Radiofrequency characteristics of capacitors and inductors and implications for shielding

• Pages: 52-55

Appendix B – Radiofrequency characteristics of

capacitors and inductors and implications for shielding

Understanding the electrical characteristics of capacitors and inductors and their

reactances at radiofrequency (RF) frequencies is essential to shield RF heaters successfully.

These characteristics will be compared at the powerline frequency (50/60 Hz) and at a typical

RF heating frequency (27 MHz). The comparisons will emphasize the dramatic differences in

these characteristics between these frequencies and their important implications for successful

shielding of RF heaters. Failure to account for these differences in designing heater shielding

and assuming that familiar 50/60 Hz or DC electrical characteristics apply can result in

unsuccessful attempts at shielding which may increase operator exposure.

Capacitors

Any two conductors that are completely electrically insulated from each other by air or

some non-conductor, such as wood, porcelain, or plastic, can store a small amount of electrical

charge between them. Electrons can be removed from one conductor (plate) and carried to the

other conductor. If this is done, a small voltage difference will be present between the two

conductors generating an electric field between the plates. The two conductors form a

"capacitor".

For DC, the voltage scene will be a battery. At power-line frequencies, the voltage

changes direction (or polarity) 50/60 times per second. At 27 MHz, it reverses polarity

27,000,000 times each second. Due to the rapid oscillation at the higher frequencies, the current

will be less inhibited (less time to charge the capacitor). Thus the current is more likely to be

larger at the higher frequency.

Ohm's law mathematically relates the amount of current flow in a circuit to the amount of

voltage in the source. Equation B1 shows how the law is normally written for DC sources:

I = V / R (B1)

where: I is the current in amperes

V is the voltage in volts

R is the resistance in ohms.

This law is similar for AC circuits:

I = V / X (B2)

This reactance is composed of two components, the resistance (R) and a second part due

to capacitive and/or inductive reactance. These latter do not cause a power loss (heating) from

current flow, while the resistance (R) does.

The value of capacitive reactance may be calculated from equation B3:

Xc = ½ ƒC (B3)

where: Xc is the capacitive reactance in ohms

ƒ is the frequency in Hz

C is the capacitance in farads.

At RF heating frequencies it is more practical to express frequency in terms of MHz and

capacitance in terms of either microfarads (10-6 farads) or picofarads (10-12 farads).

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