as long as it is ONLY 2 ports, has ONLY passive linear components, AND
is small enough to be considered a lumped element, then you can always
reverse the terminals and not know the difference.


You have pinpointed the correct issue: lumped elements.

The story began when, a few days ago, I was going to replace a trap of my HF
yagi. Not to make mistakes, I consulted the antenna assembly manual where I
found a big banner: do not invert traps otherwise the antenna will not work.

So, I thought, this is a case in which a bipole cannot be inverted.

This is clearly due to the fact that the external body of the trap (an aluminuim
can about 2 feet long), which contains two coils resonated at different
frequencies by means of built-in capacitors, is effectively part of the antenna
radiating element. So, the trap is a bipole not only comprising lumped elements,
and that is the reason why it cannot be inverted.

73

Tony I0JX
Rome, Italy