Making sense of LNA specifications

A certain 2m low noise amplifier (LNA or preamplifier) is advertised with specifications of NF<1.5dB, gain=16dB.

The manufacturer says Don’t expect your TS2000, IC706 or 7000 etc to perform as good as the older receivers...turn the radio's preamp switch off on late model radios if you are going to use the ****** 2M preamp.

I have a TS2000 K model at hand. Measured sensitivity with internal preamp ON and OFF give NF of 6.4dB and 20.5dB respectively.

TS2000 preamp	ON	OFF	ON
LNA	NO	YES	YES
TS2000 NF (dB)	6.4	20.5	6.4
TS2000 Teq (K)	976	32249	976
LNA NF (dB)	0	1.5	1.5
LNA Teq (K)	0	120	120
LNA gain (dB)	0	16	16
LNA gain	1	40	40
System Teq (K)	976	930	144
System NF (dB)	6.4	6.2	1.8

The table above shows the calculated system Teq and NF for three configurations. The LNA is co-located with the transceiver, and interconnection loss is taken as zero.

The second configuration is that recommended by the LNA manufacturer in this case (citing IMD issues), which gives a system NF of 6.2dB against the transceiver’s native NF of 6.4dB.

The manufacturer deals with IMD saying: Intermod problems don’t seem to be an issue with this kit and most problems can be directed to the receiver's design.

An interesting outcome, not one that might occur at first reading of the specifications.

Results depend on the configuration and assumptions, and the outcome may be significantly different in different scenarios.