NEC-LIST: NOISE FIGURE MEASUREMENTS

Hallo All,
          I have a question concerning measurement of the Noise figure
of a high impedance amplifier. First is this measurement accurate using
a standard HP Agilent NF meter. Does the device under test have to be
matched to 50 ohms. Second If I now add a passive circuit before the DUT
to simulate the antenna impedance , is this an accurate measurement ?
Why does the noise figure increase radically although I only add a small
capacitance and small resistance in series.?
           Any comments or ideas on this subject will appreciated .
           Thank you,
                   Zvi frank

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