RadioBanter - "Sloshing" EM Energy

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Richard Harrison wrote:

Reflected power is again reflected at the match point
because the matched source sees no reflection.

I think you put your finger on it, Richard. That's exactly what
inspired the 'sloshing energy' comment.

73, ac6xg

Jim Kelley wrote:
Richard Harrison wrote:

Reflected power is again reflected at the match point
because the matched source sees no reflection.

I think you put your finger on it, Richard. That's exactly what
inspired the 'sloshing energy' comment.

Except that the "sloshing energy" comment doesn't have the energy
sloshing from the load to the match point and back at the speed of
light. As I understand the concept of "sloshing energy" it is sloshing
back and forth rather locally between the inductance and the
capacitance.
--
73, Cecil http://www.qsl.net/w5dxp

Tim Wescott wrote:
All this is covered in a good college E&M course.

Do "good college E&M courses" cover the conservation of
energy principle applied to canceled EM waves? That topic
seems to be a black hole in the education of the average
electrical engineer.
--
73, Cecil http://www.qsl.net/w5dxp

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Ian White, GM3SEK wrote:
"But here you pick up the pace. Instead of the detailed argument above,
suddenly whole chapters rush by in a single sentence."

Fair criticism. It reflects tiring of posting before its conclusion.

The Bird wattmeter`s firectional coupler distinguishes between incident
and reflected waves by their singular difference. Upon reflection of a
wave, either the voltage or the current it generates is reversed in
phase, but not both.

Bird takes equal samples of voltage and current from the wave.. When
there has been a reflection, the samples have opposite polarity and
cancel. When there has been no reflection the samples from that
direction of travel are in-phase and the sample total is double the
contribution of either sample.

To determine reverse power flow, the polarity of one of the samples is
reversed.

You don`t need to know how it works to use it and Bird never advertised
how simple it is as far as I know.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Ian White, GM3SEK wrote:
"But here you pick up the pace. Instead of the detailed argument above,
suddenly whole chapters rush by in a single sentence."

Fair criticism. It reflects tiring of posting before its conclusion.

The Bird wattmeter`s firectional coupler distinguishes between incident
and reflected waves by their singular difference. Upon reflection of a
wave, either the voltage or the current it generates is reversed in
phase, but not both.

Bird takes equal samples of voltage and current from the wave.. When
there has been a reflection, the samples have opposite polarity and
cancel. When there has been no reflection the samples from that
direction of travel are in-phase and the sample total is double the
contribution of either sample.

Bird assumes the wave reflection model is valid, i.e.
Vsample proportional to Vtotal = vector sum of (Vfor+Vref)
Isample proportional to Itotal = vector sum of (Ifor+Iref)
Vfor in phase with Ifor, RMS Vfor/Ifor = 50 ohms, Vfor*Ifor=Pfor
Vref 180 deg out of phase with Iref, RMS Vref/Iref = 50 ohms
Vref*Iref=Pref

These assumptions are valid for a 50 ohm feedline of reasonable
length.

These assumptions are obviously not valid if no feedline exists
or if Z0 is not 50 ohms, which is, as I infer, Ian's objection.
--
73, Cecil http://www.qsl.net/w5dxp

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Richard Harrison wrote:
Ian White, GM3SEK wrote:
"But here you pick up the pace. Instead of the detailed argument above,
suddenly whole chapters rush by in a single sentence."

Fair criticism. It reflects tiring of posting before its conclusion.

Sure. If you'd kept to the original very steady pace, you'd still be
writing... which is not what we do in newsgroups.

The Bird wattmeter`s firectional coupler distinguishes between incident
and reflected waves by their singular difference. Upon reflection of a
wave, either the voltage or the current it generates is reversed in
phase, but not both.

Yup.

If I can fill this out a little...

Bird takes equal samples of voltage and current from the wave..

This is done by the pickup loop, which is both inductively and
capacitively coupled to the center line. The capacitive coupling gives
the voltage sample, while the inductive coupling gives the current
sample.

The current sample runs through a resistor, which develops a voltage
that is made exactly equal to the direct voltage sample. So now we have
two RF voltages appearing in series. In the forward direction, the thing
is built so that these voltages add in phase.

When you rotate the slug by 180deg, the phase of the current sample
reverses but the phase of the voltage sample does not, so now the two
voltages subtract. If the instrument is terminated in its design
impedance of 50 ohms, the voltages (should) cancel exactly, so the meter
reading falls back to zero. There's a small capacitive tab on the pickup
loop that allows the meter reading to be nulled exactly.

When
there has been a reflection, the samples have opposite polarity and
cancel. When there has been no reflection the samples from that
direction of travel are in-phase and the sample total is double the
contribution of either sample.

Er, yes, pretty much...

To determine reverse power flow, the polarity of one of the samples is
reversed.

And here you've made that big leap again. Where did "power" come from?
Nothing in what you or I have said above explains how come the meter can
read "Watts".

That's because it doesn't actually measure watts. It has been calibrated
in watts under certain specific test conditions, using a different kind
of wattmeter that actually does measure watts.

You don`t need to know how it works to use it

No, you don't. But if you choose to use it as "evidence" in a discussion
about waves and reflections, then you do need to know how it works.

and Bird never advertised
how simple it is as far as I know.

Possibly because it isn't actually as simple as it looks.

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

Ian White GM3SEK wrote:
Richard Harrison wrote:
To determine reverse power flow, the polarity of one of the samples is
reversed.

And here you've made that big leap again. Where did "power" come from?
Nothing in what you or I have said above explains how come the meter can
read "Watts".

Bird assumes the meter is being used in a 50 ohm environment. Bird
assumes after the two sample voltages are superposed, that the
calibration is accurate to within 5% of full scale. The calibration is
done using 50 ohm matched lines.

In a transmission line, the net power transfer is V*I*cos(theta). It
can be proven mathematically that, for a transmission line with
reflections,

Pnet = V*I*cos(theta) = Vfor*Ifor - Vref*Iref

The Bird sampling circuit allows one to read either (Vfor*Ifor) or
(Vref*Iref) by turning the slug. Bird assumes Vfor/Ifor = Vref/Iref =
50 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Ian White GM3SEK wrote:

Richard Harrison wrote:

To determine reverse power flow, the polarity of one of the samples is
reversed.

And here you've made that big leap again. Where did "power" come from?
Nothing in what you or I have said above explains how come the meter can
read "Watts".

Bird assumes the meter is being used in a 50 ohm environment. Bird
assumes after the two sample voltages are superposed, that the
calibration is accurate to within 5% of full scale. The calibration is
done using 50 ohm matched lines.

In a transmission line, the net power transfer is V*I*cos(theta). It
can be proven mathematically that, for a transmission line with
reflections,

Pnet = V*I*cos(theta) = Vfor*Ifor - Vref*Iref

The Bird sampling circuit allows one to read either (Vfor*Ifor) or
(Vref*Iref) by turning the slug. Bird assumes Vfor/Ifor = Vref/Iref =
50 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

The Bird sampling circuit certainly is a magical device if it can
allow one to "read" a power directly. Energy and power are always
calculated quantities. You don't have the math right, Cecil.
Try again.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
The Bird sampling circuit certainly is a magical device if it can
allow one to "read" a power directly.

It would be a magical device if it did that but it doesn't.
The Bird wattmeter is simply an analog calculator. When the
Bird is *calculating* power, it phasor adds/subtracts a sample
voltage proportional to the total current to/from a sample volt
proportional to the total voltage and comes up with a superposed
voltage that is proportional to either forward power or reflected
power depending upon slug position.
--
73, Cecil http://www.qsl.net/w5dxp

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