What a simple, concise, and logical explanation. Spock would be proud.
This should be standard reference material.
On 5/24/2012 4:49 PM, Gerald Youngblood wrote:
Lee,
It is a very common misconception that lower noise figure is always better.
A good article on the subject was in the June 2010 issue of QST. The
article written by Joel Hallis is titled, "Receiver Sensitivity - Can you
have too much?" The answer is yes. All you get is more noise and lower
total dynamic range. What you want is for the gain to be set optimally for
the band noise floor at your specific location.
In fact a low noise figure may actually reduce total dynamic range for a
given band and conditions. We could easily have put a<1 dB preamp for the
same cost in the radio but that would have degraded total total IMD dynamic
range. If you really care about a 0.1 dB NF preamp, it would be a total
waste to put that inside the radio because it would ruin gain distribution
and it would be swamped by the coax loss.
The FLEX-6700 can give you a 4 dB NF on 20m but that would
would ridiculous since the atmospheric noise figure equivalent in a rural
area is probably greater than 35 dB above kTb. All you would be doing is
to reduce the total dynamic range because you have too much gain. MDS of
around -120 dBm is probably appropriate for most locations on 20m On 10m
you probably need an MDS of around -130 dBm in rural areas and -122 dBm
in residential areas. On 10m you can probably use -137 dBm (10 dB NF) only
if you are in the quietest rural areas.
The bottom line is if you want lowest noise figure on 2m, put the low noise
preamp at the antenna and turn off the preamp in the radio. That will give
you better gain distribution and will overcome the coaxial line loss.
Regards,
Gerald
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