On Fri, 3 Sep 2004 17:16:48 -0300, "Another Voice" wrote:

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"Richard Harrison"
Just how short can a transmission line be
and still enforce its Zo?

The whole thing is perfectly clear if one imagines applying a step function
(rising edge) to any short, even VERY short, length of transmission line.
The current in the short line will be equal to V/Zo - at least until the
reflections (if any) start arriving back at the input. If the line happen
to be terminated with Zo, then no reflections and I=V/Zo is the steady
state.

The only issue of shortness is that a very short line means very short time
until the reflections arrive.

The step function makes things a lot easier to understand than RF. It
'enforces' the distinction between the transient period and steady state.

IMO, the length of the line is irrelevant when using a device such as the Bruene
bridge or a Bird 43. Each of those instruments are designed or adjusted to
indicate the forward or reflected power, based on three things: 1) ratio of the
foward and reflected voltages, the voltage reflection coefficient 2) the scale
numbered from 0 to 1, where 0 indicates the reflection is zero, and 1 equals
total reflection, but the significant point is that a 3:1 mismatch gives a
reflection coefficient of 0.5, which then means that the half-scale reading of
0.5 indicates the 3:1 mismatch, or a 3:1 SWR, and 3) the device is so designed
or adjusted so that the voltage ratios indicate the correct value because it's
inherent characteristic impedance, Zo, is 50 ohms.

Thus, no transmission line is necessary. For example, the device can be
connected directly to the antenna terminals, or any other device you desire to
determine the mismatch, and power it directly from the signal source--no
transmission line is needed on either port for the device to indicate the degree
of mismatch.

Walt, W2DU