I realize while writing this I am probably preaching to the choir…however this is a subject that sometimes is so common we over look it. I heard two guys talking on their CB radios years ago…one said “your not coming it too good, turn you vis-wars up a little.”

When setting up an antenna and transmitter system, it is important to avoid impedance mismatching anywhere in the system. Any mismatch means some proportion of the output wave is reflected back toward the transmitter and the system becomes inefficient. When reflection occurs, standing waves are produced in the cable.

At some points along the line the reflective wave and the forward wave will meet in phase where the resultant voltage is at a maximum (anti-node). At other points the two waves will be out of phase and therefore the resultant voltage will be at a minimum (node).
It is important to note that the resultant wave appears stationary like and does not propagate like a normal wave and does not transfer energy toward the load.

Not only is this a waste of energy, but because of the voltage nodes and anti-nodes cause damage to the transmission line if the anti-node develops a positive voltage that exceeds the transmission line voltage rating.

There is a great demonstration video that describes standing waves in an old AT&T training film.

 

The ratio of the reflected voltage amplitude to that of the forward voltage amplitude is the voltage reflection coefficient.

Reflection coefficient Γ is defined as:

ZL = The value in ohms of the load (typically an antenna)
ZO = The Characteristic impedance of the transmission line in ohms

Once we know the ratio (reflection coefficient) the actual VSWR can be found.

Power Meter Accuracy

When using a directional power meter (reflectometer) for VSWR measurement, the VSWR value is based upon the values of forward and reflected power according to the following formula:

Where, PR and PF are values of forward and reflected power as determined by the power meter. Based upon the above formula, it is clear that the basic accuracy of the power meter will have a direct effect upon the VSWR measurement.

VSWR should be one of the main items to observe during our weekly transmitter visits.

“Uh, Breaker One-Nine, this here’s the Rubber Duck… Mercy sakes alive, looks like we got us a convoy.”

I’m gone…over and out!

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