"Walter Maxwell" wrote in message
...
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

Walt,

I hope people are listening to what you are saying. I built up a Bruene
meter in SWCAD using 0% tolerance components and other ideal parts. Works
exactly like Bird claims their meter does, except that the error only
depends on the PC floating point arithmetic. Transmission line or not makes
no difference. BTW, it is kind of neat to see the directional coupler
properties, by driving the two sides with different signals, and then being
able to separate them.

Tam/WB2TT