AAVSO: Capacitor Characteristics

The capacitor used to tune the Loop Antenna can be of various types. Depending on the type of capacitor you use you will see varying degrees of tuning sharpness. This is due to a parameter called the Q of the capacitor. The Q value refers the quality of the component as far as energy loss is concerned. Stated verbally, the Q of a component is the ratio of the energy stored to the energy lost.

A Capacitor with a higher Q value is more efficient and therefore the circuit will tune more sharply and thus have a lower bandpass. This helps to tune in the desired VLF signal and reject unwanted noise and signals with a frequency close to that which you are monitoring.

Below is table of the Q value for different types of capacitors. The difference lies in the material used in the construction of each type.

	Q (1 Khz)	Temperature Coefficient (ppm)	Temperature Range (°C)	Comments
Mica	600	0 to +70	-55 to +125
Polystyrene	2000	-150 ±50	-55 to +85
NPO Ceramic	1500	±30	-55 to +125	Small Size
Polypropolene	3000	-115	-55 to +125
Glass	1500	140 ±25	-55 to +125
Polycarbonate	500	-50	-40 to +100
Mylar	100	large	-55 to +85
Polyester	100	-160	-40 to +100