SLNA 145sb installation instruction
Thank you for buying muTek limited’s SLNA 145sb transceiver-optimized
preamplifier. Although it has been designed specifically for the Yeasu/
Sommerkamp FT290 transceiver, it may find application in other transceiver for
which no complete modification is available. The installation notes below refer to
the FT290: we regret that we have no detailed information available regarding
installation of the unit in other transceivers.
There are usually two reasons for the less than adequate sensitivity of current
transceivers. Firstly, the receiver designer’s brief includes a dynamic range
specification which leads him to balance large signal handling with sensitivity.
With devices currently available at prices the transceiver manufacturer is
prepared the pay, the balance comes out to around 4db noise figure and 70db
intermodulation-free dynamic range in SSB bandwidths. The second point is that,
also to save money, designers shy a way from the use of electromehanical relays
from antenna changeover switching and tend to use various forms of diode
switch. These inevitably lead to greater insertion losses than suitable relays,
often approaching 4db! Thus it’s not usual for the overall receiver noise figure to
reach 8db or so!
At 144MHz, sky noise limits the maximum usable sensitivity of a receiver used
for terrestrial communications to about 2db noise figure (this is about the same
as 0.05uV Pd for 10db S+N/N ratio in SSB bandwidth). Lower noise figures are
easily obtainable with modern devices, but they won’t let you hear any more!
However there is distinct advantage in using a very low noise preamplifier to
improve the sensitivity of transceiver – if has been properly designed.
Overall system noise figure depends not only on the noise figure of the
preamplifier, but also on its gain and the noise of the subsequent stage (the
transceiver in this case). By adjusting the gain of the preamplifier it is possible to
set the system noise figure to any value greater than intrinsic noise figure of the
preamplifier. But, why bother to adjust the gain? It’s an unfortunate fact that a
more gain ahead of the receiver, the more susceptible it becomes to overload
effects. By putting just enough low-noise gain ahead of the receiver to set the
overall sensitivity to a level where external noise is the limiting factor an optimum
( for the system) is reached. A very low noise amplifier such as SLNA 145sb will
minimize the amount of gain required, and hence the degradation of dynamic.
CIRCUIT DESCRIPTION
A low noise relay provides the antenna changeover function. This is followed by
a BF981 in an input nose-matched, output conjugately-matched configuration for
a very low figure with optimum dynamic range. Following the output matching a
variable attenuator provides the gain control without compromising the dynamic
