"Owen Duffy" wrote in message
news
On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart
wrote:


Maybe I misunderstand, but what you seem to be saying is that if I put
Probe A at point X on a transmission line and Probe B at point X+Y
(Y0), the phase difference with respect to frequency will not change.

The directional coupler is no different, it is sampling the main line
at two physically different locations.

With respect, I think you two guys are talking about different kinds
of couplers.

Wes, your earlier description makes is clear that samples in the
coupler you described are located at equidistant from their nearest
ends, but not in the centre, so at different positions in the coupler.
I think the coupled lines type of coupler for lower frequency use
might fit this category.

Frank assumes a coupler where the samples for both ports are taken at
the same physical location on the main line. I think a crossed
waveguide coupler might fit this category (depending on the way the
coupling holes are implemented).

Owen

The type of coupler I am thinking of is indeed where the coupling lines for
forward and reflected are in the same physical region. The point I was
making refers particularly to the HP 778D; where it is specified in:
http://cp.literature.agilent.com/lit.../5952-8133.pdf
"Data can be read from the two meters of the vector voltmeter and
transferred directly to a Smith Chart". Indicating that there is no
significant phase error over the nominal bandwidth of the coupler. It is
certainly something I have always taken for granted, but in recent years
have been spoiled by Agilent's VNAs. Even Matthaei, Young, and Jones'
classic text does not get into specifics of the phase response of TEM
couplers. If I get a chance in the near future, I will run a phase response
of an HP778D coupler, and/or also run an Eagleware simulation of a co-planar
WG coupler.

Just read Wes' comments on the line stretcher on the above pdf. I must
admit I also wondered about that, but assume it is intended as a cal
adjustment for a short/open standard. Even VNAs, in their non calibrated
state, show a pretty good dot on the right/left of the Smith Chart for an
open/short in the lower hundreds of MHz.

Incidentally I have tried to access the
http://www.qsl.net/n7ws/Pages%20from%20AN77-3.pdf which is unbelievably
slow, but I keep getting error messages saying the file is corrupted. I am
using a 2.5 Mb ADSL, which is normally pretty fast.

Frank