On Sat, 15 Oct 2005 22:57:40 GMT, Cecil Moore wrote:

Dave wrote:
obviously between the tx and ant terminals it looks like a 50 ohm
transmission line.

So I repeat, what causes that characteristic? Is there
some coax inside the MFJ box?

I've been reading this thread while biting my tongue concerning the Zo of the
SWR measuring device. IMO the basis for the Zo of the device is being
overlooked, although Cecil is coming the closest to describing it.

We are discussing the Bruene version of the device in which the inductance
derives a sample of the current and C1 derives a sample of the voltage. When C1
is adjusted to obtain the correct voltage for the voltage-current ratio to equal
Zo at that point, such as 50 ohms, the device provides the correct readings when
the line impedance on the load side is Zo. The line impedance Zo on the input
side is irrelevant.

If C1 is adjusted to obtain the correct voltage for the voltage-current ratio to
equal Zo of 75 ohms it will provide the correct reading when the line impedance
Zo on the load side is 75 ohms.

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..

One can terminate the device with a lumped impedance and get the same answer as
with a transmssion line whose input impedance is the same as the lumped
impedance.

Walt, W2DU