Selectivity Hallicrafters National Hammarlund 
The selectivity system in receivers with variable selectivity, like the Hallicrafters SX115, SX101, Hammarlund HQ170, HQ180, and National NC300 and NC303, adjust bandwidth by varying coupling between circuits. This is a SPICE model the basic system. It shows values used by Hallicrafters:
L1 and L2 are the IF tuning inductors. Q is ~170 (set by resistors), based on measurements of several IF cans from old receivers. C1 is the resonating capacitor C3 sets the minimum coupling coefficient, or minimum selectivity The model source impedance is 100k ohms, the load impedance 500k ohms. The various resistors make the spice model happy by adding reasonable losses RS4 and CS4 set bandwidth, they are on a switch with the following:
The Hallicrafters receiver circuit uses (2) .01 uF capacitors, a .0047uF capacitor, or a "short" to ground:
Terminal 1of S5C is always connected to the rotor. Bandwidth selects the following shunt values for each position in descending bandwidth: 1.) 4700 pF C90 2.) .01uF C91 (this picks up and holds C91 connection for all remaining positions) 3.) .01uF C91 (now held) + 4700pF C90 (C90 is back in and holds as the notch moves beyond contact 2) 4.) .01uF C92 + (held .01uF C91 + 4700pF C90) 5.) Short to ground
With a perfect short, the 500Hz position, we have 6dB points:
49.579kHz is the 6dB starting point. Changing components to 1kHz BW we have 6dB points:
49.756 kHz is the 6dB lowest starting point.
Changing to 3kHz BW we have 6dB points: 49.709 kHz is the lowest frequency 6 dB starting point.
The passband moves upwards with a lower capacitance value (higher reactance) in CS4 (selectivity), but the lower edge starting point remains the same. The effect of this is if the absolute value of CS4 drifts higher, the upper end of the passband pulls lower. This pulls the center lower, but the actual effect is a reduction of bandwidth. Decreased capacitance adds some to the top of the passband without significantly moving the bottom. Increased capacitance (less jX) pulls the upper end lower without significantly moving the lower end. If we have a drifty capacitor for C4, a great deal of drift will alter the upper frequency points as coupling changes modify the bandwidth. Changing bandwidth capacitor value will not significantly alter the peak frequency. For one section:
