Bandwidth and Q are usually applicable to resonant circuits, not only to antennas. Q is a measure of the quality of a resonant circuit. This is the quality factor, corresponding to narrow bandwidth, which is usually ideal in resonant circuits.
If the resonant circuit contains reactance and resistance components, Q is the ratio of stored power to dissipated power:
Q = P STORED / P DISSIPATED = I 2 X / I 2 R
simplify:
Q = X / R
Where X=capacitive or inductive reactance
And R=series resistance
The equation for Q is applicable to both series resonant (also known as series variable frequency resonant) and parallel resonant circuits, where a resistor is connected in series with an inductor. Because X is in the molecule, the larger the reactance, the higher the Q. Because R is in the denominator, greater resistance means lower Q. Since I am squared, it is important in similar equations, but appears here in both the numerator and denominator, canceling out and not affecting Q or vice versa affecting the bandwidth of the circuit.
At the resonant frequency, the capacitance and inductance are equal and cancel each other out. The resistance component includes circuit impedance. Lower resistance produces higher Q and lower bandwidth.
During resonance, the impedance in a parallel resonant circuit is at its maximum. Above or below resonance, impedance decreases. During resonance, the impedance in a series resonant circuit is minimized. Above or below resonance, impedance increases. When the resistance decreases, these effects become more pronounced, resulting in a higher Q factor and less bandwidth.
By connecting the RLC resonant circuit to an arbitrary function generator sine wave at the input, these effects can be demonstrated. Using a potentiometer can change the resistance. Detect the output and connect it to the analog input channel of the oscilloscope. Change the frequency of the AFG sine wave to find resonance. If necessary, please replace the capacitor. Display the circuit output in the frequency domain using Math>FFT. Then change R by rotating the potentiometer to observe the effect on Q and bandwidth.