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HR20• 21
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. 271-265) wired in series with either outside ‘leg’ of the main
tuning control. Such a control can make fine adjustments to a CW signal so
the tone is pleasant and smooth sounding.
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.