BANDSPREAD AND TUNING RANGE:
The easiest way to enjoy finer tuning , if you plan to use your receiver only for checking your favorite segment of the band, is to adjust the oscillator coil so that your desired segment falls within the second half of the rotation range of the tuning control. The first half of the rotation range covers about 70% of the actual tuning in KHz, while the other 30% is spread over the other half of the rotation range. If your interest is in monitoring SSB, the normal alignment will work fine. For optimum CW only listening, set the lower band edge with the tuning control in the 12:00 position of its range.
In planning any ‘bandspread’ improvement, it is useful to understand exactly how varactor tuning works. The control level varies the amount of DC voltage applied to the diode. The highest produces the lowest capacitance, while the lowest voltage yields the highest capacitance. The spread is typically about 20 to 30pF from minimum to maximum.The goal in achieving bandspread or ‘fine - tuning’ is to achieve a smaller variation in capacitance with the comfortable tuning of a knob.
Any resistor in a range from 500 ohms to 3000 ohms, connected in series with the ground lead of R2, will set a limiting effect on varactor capacitance range and therefore create a “bandspread “ effect. The higher the resistance value, the shorter the available tuning range.
If you wish to reduce the tuning range of R2 down to 20 or 30KHz or even less for a favorite band segment or calling frequency, reducing the value of C4 will decrease the capacitance effect of the varactor diode, D1. Values from 2 to 5 pf may give you exactly the tuning range you are looking for.
If you are using your own enclosure and have room for a separate fine tuning control, one useful choice is a 250 ohm speaker attenuation rheostat (Radio Shack model no.
If you want the opposite of bandspread (that is as MUCH frequency range as possible over full rotation of the tuning knob ) increase the value of C4. This will cause the varactor diode to have greater effect on the tuning. Try values of 33pF up to 100pF.