Resonant conductors and waveguides: Applications to shielding

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Appendix C – Resonant conductors and waveguides:

Applications to shielding

All alternating currents and voltages can produce energy that radiates into space. At low

frequencies (e.g. 50/60 Hz), the wavelength is so long (5 × 106 m) that an efficiently radiating

antenna cannot be made. However, very good antennas can be made easily (and inadvertently)

at radiofrequency (RF) heating frequencies. Unintended antennas such as metal pipes, light

fixtures, and other metal objects can couple RF energy from nearby RF heaters and add

significantly to worker exposure. Thus a good understanding of antenna operation versus

wavelength is needed to minimize the RF energy coupled to these conductors which can

increase worker exposure near a leaking heater and at remote locations.

Resonant conductors

At certain lengths, sometimes called resonant lengths, conductors will both absorb and

radiate RF energy very readily. These lengths are related to the wavelength (or the frequency)

of the RF radiation. The resonance phenomenon occurs if the length of a conductor acting as an

antenna is one-fourth the wavelength or an integral multiple of a one-fourth wavelength. Under

resonance conditions, an electrical conductor will absorb and radiate the most RF energy. For

example, a 2.8 m (9.1 ft) long metallic conductor one-fourth the wavelength at 27 MHz) will

most efficiently absorb and reradiate RF energy from a nearby 27 MHz dielectric heater. The

RF energy reradiated from this metallic object can add to operator exposure or that of other

workers. At twice the 27-MHz fundamental frequency (i.e. the second harmonic), the length of

the metallic conductor need be only 1.4 m (4.6 ft); at three times the frequency, it need be only

0.9 m (3 ft) long, etc. This is important because many dielectric heaters radiate considerable RF

energy at harmonic frequencies in addition to the fundamental or operating frequency.

The most effective way to minimize this reradiation from metal objects is to minimize

the RF energy leakage reaching the conductor which can act like an antenna. Changing the

conductor length may be helpful in reducing reradiated RF energy, but a resonant length must

not exist over the entire frequency range (including the fundamental and harmonic frequencies)

at which the generator will operate. When harmonic frequencies (integral multiples of the

fundamental operating frequency) exist, eliminating all RF energy absorption and reradiation

for a conductor can be very difficult, if not impossible. For example, if the RF heater generator

operates with a range of fundamental frequencies of 25-30 MHz, the harmonic ranges are:

2nd harmonic 50 to 60 MHz

3rd harmonic 75 to 90 MHz

4th harmonic 100 to 120 MHz

5th harmonic 125 to 150 MHz

6th harmonic 150 to 180 MHz

7th harmonic 175 to 210 MHz

8th harmonic 200 to 240 MHz

... etc.

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