Ian, G3SEK wrote:
"What are those V`s and I`s doing in your reply?"

Cecil is right on target. Power is VxI. The Bird takes carefully
balanced samples of V&I. In the incident wave the samples are in-phase.
In the reflected wave the samples are 180-degrees out-of-phase. By
exactly balancing the two samples, the reflected wave cancels. Only the
incident wave is sensed. To sense the reflected wave only, the polarity
of one of the two samples is switched.

I am now more dangerous. Until today, I had never seen one of Kraus`
books. I now own the third edition of "Antennas (for all applications)".
Kraus says there are "fundamental" units and "secondary" units. The
"Ampere" is a fundamental unit. All secondary units can be derived from
the fundamental units.

In the Bird, the given is a 50-ohm line. Power is 50(I)(I), since I=E/R
= E/50, and E = 50(I) = V.

Power = EI

The Bird is accurate enough using a sample of E and a sample of I and
scaling to determine power. Multiplication is just scaling or the number
of times you repeat a number in an addition column. In the Bird, scaling
is an electrical adjustment.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Ian, G3SEK wrote:
"What are those V`s and I`s doing in your reply?"

Cecil is right on target. Power is VxI. The Bird takes carefully
balanced samples of V&I. In the incident wave the samples are in-phase.
In the reflected wave the samples are 180-degrees out-of-phase. By
exactly balancing the two samples, the reflected wave cancels. Only the
incident wave is sensed. To sense the reflected wave only, the polarity
of one of the two samples is switched.

Correct. I have said that, or near, several times in this and preceding
threads. An article saying the same has been on my web site for almost a
year.

I am now more dangerous. Until today, I had never seen one of Kraus`
books. I now own the third edition of "Antennas (for all applications)".
Kraus says there are "fundamental" units and "secondary" units. The
"Ampere" is a fundamental unit. All secondary units can be derived from
the fundamental units.

That is of course true... but if you believe it relevant to this
discussion, you are indeed much more dangerous.

In the Bird, the given is a 50-ohm line. Power is 50(I)(I), since I=E/R
= E/50, and E = 50(I) = V.

Power = EI

The Bird is accurate enough using a sample of E and a sample of I and
scaling to determine power.

As you yourself say, the Bird senses current and voltage samples; and
these are then added (forward) or subtracted (reverse). There is no
physical mechanism inside the Bird by which E x I multiplication takes
place, so power is not directly sensed or measured.

Multiplication is just scaling or the number
of times you repeat a number in an addition column.

That is a kindergarten concept that only works for integers (how do you
repeat a number 3.14159 times?) and anyway, it physically does not
happen inside a Bird.

In the Bird, scaling
is an electrical adjustment.

No, the meter scale comes from an external calibration.


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

On Sun, 27 Jul 2003 07:49:22 +0100, "Ian White, G3SEK"
wrote:
As you yourself say, the Bird senses current and voltage samples; and
these are then added (forward) or subtracted (reverse). There is no
physical mechanism inside the Bird by which E x I multiplication takes
place, so power is not directly sensed or measured.

In the Bird, scaling
is an electrical adjustment.

No, the meter scale comes from an external calibration.


Hi Ian,

If you boil down power to heat, it still takes the scale on a
thermometer to confirm the magnitude. Scales are a form of
quantifying into scientific rigor the qualitative utterances of "not
much power," "some power," "more power," "a lot of power...." Denying
that multiplication is performed through the observance of established
boundary conditions and the movement of a coil in a magnetic field
(the resulting torsion which easily conforms to evidence of power) by
nay-saying is not compelling argument.

I've seen antennas described as "transducers" and with this current
side-topic, the Bird Wattmeter is more so such a transducer in its
translation of electrical power to physical power. The Bird Wattmeter
consumes power at a known ratio to the applied power. Ratios are one
of the simplest and most resolved quantities to confirm. There is
very little, short of serious physical abuse, that can disturb this
ratio (such as overpowering the line to create breakdowns such that
there are destructive arcs or fusing current). Efficiency aside (that
power lost to resistors within the sensing circuit), power of a known
ratio magnitude is invested in the needle's angular deflection and
holding it against spring tension. Newton would easily accept this as
"Power." It is basically what he called the ballistic pendulum.

If one cannot measure Power with a Bird Wattmeter, then prayer is the
only solution to the enigma of its reckoning. If you have issue with
the systemic failures that abound in Power determination, then argue
the cause not the instrument. I have described in the past how Power
determination can be upset drastically in the face of measurements
performed between two mismatches (both the source and the load); but
it is not the fault of the instrument in the Power determination's
failure. No other instrument nor method (short of acknowledging the
indeterminacy of phase) will improve over sure failure.

73's
Richard Clark, KB7QHC

Ian, G3SEK wrote:
"No, the meter scale comes from external calibration."

Who said it didn`t?

The scale is mass produced. In the Bird element, a resistor is selected,
in some elements it is a variable resistor, called the "calibration
resistor". A suitable replacement is the Helitrim 7138 mini variable.

See "Calobration and Repair for Bird Wattmeter Elements, page 48, April
1989, 73 Amateur Radio.

Best regards, Richard Harrison, KB5WZI