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# Current transformers as SWR meter may be?

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#### carpenter

##### Full Member level 6

I am trying to understand this circuit and I really can not.

Red line is transmission line from ~20MHz 200W Amplifer to load (on schematic L1)
On this line is current transformer small toroid with 2 turn to primar and 2x10 turn to sec..
I thing, current on secondary side is on RV1 convert to voltage on C3.
if all the energy is transferred from PS to Load and none is reflected from the Load is voltage on C3 neative (over S1 and D1 B)
As increasing the size of the reflected energy from the Load back to Ampliier, flips the voltage on C3 to positive (over S2 and D2 B).
Unfortunately it is not why this is so?
What is the meaning R1 + C1 + C2
Is it possible to derive a relationship between the voltage on C3 and energy consumed in Load?
As will be wound windings S1 and S2?

Thank you for every kick

R1 + C1 + C2 inject RF with a phase shift of 90 degs. So the end of each winding will have two components to it :- say 0 degs + 90 degs, the other + 180 degs + 90 degs. Now if you balance the amplitudes so the C1 voltage equals the voltage at the end of the transformer when operating into a 50 ohm load, you will get one voltage at a phase of +45 degs, the other at -45 degrees with equal amplitudes, so when you detect them the + and - voltages cancel. As the reflected voltage increases, the voltages change in amplitude and phase so the outputs do not cancel so you can then calibrate the meter in VSWR units. Some where down the line you must calibrate for the actual power (operating into an open circuit ?).
Frank

- - - Updated - - -

Just thinking about it a bit more. The circuit really compares the current and voltage PHASES on the line. With any line resistive impedance the Vtransformer plus Vline will be equal at both diodes, so equal cancelling DC outputs.
Frank

carpenter

### carpenter

Points: 2
No it's not a CT. It's a modified RF splitter.

It is similar to modifying a (hybrid type) 3dB Splitter with 100R from center tap to ground and re-arranging to add a bridge to rectify the signal returned due to imbalanced impedance between source and load and outputting a Vdc to the combiner port (= splitter input port)

Thank you for your answer, the idea with compares the current and voltage PHASES I also challenged too.
Unfortunately, I do not know how the energy reflected from the load changing phase current to voltage on this cicruits.

Another story.
I would like to measure the current over RF line.
My idea is add on this toroid new winding. see Picture.
You think it has a chance to work or not?

Your new winding needs to have one end earthed and the other fed through a diode to R5.
Frank

Another story.
I would like to measure the current over RF line.
My idea is add on this toroid new winding. see Picture.
You think it has a chance to work or not?

You shouldn't do that.

If you consider the torroid circuit as current transformer for the time being, you realize that the n*I sum of the individual windings balances itself. Adding another secondary winding changes the current in the other windings and ruins SWR bridge operation. You need to add a separate current transformer. Or redesign the SWR bridge.

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