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RF power vswr meter how does it work?

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neazoi

neazoi

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How does this circuit work?
**broken link removed**
It does not have a capacitive or a resistive tap to the main antenna conductor, like ordinary meters do. But it has a tap at the center (200R) how does this work?

Van I alter the resistor (200R) and use it also for higher impedance lines?
 

An LTSpice simulation shows that the 200 ohms resistor increase a bit the forward and reflected voltages to the diodes, and also flatness the transformer response for wider frequency bandwidth.
Most of the similar schematics don't use this resistor, but seems that is doing something good.
 
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    neazoi

    neazoi

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vfone said:
An LTSpice simulation shows that the 200 ohms resistor increase a bit the forward and reflected voltages to the diodes, and also flatness the transformer response for wider frequency bandwidth.
Most of the similar schematics don't use this resistor, but seems that is doing something good.
Click to expand...

So This simple schematic does work indeed as a simple FW/REF power monitor. All right that's good news.

I wonder, will it work on higher input/output impedances as well (and not just 50R)?

Also, what if I just use LEDs instead of diodes and connect them to ground directly, will this still work?
 

A VWSR meter can't work without relating antenna line voltage and current. If the shown circuit works, it does by parasitic capacitive coupling between unshielded antenna line and winding. If so, it will be sensitive to the exact wire position. I doubt that it can compete with classical VWSR bridge designs.
 
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    neazoi

    neazoi

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Totally agree, this circuit is very sensitive to the layout construction.
Otherwise, I've seen many commercial VSWR meters using this kind of circuit, which should work if is build in concordance. Bridge type VSWR meters are better.
Higher impedances will provide higher voltages into the circuit, and these voltages are harder to deal with. I would recommend to stay on low impedance values.
LEDs do not detect high frequencies due to high junction capacitance (I didn't try the latest LED types), so you need the 1N60 diodes in place.
If want to use LEDs, best idea would be to replace the meters with LEDs, and adjust the series 1k resistor for proper work.
 
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