> 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.
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.
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...
> 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.
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.
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.
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
|