Mutek SLNA 145sb Installation Instruction

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

performance, which would be case if the usual amateur practice of providing gain
control by varying the bias on G2 of the FET was followed.
After the attenuator, a property designed bandpass filter provides very
substantial rejection of out-of-bands signal.
The preamplifier is designed and tested to very high standards. A platted throw
hole epoxy fiberglass pcb is employed, and bushed mounting are provided for
mounting in the FT290.

INSTALATION
Before attempting installation of the SLNA 145sb it is very strongly recommended

that the manual and circuit diagrams supplied by Yaesu/Sommerkamp are
studied thoroughly. If you are at all unsure of your abilities muTek strongly
recommends that you find a competent technician to perform the installation: we
can not accept responsibility for any damage however caused.
If any difficulties are encountered than please get in touch with muTek – we want
to make sure that you are happy

The preamplifier mount on the lugs provided for mounting for optional tone
encode / tone squelch assembly. With the bottom of the unit removed, those may
be found close to the “switch B” unit (see mage 33 of the manual).

1. Remove both top and bottom cover of the transceiver.

2. Remove battery compartment (4 screws).

3. Locate and remove C101. This (normally 470pF or 1nF) capacitor is
located on the main pcb near the PA compartment close to the back panel
of the transceiver. This is probably best done by crushing the component
with a pair a long-noised pliers and cutting clear remaining debris – it may
sound crude, but it’s better than wrecking the reverse side of board –
easily done!

4. Locate and remove L02 – this is usually yellow sleeved toroidally wound
inductor going between the stand-off in the PA compartment and the
adjacent tag-strip. Keep this component – in the unlikely event of the
SLNA 145sb having to be returned for service, the FT290 can fairly easily
be returned to its unmodified state if the removed components are
retained (almost any sub-min plate ceramic capacitor of around 1nF would
be suitable for the C101 in this eventuality).

5. Remove telescopic antenna.

6. Remove the anodized aluminum stripes from each side of the transceiver
case.

7. Unwrap the SLNA 145sb and the kit of cables. Refer to the diagram below
and solder the cables to the preamplifier as shown. BE VERY CAREFULY
NOT to allow small whiskers of the braid for example to short across pins.