RF AMPLIFIERS AND CONVERTERS144MHz Preamplifier
This preamplifier can be used with existing 2 metre equipment,
or ahead of the 144MHz converter described later. TR1 is the
40602 or 40673.
Aerial input is to a tapping on L1, and will generally be by
co-axial feeder. In some circumstances a short vertical aerial or
whip may be used and may provide sufficient signal strength. A
high aerial will naturally increase range and many different types
of aerial for 2m reception can be obtained. Alternatively, if a start
is being made on this band, a simple dipole may be constructed.
This can be self supporting, or of stout wire, and can be about
38½in in length overall, with the feeder descending from the cen
tre. Such an aerial will have little directivlty so need not be ro
tated, and can be raised on a light pole or mast.
For 144-146MHz reception, L1 is permanently tuned to about
145MHz by T1. Input is to gate 1, from a second tapping, and R3
with the by-pass capacitor C2 provide source bias. Gate 2 is op
erated at a fixed potential derived from the divider R1/R2. Output
from TR1 drain is to the tapping on L2, which is tuned by T2. For
a narrow range of frequencies such as the 2m Amateur band,
variable tuning is not justified, especially as L1 and L2 do not
tune sharply L3 couples to the existing 2m equipment - generally
a converter working into a lower frequency receiver.
L1 is wound with 18swg or similar stout wire, enamelled or
tinned copper. It has five turns and is tapped at one turn from the
upper end in Figure 4 for G1, and two turns from the grounded
end for the aerial. The winding is 5/16th in in diameter and turns
are spaced so that the coil is ½in long. L2 is wound in the same
way with five turns, but is ¾in long and has a centre tap for the
drain. L3 consists of a single turn of insulated wire, wound over
the lower end of L2.
When building VHF units of this and similar type, a layout per
mitting short radio frequency and by-pass return connections will
be required, and Figure 4B shows a layout for Figure 4. (Note
that TR1 is shown from the top.) A printed circuit can be pre
pared to take the components, or plain perforated board (0.15in
matrix) can be used, wired below. It is convenient to insert pins to
take L1 and L2. A small aluminium box will house the amplifier,
and this allows the co-axial aerial and output sockets to be
mounted as shown.
The screen to divide the box into two sections, to separate
gate 1 and drain coils, can generally be omitted as the layout
does not allow much feedback from L2 to L1. Tapping Gate 1
and drain down L1 and L2 also contributes to stability.
A 12v supply is preferred, but this can be 9v if other equip
ment provides this voltage and is also to supply the amplifier. The
amplifier can be self-contained if a battery is included in the box,
with on-off switch in series.
The bolts MC pass through the board and box, so that they
can provide a ground return, and they require spacers or lock
nuts. (Should it be felt that full details of preparing a printed cir
cuit, or wiring on perforated board, are required, reference can
be made to handbooks No. BP30 and No. BP35, Babani Press.)
Should a resonant dipper be available, or be constructed as
shown later, this will allow L1 and L2 to be set to about 145MHz.
If the coils are made exactly as described, adjustment of T1 and
T2 should give resonance in the 2m band. However, slight
changes in the length of leads, and similar points which arise in
construction, can influence the frequency. So should either trim
mer, be fully open, stretch the associated coil slightly to separate
its turns. On the other hand, if either trimmer is fully closed, com
press the coils to bring the turns nearer together. It is possible to
experiment with the taps, for best individual results, if the coils
are wound with tinned copper wire. Resonance can later be
checked when signals are being received through the amplifier.
To do this, or tune with no dipper, adjust the trimmers (and coils if
necessary as mentioned) for best volume
