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Why VSWR value change when keep the close ground(floor)?

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emreer3442

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Hello everyone, last night i was testing my 145MHz Yagi antenna for VSWR value. I realized something stupid. When I was keeping the antenna close to ground, VSWR value was changing. Also, I stanted in front of the antenna same thing was happen. I search about that on internet but i am not be satisfied. If any one know the reason about this, could you tell me please. I will be really appriciated. Thanks a lot.
 

BigBoss

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VSWR or Input Impedance of an Antenna is also a function of Boundary Conditions of that antenna.
So it's very normal because the Ground is the Mirror of the Antenna in regard of EM Theory.
 

FvM

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The general answer is simple: Any objects (conducting, dielectric or lossy) within the near field range of an antenna will affect its input impedance and radiation characteristic. The ground distance of a dipole must be e.g. above 2 wavelengths to keep the impedance variation below 10%.
 

vfone

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I realized something stupid. When I was keeping the antenna close to ground, VSWR value was changing.
Therefore, if the VSWR doesn't change when you move the antenna close to the grond, THEN is a problem..
 

D.A.(Tony)Stewart

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The antenna is bidirectional, and the near-field reflection of weak signals opposes the transmitted voltage and increases current, thereby reducing impedance and raising the VSWR relative to ideal.

This is also what makes it a great motion detector if you have a directional coupler and very sensitive DMM on a Schottky signal diode in the return path for more than 10 wavelngths.
 

volker@muehlhaus

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Think of your setup as a radar system: you get the internal reflection (in the antenna) plus the reflection from any objects where you point the antenna.

If you point the antenna in the sky, there are no reflecting objects and then you see the internal reflection (=antenna matching) only.
 
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