All noise is reduced in proportion to the bandwidth,
both external and equipment (thermal) noise. The power of thermal
noise is given by kTB, where k is Boltzmans constant, 1.38E-23, and B in
degrees K. Gives in -174dBm per
all but very tiny, lossy, antennas, atmospheric noise usually dominates so
a fantastically low noise figure is not essential. A tuned loop gives an
output vastly greater than thermal noise, but when untuned and if it is
particularly lossy as well, then the levels may be comparable. The MAR6
has a noise figure better than 4dB at frequencies up to 1GHz so gives a good
account of itself at LF, but this level of performance is unnecesasry at LF if
using a tuned loop.
I have been thinking about weak signals on 136
using QRS and DFCW. These techniques enable signals to be read which are
buried 30db down in the noise(see Extreme narrow bandwidth techniques ON7YD )
Does this mean that signals 30db down on atmospheric noise are seen,if so what
about the noise from the RX pre-amp ? In my case this is about 20/30 db less
than the Ant. noise,so would I be looking at the pre-amp noise or is this also
reduced by the narrow bandwidth ?
I should like to improve the noise performance of
filter combination. and would appreciate any
information on high performance circuits or amp. units. Is the MAR6 likely to
be worth while.
By the way if my thoughts are correct this may be
why high Q loops are reported to be superior to lower Q types, higher signal
output overriding amp. noise. Any ideas ?
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