Walter Maxwell wrote:
The line impedance of either the input or load transmission line is irrelevant
to the basis for the Zo of the device, therefore a length of line is unnecessary
to establish a Zo environment, the current sample and voltage sample establish
it..

Walt, there's one thing I don't understand about what Dave reported.
As I understand it, these were the test setups.

TDR---50 ohm coax---MFJ---50 ohm coax---50 ohm load

The TDR doesn't see the MFJ wattmeter.

TDR---75 ohm coax---MFJ---75 ohm coax---75 ohm load

The TDR sees the MFJ wattmeter.

If the MFJ in the second setup were recalibrated for 75 ohms,
would the TDR not see it?

************************************************** **************

The threads for the past few days have all diverged from the
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.
--
73, Cecil http://www.qsl.net/w5dxp

On Sun, 16 Oct 2005 14:58:52 GMT, Cecil Moore wrote:
How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.
Already back peddling I see.

On Sun, 16 Oct 2005 14:58:52 GMT, Cecil Moore wrote:

Walter Maxwell wrote:
The line impedance of either the input or load transmission line is irrelevant
to the basis for the Zo of the device, therefore a length of line is unnecessary
to establish a Zo environment, the current sample and voltage sample establish
it..

Walt, there's one thing I don't understand about what Dave reported.
As I understand it, these were the test setups.

TDR---50 ohm coax---MFJ---50 ohm coax---50 ohm load

The TDR doesn't see the MFJ wattmeter.

The TDR doesn't see the wattmeter because there is no discontinuity.

TDR---75 ohm coax---MFJ---75 ohm coax---75 ohm load

The TDR sees the MFJ wattmeter.

It seems to me the amount of the discontinuity the wattmeter calibrated to 50
ohms the TDR sees would be determined by the tightness of coupling of the
sampling inductance and capacitance to the main line.

If the MFJ in the second setup were recalibrated for 75 ohms,
would the TDR not see it?

I believe the TDR would not see it.

************************************************* ***************

The threads for the past few days have all diverged from the
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.

Cecil, as I explained earlier, if we're considering the Bruene type SWR
indicator, the setting of the sampling cap in relation to the inductive sampling
determines the measurement reference, not the length or Zo of the coax. So I
repeat, with this SWR indicator zero coax length is sufficient. It will produce
the same answer with a lumped impedance as the load as it will with the same
impedance appearing as the input impedance of a coax.

Walt, W2DU

Walter Maxwell wrote:

Cecil Moore wrote:
How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.

Cecil, as I explained earlier, if we're considering the Bruene type SWR
indicator, the setting of the sampling cap in relation to the inductive sampling
determines the measurement reference, not the length or Zo of the coax. So I
repeat, with this SWR indicator zero coax length is sufficient. It will produce
the same answer with a lumped impedance as the load as it will with the same
impedance appearing as the input impedance of a coax.

But Walt, there no SWR possible on a zero length line. I think it
was Reg who said that first. I can now see how the initial confusion
arose and why the thread drifted.

Source---75 ohm line---SWR meter---load

The SWR meter will not report the actual SWR on the external 75 ohm line.

Source--75 ohm line--50 ohm line--SWR meter--load

The SWR meter will report the actual SWR on the external 50 ohm coax if
the external 50 ohm coax is _______ long.

Turns out the answer is a lot smaller than I previously thought.
The answer is less than one inch for the most popular 50 ohm coax's.

I didn't mean for the original question to be a trick question but it
apparently turned out that way.
--
73, Cecil http://www.qsl.net/w5dxp