dykesc wrote in news:1178814532.207062.89230
@h2g2000hsg.googlegroups.com:

I just aquired an MFJ 259b analyzer. I'm using it to measure the
impedance at the input of the 300 ohm twin lead I have feeding a
sloping off center fed dipole. I measured 75 -j236 at 3.94 Mhz. When i
reverse the txmsn line leads where they connect to the 259b I get 175 -
j237. Does anyone have an explanation as to why the resistance value
changes simply by reversing the way the txsmsn line is attached to the
analyzer? I have repeated this numerous times, attaching and
reattaching always with very near the same results. Does the fact that
the antenna has unequal leg lengths somehow explain this? If it
matters, there is a 1:1 current balun between the txmsn line and the
antenna feedpoints.

You haven't mentioned a balun at the 259B, or any other device to float
the measurement terminals to make a true differential mode impedance
measurment without significantly disturbing the thing you are measuring.

There are issues regarding balance of the feedline with an OCF dipole,
but reversing the meter for different readings suggests that the meter
terminals are not sufficiently isolated from the environment (ground, the
adjacent transmission line, possibly a power cord).

What have you done to make the 259B appear as an isolated impedance
meter?

Owen