[Amps] Re: [Amps] Bird® 43 Manual

[Ron]

Tom Rauch wrote:

> > The bird only picks up voltage or current but not both.
>
> Respectfully Ron, that simply is not true in the least.
>
> If it were true, the meter could not possibly have directivity. Turning
> the slug around would result in the same reading. The reason is
> simple, current can NOT flow both directions in a conductor at the
> same time.
>

Hi Tom/Gary/Ian

The discussion has moved from " Where does the power go" to "How does the
Bird 43( trade mark) work".

I have to admit that I am not the expert on the technique of how the meter
internals work and so I will ask a fellow who works for them as they are
local to Cleveland to see if he has the expert answer ....that is unless
you are an expert on the meter and in that case I should be quiet and
listen.

These directional couplers use a pickup loop and a diode to separate the
current pulses for forward and reverse power.  Several are shown in
schematics of many popular radios.  These can only sense current pulses in
the line.

There is a diode in the Bird 43(tm) slug.
Measure the conductivity from the base to either of the recessed terminals.

You will see a diode and a series resistance.
On the slug I had loose it's cap , there is a coil as you describe who's
central axis is perpendicular to the line section.
On one end is a diode in series with the coil and then goes to the recessed
connections.  The other end of the coil is a 155 ohm resistor that goes to
the base . At the head of the resistor is a small wire and I agree that
that could be a capacitive pickup.
It could also be used for frequency trim.
Rotating the slug reverses the positive sensing of the current wave where
the diode blocks the negative current wave.

If the little wire is a capacitive pickup , it sums voltage with current at
the slug position. Power is voltage TIMES current.
You would have to take the current magnitude and multiply it by the voltage
magnitude at that instant to get instant power.

In a true watt meter, there are three terminals minimum to measure voltage
and current.
Using a mechanical, passive device is a two coil meter movement where the
current drives the armature and the voltage drives the stator. Both must be
present at the same instant for the meter movement to develop torque to
oppose the calibrated spring and scale to read out true power. This is
where the multiplication takes place. If there were reverse power , the
meter would move backwards.
If there is true power flowing in a pulsed or sinusoidal fashion , the
torque pulses are smoothed out by the mechanical constants and average the
meter position against the meter spring.

This is obvious as when you place the slug in the half position , you short
out the coil winding to the meter and if you try to shake the needle , it
is damped as the induced currents are going through the short ....try it.
Pull the slug out and the needle is free to move. The meter is a permanent
magnet type.
The bird just does not have the type of meter movement that is required to
measure true power unless there were a multiplier IC inside the slug.

Current CAN flow in a conductor in different directions at the same time!.
If you have a standing wave on a line , there is a node that has a voltage
null. There is also a voltage peak where the standing wave voltage swings
from it's extreme positive peak to it's extreme negative peak voltage. The
Charge on the line at the positive peak flows through the node to the
negative peak and repeats this every cycle. The voltage node is  a current
peak as current washes from the left voltage peak to the right voltage
trough on the line. Further down the line , the opposite current is doing
the came thing but in a different direction even though it is the same
conductor.  The net current through the entire length of the line is zero
but there are currents and voltages within the line as it is a reactive
element.



>
> Also if what you said was actually correct, power would be reduced
> when line impedance increased because current would be reduced.
> That is very clearly not true.
>

Current washes up and down the line setting up a standing wave similar to a
string tied to a doorknob.   Depending where you place the meter along the
line, you will read different but equal readings for forward and backward.
Your F-B is correct.


>
> If you open an element, you will find a small inductive pickup that
> forms a current transformer (low frequency slugs actually use a
> transformer with a core), and a capacitive divider formed by a small
> metal plate and/or a trimmer capacitor that samples voltage.

>
>
> Without sampling both current and voltage it is impossible to make
> a directional coupler. You would only have a non-directional volt
> meter, or current meter. Read the test I performed, or better yet
> make some actual measurements yourself.

The tests you performed seem to prove my point.
If this were true , it wouldn't matter where you placed the meter in the
line, it would always read the correct power through the line without
regard to position of the instrument as you show using your test setup. On
your setup , move the meter on it's position of the line.  You will see
both readings change dependent on position but the difference will always
be the correct forward power to the load.  If you find the current node ,
you won't have any reflected power. The simple fact that you can separate
the forward and reflected wave indicates that you have a directional
current meter.

Forget the meter ....think about it.  If current were washing up and down
the line ....but no net power went through the line( ie , the line is open
at the end as in the first example) , there would positive and negative
torques that would cancel exactly and the meter would read zero power.
---
Ron


>
> 73, Tom W8JI
> W8JI@contesting.com