chuck wrote:
Ian White GM3SEK wrote:
[SNIP]
The disagreement is entirely about the interpretation - in other
words, it's about the theory about standing and travelling waves.
Richard habitually misses out this step, which makes it look as if
the Bird wattmeter "proves" the physical existence of forward and
reverse travelling waves of power.
It doesn't. Everything that you see printed on the Bird's meter
scale, and in the Bird literature, represents that company's
particular interpretation of theory about waves on transmission lines.
details of that theory are *not* agreed within this newsgroup, which
means that - to some people - the two halves of Richard's claim do
not join up.


Ian, I've not detected this particular disagreement about waves on
transmission lines in the group. I would be most grateful to see a
brief statement of where and how Bird's interpretation of theory is
found infirm.

The Bird 43 has only one scale calibration: power. Readings on that
scale represent the power delivered to a 50-ohm load when the sensor is
turned to the forward direction. However, the instrument internally
senses only the voltage and current on the line. The "power" reading is
only a calibration, and is only completely meaningful when you use the
instrument in the same circumstances as when it was calibrated.

Now what happens if the load is not exactly 50 ohms? What happens when
you turn the sensor around? You then get some new and different readings
which have to be called "forward power" and "reflected power" - because
"power" is the only thing the Bird's meter scale is calibrated to
indicate.

The Bird Corporation's Application Note "Straight Talk About
Directivity" discusses the meaning of "forward and reflected power"
indications with a mismatched load. That document does not directly
address your question, Chuck, but takes the subject to a further level
of detail about the accuracy of the real-life instrument, and its
limited ability to discriminate between forward and reflected waves.
http://www.bird-electronic.com/app_n...irectivity.pdf

One notable thing about "Straight Talk" is how all the calculations
begin by taking the square root of the power indications. In other
words, all the relative RF power indications from the meter scale are
converted into relative RF voltages, and all the real calculations are
done on the voltages.

Another interesting observation is that if you have a mismatched load,
such that the meter indicates say 93W with the arrow on the sensor
pointing forward and say 23W with the sensor rotated 180deg, then you
would find that 70W is being delivered into the resistive part of the
mismatched load impedance (assuming perfect directivity and no errors of
any other kind).

There are two schools of thought about the physical meaning of all this.

One is that if the meter scale says "power", then there genuinely are
forward and reflected traveling waves of power on the line. In the "93 -
23 = 70W" example, the belief is that there genuinely is a power flow of
93W towards the load, only 70W of which is accepted and 23W is returned.

The other school of thought is that that's not true. The meter may
*read* more "forward power" than is actually being delivered to the
load, but that is a false indication because the instrument is not being
used in the situation for which the power scale was calibrated.
Therefore it is not to be taken at face value - and above all, the
letter "W" on the meter scale does not prove the physical existence of
forward and reflected waves of power.

Every detail about a Bird 43 or similar "directional wattmeter" can be
explained quite simply in terms of travelling waves of voltage and
current. And I do mean every detail - including why a meter that happens
to have been calibrated in "power" will read as it does.

The classic explanation was by Warren Bruene, W5OLY, of the Collins
company. He invented the familiar "Bruene bridge" directional coupler
which samples current through a toroidal transformer, and voltage by a
voltage divider. The principle of the Bird 43 is the same, but the two
separate sampling functions are easier to see in the Bruene bridge.

After a previous incarnation of this debate in 2002, I wrote a 2-page
article which summarised Bruene's original article (from QST, April
1959) and explained the crossover to the Bird sampling technique:
http://www.ifwtech.co.uk/g3sek/in-pr...-of.htm#bruene

An equally good explanation of those same meter readings can be
constructed by regarding the Bruene "bridge" literally as an impedance
bridge. There is no inconsistency between the two approaches - they are
just two different viewpoints looking at the same reality.

On the other hand, we have yet to see an explanation in equivalent
physical detail that is based entirely and exclusively on the viewpoint
of travelling waves of power (with no borrowing from explanations based
on voltage and current).

I am not blaming the Bird Corporation for any of these
misunderstandings. They are simply telling users how to work with the
available "power" markings on the meter scale. The problem is when some
users take them too literally.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek