Applying high-frequency voltage of a few to tens of MHz (typically 13.56, 27.12 or 40.68 MHz) between two electrodes to heat the material between them internally by dielectric loss. It's used to dry and bond wood, weld plastics, and thaw or pasteurise food. Lower frequency and longer wavelength than microwaves make thick materials easier to heat uniformly.

How differently do materials heat?

Heating is proportional to the loss factor ε''. At 100 kV/m, wet wood (ε'' 0.5) heats at 8.9 K/s and PVC (0.15) at 1.6 K/s, but polyethylene (0.0005) barely warms at 0.003 K/s. Selective heating is possible, melting only lossy PVC without heating surrounding materials.

Do gaps between electrode and material matter?

Greatly. Air's relative permittivity of 1 is below the material's (about 4 for wood), so as series capacitors the air takes much of the voltage and the field in the material drops. With a 5 mm gap in a 50 mm electrode spacing, the field in wood fell to 77% and heating to 59%. Thickness variation and gaps are major causes of uneven heating.

RF Heating Simulation

Category: Coupled Analysis – Electromagnetic-Thermal | Revised 2026-10-01

Theory: heating from inside with the field between electrodes

Overview

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How does RF heating differ from microwave heating?

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Both generate heat through dielectric loss, but frequency and apparatus differ. RF heating applies a low frequency such as 27 MHz across two parallel electrodes. Its 11 m wavelength is far longer than the equipment, so the gap behaves like a capacitor with a uniform field, heating even thick materials evenly throughout. Microwaves at 2.45 GHz have a 12 cm wavelength, form standing waves in the cavity and weaken within a few centimetres in moist material. In RF heating the field is just voltage over gap, but gaps between material and electrodes and thickness variation change it greatly, which analysis must capture.

Heating and field

$$ P = 2\pi f\,\varepsilon_0\,\varepsilon''\,E^2,\qquad E = \frac{V}{d},\qquad E_{mat} = \frac{V}{t_{mat} + \varepsilon'_{mat}\,t_{air}} $$

f = 27.12 MHz, ε'' is loss factor, V electrode voltage (rms) and d electrode spacing. The last formula gives the field in a material of thickness t_mat in series with an air gap t_air.

Coffee Break Trivia

RF welding and plywood

RF dielectric heating entered industrial use in the 1930s and 40s; during the Second World War it rapidly cured adhesives in aircraft plywood. After the war RF welders that melt and join PVC sheet spread widely, making raincoats, medical bags and stationery. Polyethylene can't be RF-welded because its loss factor is tiny and it barely heats.

Worked examples

Example 1: heating by material (27.12 MHz, 5 kV, 50 mm gap, 100 kV/m)

Material	ε′	ε″	Power density	Heating rate	Penetration depth
Wet wood	4	0.5	7.5 MW/m³	8.9 K/s	7.1 m
PVC	3.0	0.15	2.3 MW/m³	1.6 K/s	20 m
Polyethylene	2.3	0.0005	7.5 kW/m³	0.003 K/s	very large

The same wood at 2.45 GHz microwaves has a 78 mm penetration depth. PVC heats 300 times more than polyethylene. Permittivities are representative.

Example 2: effect of an air gap (50 mm spacing, wood ε′ = 4)

Air gap	Field in wood	Field ratio	Heating ratio
0 mm	100 kV/m	100%	100%
1 mm	94.3 kV/m	94%	89%
2 mm	89.3 kV/m	89%	80%
5 mm	76.9 kV/m	77%	59%

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Even a 2 mm gap cuts heating by a fifth.

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Wood's permittivity is four times air's, so for equal thickness air takes four times the voltage: 2 mm of air takes as much as 8 mm of wood. Heating scales with field squared, so a 10% field drop means a 20% heating drop. Real boards vary in thickness and warp, so heating differs where they touch the electrodes and where they float, causing uneven heating and scorching. Wetter regions also have higher loss and heat more, and lose loss as they dry, which naturally evens out moisture. RF heating simulation predicts such effects by coupling field, temperature and moisture.

Simulation workflow

Prepare permittivity (ε′, ε″) as functions of frequency, temperature and moisture.
Solve the field quasi-electrostatically (or in frequency domain) including electrodes, material and gaps.
Use heating as the source for temperature and, if needed, moisture transport.
Update permittivity with temperature and moisture and iterate field and heat.
Check field concentration at electrode edges and overheating at material corners.

Coffee Break Trivia

“Bonding scorched only at the edges”

In one RF wood-bonding process, board edges scorched while adhesive in the centre didn't fully cure. Field analysis showed electrodes slightly larger than the board, concentrating the field near their edges, and the board centre slightly warped away from the electrode, lowering the central field through an air gap. Matching electrode size to the board and pressing out the warp reduced temperature non-uniformity and eliminated the defect.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
Ignoring air gaps	Heating overestimated	Include gaps
Constant permittivity	Drying misjudged	Moisture/temperature functions
Missing electrode-edge concentration	Edge scorching	Match electrode size
Mixing peak and rms voltage	Heating off by 2×	Use rms
Thinking in microwave terms	Misjudged penetration	Treat quasi-statically

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