The response of the display to switching on the rig preamplifier or attenuator may be puzzling at first because
it is not intuitive. NaP3
compensates for the published preamp gain(s) and attenuator loss(es) of the connected rig. This keeps the display calibration correct, a signal
remaining at the same level whatever the post-antenna-connector settings may be. The first picture below is of an (XG2) -73dBm signal with
no preamp or attenuator.
Switching on a preamp (in this case the K3, +11dB) does not increase the displayed noise floor - it *decreases* it by the amount of gain to
keep the signal level correct:
Switching off the preamp and switching on the attenuator (K3, -10dB) raises the displayed noise
floor by the amount of loss to keep the
calibration constant:
These responses can be useful when monitoring band conditions. For example, on a high band,
if the noise floor does not drop when you
add preamp gain, the sensitivity has not increased - so you don't really need it! Best dynamic range can be retained by leaving it off.
Similarly, on a low band, if the noise floor does not rise when you add attenuator loss, your
minimum detectable signal is limited by the
external noise floor and the attenuator can be left on to maximise dynamic range. In this way, with a directional antenna, it is possible to
build up an idea of the noise 'map' around the QTH. At the author's QTH the preamp sometimes improves sensitivity on 40m in one
direction but not the other.