
approximately zero on a 50 ohm load.

I should think, though, that one could calibrate such a
reflectometer/directional wattmeter. That is, you could test it with a
suitable variety of source and load impedances and develop a fairly
simple arithmetic correction that would be accurate.

The interesting question might be whether you could unambiguously take a
particular fwd and rev reading and turn that into a true fwd and true
rev, essentially solving for the mismatch.

Down in the lab here at work we have a whole rack of precision
misterminations (1.1:1, 1.2:1, 1.5:1, etc.) that some talented engineer
built and calibrated some decades ago. They're built on the Maury
bluedot N terminations.



Owen

Jim Lux wrote in news:f9i1i3$8v5$1
@nntp1.jpl.nasa.gov:


BTW, directional wattmeters for the ham market are often not capable
of
reasonable accuracy on loads other than the nominal 50 ohm load. There
are a range of tests that such an instrument should satisfy, but for
hams, it is usually considered sufficient if the "reflected" reading
is
approximately zero on a 50 ohm load.

I should think, though, that one could calibrate such a
reflectometer/directional wattmeter. That is, you could test it with a
suitable variety of source and load impedances and develop a fairly
simple arithmetic correction that would be accurate.

Yes Jim, some of the deficiencies of the instrument fall to things like
an equal response from the separate forward and reverse couplers. Scale
shape is an issue (especially where the sensitivity is continuously
adjustable using a pot). Phase and amplitude response of the coupler over
the frequency range is another issue not so readily calibrated out. A
coupler that is long will underestimate rho, and some couplers insert
more mismatch than they pretend to measure.

In my experience, many of the instruments that are claimed to work up to
144MHz band might well indicate close to 1:1 on a dummy load, but they do
not indicate rho=1 on a s/c or o/c. Whilst they may serve their purpose
as a null indicator on a 50 ohm load, they are not suited to the loss
measurement such as Jimmie is performing.


The interesting question might be whether you could unambiguously take
a
particular fwd and rev reading and turn that into a true fwd and true
rev, essentially solving for the mismatch.

I don't think you can compensate for lack of f/b ratio in the coupler,
for example because the coupled lines are too long.


Down in the lab here at work we have a whole rack of precision
misterminations (1.1:1, 1.2:1, 1.5:1, etc.) that some talented engineer
built and calibrated some decades ago. They're built on the Maury
bluedot N terminations.

I have always though that a budget priced set of mismatches would be real
handy, and have wondered why MFJ (or someone else for that matter) don't
offer a set for checking / calibration of the MFJ259B etc.

Owen

On Aug 10, 2:28 pm, Owen Duffy wrote:
....
I don't think you can compensate for lack of f/b ratio in the coupler,
for example because the coupled lines are too long.
....
I'm curious what you mean by that, Owen...

Cheers,
Tom

K7ITM wrote in news:1186788470.852002.260460
@b79g2000hse.googlegroups.com:

On Aug 10, 2:28 pm, Owen Duffy wrote:
...
I don't think you can compensate for lack of f/b ratio in the coupler,
for example because the coupled lines are too long.
...
I'm curious what you mean by that, Owen...

Tom, I was thinking of several instruments, all of the coupled lines type
of construction, that on a s/c and o/c failed to indicate rho=1, and showed
similar readings when physically reversed, suggesting it was not just a fwd
/ rev matching issue, there was something about the coupler that was too
dependent on the location of the SWR pattern relative to the coupler. Since
they worked better at lower frequencies, the length of the coupler was
likely to be a contribution.

Owen

"Owen Duffy" wrote in message
...
K7ITM wrote in news:1186788470.852002.260460
@b79g2000hse.googlegroups.com:

On Aug 10, 2:28 pm, Owen Duffy wrote:
...
I don't think you can compensate for lack of f/b ratio in the coupler,
for example because the coupled lines are too long.
...
I'm curious what you mean by that, Owen...

Tom, I was thinking of several instruments, all of the coupled lines type
of construction, that on a s/c and o/c failed to indicate rho=1, and
showed
similar readings when physically reversed, suggesting it was not just a
fwd
/ rev matching issue, there was something about the coupler that was too
dependent on the location of the SWR pattern relative to the coupler.
Since
they worked better at lower frequencies, the length of the coupler was
likely to be a contribution.

Owen


Would this be a problem for a directional coupler designed for a specific
frequecy?


Jimmie

"Jimmie D" wrote in
:


"Owen Duffy" wrote in message
...
K7ITM wrote in news:1186788470.852002.260460
@b79g2000hse.googlegroups.com:

On Aug 10, 2:28 pm, Owen Duffy wrote:
...
I don't think you can compensate for lack of f/b ratio in the
coupler, for example because the coupled lines are too long.
...
I'm curious what you mean by that, Owen...

Tom, I was thinking of several instruments, all of the coupled lines
type of construction, that on a s/c and o/c failed to indicate rho=1,
and showed
similar readings when physically reversed, suggesting it was not just
a fwd
/ rev matching issue, there was something about the coupler that was
too dependent on the location of the SWR pattern relative to the
coupler. Since
they worked better at lower frequencies, the length of the coupler
was likely to be a contribution.

Owen


Would this be a problem for a directional coupler designed for a
specific frequecy?


Jimmie

Jimmie, I am talking about the el-cheap inline SWR / Power Meter that is
often sold to hams with unrealistic specs.

You can / should always test the performance of the kit you are using to
determine if you should have confidence in it.

There are a bund of notes on testing a directional wattmeter in the
article at http://www.vk1od.net/VSWR/VSWRMeter.htm .

BTW, for your purposes, if you had a Bird 43 with an element that read
upscale on fwd power (250W element for your application), it is all you
should need to form a reasonable estimate of line loss and set the
transmitter to deliver 100W to the antenna. You might need a smaller slug
to make a measurement of RL on a s/c or o/c termination.

Owen

"Owen Duffy" wrote in message
...
"Jimmie D" wrote in
:


"Owen Duffy" wrote in message
...
K7ITM wrote in news:1186788470.852002.260460
@b79g2000hse.googlegroups.com:

On Aug 10, 2:28 pm, Owen Duffy wrote:
...
I don't think you can compensate for lack of f/b ratio in the
coupler, for example because the coupled lines are too long.
...
I'm curious what you mean by that, Owen...

Tom, I was thinking of several instruments, all of the coupled lines
type of construction, that on a s/c and o/c failed to indicate rho=1,
and showed
similar readings when physically reversed, suggesting it was not just
a fwd
/ rev matching issue, there was something about the coupler that was
too dependent on the location of the SWR pattern relative to the
coupler. Since
they worked better at lower frequencies, the length of the coupler
was likely to be a contribution.

Owen


Would this be a problem for a directional coupler designed for a
specific frequecy?


Jimmie

Jimmie, I am talking about the el-cheap inline SWR / Power Meter that is
often sold to hams with unrealistic specs.

You can / should always test the performance of the kit you are using to
determine if you should have confidence in it.

There are a bund of notes on testing a directional wattmeter in the
article at http://www.vk1od.net/VSWR/VSWRMeter.htm .

BTW, for your purposes, if you had a Bird 43 with an element that read
upscale on fwd power (250W element for your application), it is all you
should need to form a reasonable estimate of line loss and set the
transmitter to deliver 100W to the antenna. You might need a smaller slug
to make a measurement of RL on a s/c or o/c termination.

Owen

Well it a done deal,
Engineering support came out last night and ran the checks for us while Im
on vacation and recovering from minor surgery, Yaaay. They did it the normal
way and by measuring the return loss and they decided the "return loss
method" worked better. Not sure what better means at this point. accurate
enough and easier and faster would constitute better.


Jimmie

On Aug 10, 5:29 pm, Owen Duffy wrote:
K7ITM wrote in news:1186788470.852002.260460
@b79g2000hse.googlegroups.com:

On Aug 10, 2:28 pm, Owen Duffy wrote:
...
I don't think you can compensate for lack of f/b ratio in the coupler,
for example because the coupled lines are too long.
...
I'm curious what you mean by that, Owen...

Tom, I was thinking of several instruments, all of the coupled lines type
of construction, that on a s/c and o/c failed to indicate rho=1, and showed
similar readings when physically reversed, suggesting it was not just a fwd
/ rev matching issue, there was something about the coupler that was too
dependent on the location of the SWR pattern relative to the coupler. Since
they worked better at lower frequencies, the length of the coupler was
likely to be a contribution.

Owen

Hi Owen,

I've recently done at least a cursory study of the coupled-line
hybrid, and I found nothing to indicate that directionality is
affected by the line length. In fact, the usual length where it's
practical is 1/4 wave, since that's the length that provides maximum
coupling, and the coupling near that frequency changes only gently
with changes in frequency (falling off on either side). I was
particularly interested in finding that the directionality is
independent of the length, assuming uniform cross-section at least.
If this is in error, I'd really like to know about it, because it
affects something I'm working on.

I'm not sure exactly what sort of bridge is used in microwave network
analyzers; I do know that the ones we build out to a few hundred MHz
use resistive bridges, which are relatively frequency insensitive. (A
key trick is how to read the bridge imbalance without introducing
errors...)

Cheers,
Tom

K7ITM wrote in
ups.com:

....
If this is in error, I'd really like to know about it, because it
affects something I'm working on.

Interesting findings Tom.

The way I think of these couplers is that you are trying to sample V and
I at a point on the main line, and a longish coupler of that type departs
from that ideal.

The effect I observed, and in several instruments, was obvious and
repeatable. I wonder that if the length of the lines is not the cause, if
it was the untidiness of the way in which the detector circuit was
implemented at each end of the coupler section. Of relevance also, is
that insertion of the instruments also caused significant SWR (1.2 in
the case of one of them) at the extreme uppoer end of their specified
range. IIRC two of the instruments had no equalisation / compensation,
they had a resistor at one end of the coupled line and a cap/diode at the
other end.

I still have one of the things that did this, and I have since nulled it
for 75 ohms, but I will have a play with it when I get home next week.

Owen

K7ITM wrote in
ups.com:

....

Tom, for avoidance of doubt, I am not talking about the type of directional
coupler that uses a couple line and that you would terminate with matching
load. I am talking about the cheap VSWR meters that have about 100mm long
coupled line, that is quite tightly coupled, and the resistor at one end of
the line is adjusted to balance the electric field sample with the magnetic
field sample for a null reading with V/I=Zn.

Owen