Looking back through my old notes, the ENR at 1MHz was lower than 5.20dB so this may be a typo on my ENR table stored in the analyser. However, I possibly boosted it to prevent 'pulling' of the NF results when operating just below 2MHz. My notes suggest that the ENR drops quite rapidly below about 1.5MHz. I should probably have added an extra ENR table entry at 1.5MHz because of this.
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I wouldn't trust it below about 2MHz anyway. There are too many contributors to overall uncertainty. The analyser uncertainty will be quite significant here although the use of an external LNA helps in this respect. Not many modern analysers from HPAK are going to be usable below 10MHz unless an external LNA is used. The noise floor and VSWR is going to degrade below 10MHz with the internal factory preamp enabled. My analyser manages a noise floor of about -170dBm/Hz down to a few MHz with my external preamp fitted and the preamp input VSWR is about the same or better than the 346A noise source VSWR at these sub 10MHz frequencies.
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