[Elecraft] Receiver impedance?

[Ed Cole]

Most of you are HF operators; translation: you receive strong signals.

But if you design VHF/UHF/mw preamps, achieving low Noise figure (NF) is 
desired.  Advanced receiver designers are aware lowest NF does not occur 
at low-Z (eg. 50-ohms).  Input Z is lowest in NF at Z near 200-ohms 
(varies by device, ckt design, individual units).

This why achieving low NF is difficult when device is used on real 
antennas vs ideal NF meter 50-ohm setups.

But as I stated at beginning, HF receivers are designed with other 
parameters being more important (operating in a high RF environ, for 
example).

My eme preamps try to detect signals at -155 to -160 dBm (K3+PR6 sens is 
about -145dBm @ 500-KHz bw (NF=4 dB), which is way more than needed on 
most HF bands (exception 10m and 6m, where sky noise begins to drop 
making preamp NF become more important).  But local produced noise is 
still the limiting factor for most of us (and getting worse with more 
"wireless" devices encroaching our neighborhood).

On 6m I see S5 noise with PR6 (21 dB gain) on my K3. With 6m antennas 
elevated above horizon s-meter drops to S3.  80m is S8 with preamp off. 
With 50-ohm term instead of antenna I see S0

73, Ed - KL7UW