Tom Rauch wrote:
> > 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.
>
> No, that is NOT true.
>
On the above comment , I mention couplers that are in radio diagrams ....not
the Bird.
They switch the diode to measure forward and reverse readings. If there is
any capacitive coupling into the link , it is not sensed as the differential
pair float with respect to the meter....the capacitive coupled voltage would
be common mode.
>
> If the slugs only sense the current pulse, it would be technically
> impossible for them to have directivity. Current reverses direction at
> the frequency of operation, during the first half-cycle it flows one
> way...during the second half-cycle the other.
>
With a diode in series with the secondary winding, it would be difficult for
the wrong direction current in the primary to cause a current flow in the
secondary.
> If a current-only sampling system measured the direction of current
> flow it would read zero current for any symmetrical AC waveform,
> because the average current would be zero.
>
> The directional effect comes from a voltage-reference sample,
> which even in the case of a pickup wire occurs when the wire is
> placed next to the transmission line center conductor.
>
> That is why the sampling system requires a termination resistor,
> and why line to pickup-loop spacing changes affect the null.
>
> When you reverse the slug, that capacitance does not change. It
> remains the same. The current sampling changes phase by 180-
> degrees.
>
> All of this is consistent with everything you will read in a scholarly
> text dealing with directional couplers and "power flow", and what
> you even say below...
>
I have agreed that you are correct on the capacitive coupling from Bill's
explaination.
I could even see where the diode could be used to obtain the log of the
current and voltage magnitude and sum then but the Bird(tm) isn't set up
that way. The meter movment responds to the series current through the
diode.
It all helps to explain the limits that one can expect when using it.
>
> > There is a diode in the Bird 43(tm) slug.
> > Measure the conductivity from the base to either of the recessed
> > terminals.
>
> > for frequency trim. Rotating the slug reverses the positive sensing of
> > the current wave where the diode blocks the negative current wave.
>
> That is just plain silly. There is no "positive current wave" or
> "negative current wave". It is symmetrical AC. At 7 MHz, we have
> exactly equal and opposite currents reversing directions 7million
> times a second. The current reverses directions about 1.8 million
> times a second when we operate 160 meters.
Why not . Its called superposition.
Maybe a physical explaination will help.
Take a fish tank........water volumn=charge, voltage=pressure, water flow =
current............apply standing waves..........when the crest of one wave
turns into a trough at the next 1/2 cycle, where did the water/current go?
It goes to fill the trough that now becomes the peak at the next 1/2 cycle.
Currents flow in both directions of the line. The line has two reactive
elements that swap energy every 1/2 cycle.
>
>
> How the heck could the meter, if it only samples current, tell what
> direction the source was from?????
>
> What is a negative current wave, and a positive current wave?
>
> Think about it.
>
> > 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.
>
> Now you have the answer. The slug sums voltage and current. The
> meter is calibrated so that sum represents power.
>
I have agreed. It represents power as long as the phase is not far off.
>
> Please look at the measurements I made, read the Bird manual,
> read articles on directional couplers, or do the experiment yourself.
>
> The math works out perfectly in a way that when when we subtract
> reflected from forward power readings, we are left with true power.
>
> > In a true watt meter, there are three terminals minimum to measure
> > voltage and current.
>
> Actually the requirement is the meter respond to both across and
> through vectors, directly sum them including phase.
>
> A conventional power meter on a power line does something very
> similar to the Bird slug.
>
> It normally has an inductive pickup that sample current, and sums
> that with a transformer pickup from the line to ground that samples
> voltage. The results are summed, and the meter is calibrated so
> that sum represents watts.
>
>
> The transformer pickup can be used in RF wattmeters, there have
> been many articles about that. But most, because of the high
> frequency, use a capacitive divider.
>
> First you insist there are not both sample, that the meter only
> samples current.
>
> Then you explain it also samples voltage and including phase
> sums the current and voltage as you describe how it is constructed.
>
I didn't explain that, I observed it and made a reference that it would be a
possibility that I felt didn't give an accurate result under a perfect
standing wave situation. Your putting words into my mouth.
The initial discussion was about where the power went under an open line
situation /High SWR.
Then the Bird(tm) thing got in here.
As I said , I agree that the instrument is sensing voltage.
I had a good time discussing and learned some new things so I am happy.
Please don't get frustrated.
---
Ron
>
> Now you seem to insist again it only samples the through vector,
> and not the across vector.
>
> Which way is it? It can't be both!
> 73, Tom W8JI
> W8JI@contesting.com
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