[NLRS] 8478b thermister head

[Mike]

Hi all, I have a question.  I have an 8478b thermister head that goes 
with a HP 432 power meter.  The head works fine when used alone, that is 
to say it agrees with my frequency generator and the 1mw output from a 
hp435.  When you put any kind of an attenuator on it though, which you 
almost always have to do to read any kind of useful power, the meter 
pegs right away with nothing else connected.
Why is this???

Thanks for your help

Mike
KB0ozn