Topband: Intrinsic impedance of free space

[John Harden, D.M.D.]

Dear Ford,

The intrinsic impedance of free space was described by the late (great) John
Kraus (W8JK) in his book ELECTROMAGNETICS in 1952. He was a professor of
electrical engineering at Ohio State University for many years. My late
father gave me this book when I was a student at the Georgia Institute of
Technology in the 1960’s.

This has do with various reflection and transmission situations at a
“boundary”. A special case exists in which the incident wave is terminated
so that no wave will be transmitted or reflected.

The intrinsic impedance of free space is 376.7 ohms. This concept of an
impedance for free space takes on more physical significance if we consider
the properties of a resistive sheet having a resistance of 376.7 OHMS PER
SQUARE. Material so treated is often called SPACE PAPER OR SPACE CLOTH. It
should be noted that the resistance is NOT per square centimeter but simply
per square. This is equivalent to saying that the resistance between the
edges of any square section of the material is the same.

The conductivity of the material required for a sheet of the space cloth
depends on the thickness of the sheet. Without going through all of the
derivations the impedance presented to the incident wave at the sheet is the
resultant of the space cloth in parallel with the impedance of the space
behind it. This is ½ of 367.7 or 188.3 ohms. It is apparent that a sheet of
space cloth is insufficient to terminate a wave.

In order to completely absorb or terminate the incident wave without
reflection or transmission one can place a perfectly conducting sheet
parallel to the space cloth and ¼ wave behind it. Now the impedance
presented to the incident wave at the sheet of space cloth is 367.7 ohms,
being the impedance of the sheet in parallel with an infinite impedance. As
a consequence, this arrangement results in the total absorption of the wave
by the space cloth. However, there is a “standing wave” and energy
circulation between  the cloth and the conducting sheet.

This is like a shorting bar on a transmission line reducing the wave beyond
it to a small value. Of course a transmission line may also be terminated by
placing an impedance across the line which is equal to the characteristic
impedance of the line, and disconnecting the line beyond it. 

Although this provides a practical method of terminating a transmission
line, there is no analogous counterpart in the case of a wave in space. This
is because it is not possible to “disconnect” the space to the right of the
termination. A region of space may only be isolated or shielded as by a
perfectly conducting sheet.

Ford, you heard right, and the phenomenon does exist. However, it’s “all in
the books”.

73,

John, W4NU

Atlanta, K4JAG (1959 to 1998)