>>
A 0.5 dB noise figure front end amplifier with NO other losses would produce
-149.5 dB MDS. That is the absolute maximum MDS sensitivity obtainable with
250 Hz BW and 0.5dB total input noise figure.
<<
Correct agree
>>
If we include the receiver's noise figure, 14.5 dBm gain would result is a
system composite noise figure of 3.44 dB.
At someplace around 20-25 dB, you get into system limits. The improvement
from 30 dB to 40 dB is only 0.11 dB. No one will notice that.
>>
Agree when the signal is above noise floor
The issue is most internal preamps are not designed for 160m, they need to
cover up to 50 MHz Even the ICOM Norton preamp does not have the muscle to
handle the signals level on 160m. If you turn on you internal preamp and use
a better preamp outside the radio with a good BPF or pre-selector you will
always have the flexibility to adjust the system NF reducing RF gain. But
there is no way to improve NF it is all about less degradation of the signal
to noise ratio.
The low NF is required only when the propagation noise is very low, the
concept of degradation is based on human skills and vary from operator to
operator, Most of us can copy CW signal 3 db above noise, 2 db above noise
is hard to copy, and 0 db SNR is very hard but some of us can compensate
that in their brain.
What I don't agree is that :
When the signal is at noise level. 3 db improvement on signal to noise ratio
can make a QSO possible, you can hear the DX early and for a long time
before it fade. 3 db degradation means 50% power noise and 50% signal power
noise. Near the MDS 3 db signal to noise is close to 3db NF improvement.
At this point the difference is copy or no copy. QSO or no QSO. Log the DX
or wait the next DX expedition.
<<
There is a point where inevitable system flaws make using an antenna with
such negative gain to require less than 1 dB NF impossible for "copy this
plan". This is why Beazley's out-of-phase small horizontal elements were
mostly met with didn't work. The problem with models is we can build perfect
systems that we cannot repeat in the real world.
>>
Agree , the practical noise figure for the system is around 2 dB NF, The way
to drop the noise floor few db more is reducing the BW to 100 Hz or 50 Hz.
An EME experienced CW operator knows that some signals you can copy using 50
Hz and no copy with 100 Hz. It is that simple in real weak signal DX.
The MDS can be low as -157dbm for 50 Hz BW, and this is almost -s9, if
using 6db for each S unit. There is a lot of dynamic range bellow s0.
3db makes a lot of difference, I fight for every .1 dBm I can get.
<<
Again my example of the small commercial loop I have.
>>
The HWF is not so small, for the WF300 (300 sq Ft.) I use 44 ft boom and
loops 24ft.x12ft.
However for a noise location there is one ideal size, the trick is to drop
local manmade noise bellow or near the MDS. During the day the HWF looks
like a dummy load noiseless antenna. My system's uses different gain for
each band and tuned for a radios with 20 NF (no internal preamp on).
For noise locations the WF200 with 24 ft boom is a good choice.
Like I said , it's not a weekend project but can be done and it performs
very well. Polarization filter is a powerful tool for urban stations that
cannot be disregarded.
There is no commercial loop compatible or comparable to the HWF antenna, it
is a different ball game.
The results worth the time and money to implement it.
It all a good debate and we agree in most of it. Or at least we agree we
disagree.
Regards
JC
N4IS
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