Baron wrote:

Thankyou, Jim & Roy.
Your explanations were most enlightening. I just couldn't get my head
around the "how & why" the VF should be different. I have also
realised why I have sometimes seen more than one TDR reflection from a
perfectly good transmission line.

You can easily excite a common mode wave on coax with a TDR -- or
transmitter -- simply by connecting to it with pigtails. This provides a
path between the inside and outside of the shield, unlike a proper coax
connector which preserves the integrity of the shield.

Roy Lewallen, W7EL