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 centre. Such an aerial will have little directivlty so need not be rotated, 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 operated 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 permitting 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 prepared 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 equipment 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 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 trimmer, be fully open, stretch the associated coil slightly to separate its turns. On the other hand, if either trimmer is fully closed, compress 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.
