One should compare a terminated RF stage to the antenna, not an open
antenna connector. In many RF stages, especially those critically
matched for best noise figure, changing the source impedance changes the
gain and the noise from the RF stage. A small change is critical to a
very good RF stage and can confuse the measurement a great deal. The
massive change from antenna to open can have a much greater effect. An
oscillating stage (on that open circuit) might block the mixer and make
the receiver unusually quiet but not be a realistic operating condition.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.
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