What does "Q" stand for or represent?
==========================

Q is a universal measure of the selectivity, the sharpness of tuning, of a
tuned circuit at resonance.

It is also the ratio of reactance to loss resistance of a coil or capacitor
which forms part of a tuned circuit. Numerically, it is the ratio Q = X
ohms / R ohms. It is a measure of the "Quality" of an L or C component.

Values of low Q are 20 or less. Typical values of Q for HF tuning coils are
70 to 400. The physically large coils have the higher Q values. HF
capacitors can have Q values as high as several thousands.

In the present context a radio antenna, having distributed inductance and
capacitance, behaves as a tuned resonant circuit and has a low value of Q.
The loss resistance is the radiation resistance.

A 160 meter dipole of thin wire will have a value of Q around 14.

A 2 meter dipole made of aluminium tubing will have a Q around 5.

The working bandwidth of an antenna can be defined in different ways. But
it is always inversely proportional to Q. High selectivity = narrow
bandwidth. Low selectivity = broad bandwidth, a desirable property of an
antenna.
----
Reg, G4FGQ