On Mon, 22 Sep 2003 16:09:09 +0100, "Ian White, G3SEK"
wrote:

I'm sorry to keep picking away at this one, but it seems to be
necessary...

The statement that a directional coupler can "separate energy by its
direction of travel" involves some unaware assumptions involving
transmission-line theory.

If we're trying to get that theory right, we have to avoid using it
unawarely in order to prove itself... because that way would let us
"prove" just about anything.

A directional coupler only senses the current (directionally) at a
particular location on the line, and the voltage between the two
conductors at that same location. The directional coupler tells us
NOTHING else. We have to be very literal-minded about that.

We cannot determine the reflection coefficient, the SWR, or what is
happening to the energy, without applying some flavor of
transmission-line theory. When the whole discussion is about getting
that theory right, we have to be very careful to avoid unawarely arguing
in circles.

Hi Ian,

It seems every time you come into conflict, you reject other's
statement as issues of circularity and confusion.

A Directional Coupler is principally a transmission line in itself, a
paired one in fact with controlled leakage between the two. There is
nothing inherently restrained in its operation that enforces this
curious complaint of
A directional coupler only senses the current (directionally) at a
particular location on the line, and the voltage between the two
conductors at that same location. The directional coupler tells us
NOTHING else. We have to be very literal-minded about that.
which as a statement means little beyond the obvious coupling that is
necessary. And to state that NOTHING else is told begs the question:
So What? Nothing else was implied, inferred or demanded, and you
offer nothing to illustrate just what it was you objected to. You
call them "unaware presumptions." WHAT presumptions are they?
Certainly not the same observation I quoted just half a dozen lines
above.

The Bird element is indeed a primitive implementation of a Directional
Coupler. It even discards phase information (in fact, it is
unavailable, but it would still be discarded through rectification and
filtering). The Bird element constitutes a three port Directional
Coupler where a four port coupler would return that phase information
(if it were not then immediately discarded through the same
rectification and filtering). The difference between what is
available and what is not is a design choice enforced by the
application of the instrument, not a shortfall of Directional Couplers
as a class of device.

Directional Couplers are literal transmission line components and the
heart and soul of network analyzers.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
Hi Ian,

It seems every time you come into conflict, you reject other's
statement as issues of circularity and confusion.

Now that you mention it, that could indeed be a common factor at the
root of this newsgroup's chronically unresolved arguments.

A Directional Coupler is principally a transmission line in itself, a
paired one in fact with controlled leakage between the two.

Some UHF/microwave directional couplers consist of a primary
transmission line (the 'through' line) and a secondary transmission line
for sampling; but not all directional couplers are of this type. Many
types of directional coupler contain no kind of secondary transmission
line. Some have a bridge configuration - for example the Bruene bridge
and the resistor bridge.

At HF through VHF, even the Bird element is better analysed as an
electrically small loop that samples V and I components from the main
line, and not as a section of secondary transmission line. You only need
to consider a Bird element as a secondary transmission line at
frequencies where the loop dimensions are a significant fraction of a
wavelength, so its distributed properties become important.

There is
nothing inherently restrained in its operation that enforces this
curious complaint of
A directional coupler only senses the current (directionally) at a
particular location on the line, and the voltage between the two
conductors at that same location. The directional coupler tells us
NOTHING else. We have to be very literal-minded about that.
which as a statement means little beyond the obvious coupling that is
necessary. And to state that NOTHING else is told begs the question:
So What? Nothing else was implied, inferred or demanded, and you
offer nothing to illustrate just what it was you objected to.

It was all there, but you missed it. Possibly your mind was on your own
reply.

You
call them "unaware presumptions." WHAT presumptions are they?

The presumption is that a directional coupler directly samples power,
when in fact it doesn't. It samples voltage and current separately. The
sampled current is passed through a resistor to develop a second
voltage, and then these two RF voltages are either added or subtracted.
Finally the resultant RF voltage is detected.

Nowhere in this process is there anything that could be described as
directionally sampling power. So any argument about transmission theory
that calls upon that unfounded notion as part of its "evidence" is not
going to get us anywhere useful.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

On Mon, 22 Sep 2003 22:46:24 +0100, "Ian White, G3SEK"
wrote:
You
call them "unaware presumptions." WHAT presumptions are they?

The presumption is that a directional coupler directly samples power,
when in fact it doesn't. It samples voltage and current separately.

Hi Ian,

A Directional Coupler consists of two transmission lines.
Transmission Lines are the media through which B/H waves migrate
inexorably fixed together. The premise (which you alone bring as a
clouded presumption) that the Bruene bridge somehow works with
independence from this is simply a convenience in discussing its
operation, a convention of discussion at best and not a reality.

73's
Richard Clark, KB7QHC

Richard seems to have a great deal of respect for Terman. So I suggest
that he read Terman's explanation of directional coupler operation in
_Radio Engineering_. In the Fourth Edition, at least, he does so without
a single mention of power, let alone "directional" power or "power waves".

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
Richard Harrison wrote:

Keith wrote:
"I agree completely when the volts and amps are the measurable resultant
voolts and amps."

There is a big problem with resultant volts and amps. It is the
resultant variation in amplitude which is position dependent.

The only average variation in forward and reflected powers is a decline
with distance caused by line loss. Power flows at a constant average
rate into, through, and out of a transmission line. Line loss causes
decline in power along a lossy line.

The convenient way to get useful numbers is to separate energy by its
direction of travel and to measure these. A directional coupler is
needed and the Bird among others does this.


I'm sorry to keep picking away at this one, but it seems to be necessary...

The statement that a directional coupler can "separate energy by its
direction of travel" involves some unaware assumptions involving
transmission-line theory.

If we're trying to get that theory right, we have to avoid using it
unawarely in order to prove itself... because that way would let us
"prove" just about anything.

A directional coupler only senses the current (directionally) at a
particular location on the line, and the voltage between the two
conductors at that same location. The directional coupler tells us
NOTHING else. We have to be very literal-minded about that.

We cannot determine the reflection coefficient, the SWR, or what is
happening to the energy, without applying some flavor of
transmission-line theory. When the whole discussion is about getting
that theory right, we have to be very careful to avoid unawarely arguing
in circles.