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

On Aug 11, 1:37 am, Owen Duffy wrote:
K7ITM wrote roups.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

Hi Owen,

I'm not sure I see the difference. The load on the cheapie you
describe is just the load required to terminate that line. I have a
freely redistributable field solver program that will calculate the
even and odd mode impedances for you from the geometry and the
dielectric's permittivity, and from those impedances and the length
you can predict the proper termination impedance of both the "through"
and the "coupled" lines, and the coupling at any particular
frequency. It IS a problem if you try to do it in microstrip because
the propagation velocity for the even and odd modes is different, but
in true TEM configurations, I believe the directionality is fine if
you maintain uniform cross-section. Actually, a way that they make
broadband coupled lines is to have a central section tightly coupled,
and another section on each end of that which is less tightly
coupled. You can extend it to 5 sections or more, to get even broader
bandwidth.

Info about them is out there, but it wasn't as easy for me to find as
I figured it would be. ;-)

Cheers,
Tom

I haven't read all the posts in this thread, so I'm not sure that the method I'm advancing for measuring line
loss has not already been discussed.

The method I'm suggesting measures the input impedance (R and jX) of the line in question with the opposite
end of the line terminated first with an open circuit and then with a short, at a frequency where the line
will be close to lambda/8. With low-loss lines the R component will be very small, requiring an RF impedance
bridge that can produce accurate values of R in the 0.2 to 2-ohm range. (The General Radio GR-1606A is a
typical example, which, while using this procedure, will yield answers of greater accuracy than the methods
described in the current posts.) With the 1/8wl line the + and - reactances appearing in the measurements will
be approximately the value of the line Zo.

The open and short circuit values are then plugged into a BASIC program that I wrote years ago, which appears
in Chapter 15 of both Reflections 1 and 2. It also appears on my web page at www.w2du.com. The program outputs
the line attenuation, the complex Zo, and the electrical length of the line. The program solves the equations
appearing in Chipman's "Theory and Problems of Transmission Lines," Page 135.

On my web page go to 'View Chapters of Reflections 2' and click on Chapter 15 to see the detailed explanation
of the procedure.

The BASIC program TRANSCON (Transmission-line Constants) is listed there, but to save your having to load the
program from the typed list I will email a copy of the actual operable program to anyone who requests it,
addressing your request to .

I hope this suggestion will prove to be of value.

Walt, W2DU

On Sat, 11 Aug 2007 21:44:08 -0400, Walter Maxwell wrote:

I haven't read all the posts in this thread, so I'm not sure that the method I'm advancing for measuring line
loss has not already been discussed.

The method I'm suggesting measures the input impedance (R and jX) of the line in question with the opposite
end of the line terminated first with an open circuit and then with a short, at a frequency where the line
will be close to lambda/8. With low-loss lines the R component will be very small, requiring an RF impedance
bridge that can produce accurate values of R in the 0.2 to 2-ohm range. (The General Radio GR-1606A is a
typical example, which, while using this procedure, will yield answers of greater accuracy than the methods
described in the current posts.) With the 1/8wl line the + and - reactances appearing in the measurements will
be approximately the value of the line Zo.

The open and short circuit values are then plugged into a BASIC program that I wrote years ago, which appears
in Chapter 15 of both Reflections 1 and 2. It also appears on my web page at www.w2du.com. The program outputs
the line attenuation, the complex Zo, and the electrical length of the line. The program solves the equations
appearing in Chipman's "Theory and Problems of Transmission Lines," Page 135.

On my web page go to 'View Chapters of Reflections 2' and click on Chapter 15 to see the detailed explanation
of the procedure.

The BASIC program TRANSCON (Transmission-line Constants) is listed there, but to save your having to load the
program from the typed list I will email a copy of the actual operable program to anyone who requests it,
addressing your request to .

I hope this suggestion will prove to be of value.

Walt, W2DU

Some additional information that can make it easier to follow the procedure. The section of Chapter 15
describing the line attenuation measurement procedure is Sec 15.3.1, Calibration of the Feedline, Page 15-3,
Chapter 15, and the table showing the results using the TRANSCON program is Fig 15-1, Page 15-4 The TRANSCON
program listing is on Page 15-22.

Walt, W2DU