Heat Sinks and Cooling for Ham Radio Transceivers

A transceiver rated at 100 watts output does not turn all the energy it consumes into radio waves. The power amplifier in a typical linear design converts only about half of its DC input into RF, and the rest becomes heat in a handful of transistors that are only a few centimetres across. If that heat cannot leave, the temperature of the silicon rises until the radio reduces its power, switches itself off or, in the worst case, fails. A good heat sink (see the general article on the heat sink) and a clear path for air are therefore as much a part of a radio as the filters and the oscillators. This guide explains the idea of thermal resistance with simple numbers, shows how to estimate the temperature of a device, describes how to mount and cool a transistor properly, and gives practical advice for the shack, the field and long digital transmissions.

Where the Heat Comes From

An amplifier stage dissipates the difference between the power it takes from the supply and the power it delivers. For a class AB amplifier of 100 watts output with an efficiency of 45 percent, the DC input is about 222 watts, and the dissipation is about 122 watts. Class A stages are far worse, and class C or switching stages are far better, as explained for the classes in our overview of the amplifier families. The consequence is that a few watts of RF at the antenna require a large flow of heat from the final transistors to the outside air.

Heat is not spread evenly. In an SSB transmission the average power is only a fraction of the peak, since speech has gaps and a high peak-to-average ratio, so the average dissipation is maybe a third of the maximum. For a continuous mode, such as FT8, RTTY, a long CW carrier or an FM transmission, the dissipation is the full amount, and the temperature rises without pause. That is the reason why a radio that is comfortable on SSB can overheat on a digital mode, an effect described in the article on the FT8 duty cycle.

Thermal Resistance in Plain Numbers

Thermal resistance tells you how many degrees the temperature rises for every watt of heat that flows through a path. It is written in degrees Celsius per watt and is used exactly like electrical resistance in Ohm’s law, with heat flow in place of current and temperature difference in place of voltage. The heat flows from the junction of the transistor to its case, then through a layer of contact material to the heat sink, and then from the heat sink to the air. The total is the sum of the three: junction to case, case to sink and sink to ambient. The junction temperature is the ambient temperature plus the power times the total resistance.

Take a transistor that dissipates 50 watts. Its data sheet gives a junction-to-case resistance of 0.5 degrees per watt. The thermal paste or pad between case and heat sink adds 0.2 degrees per watt, and the heat sink, with a fan, has 0.6 degrees per watt to the air. The total is 1.3 degrees per watt, so the junction is 65 degrees above the air. In a room at 25 degrees Celsius the junction reaches 90 degrees. With the fan stopped, the heat sink’s resistance may rise to 2 degrees per watt, the total becomes 2.7, the rise is 135 degrees and the junction reaches 160 degrees, which is beyond the limit of many devices, typically 150 to 175 degrees. The same arithmetic works for any component and shows why a fan is not a luxury. The structure of the calculation is no more difficult than Ohm’s law for ham radio.

Mounting: The Joint Is Half the Job

The thermal resistance between the case and the sink depends on the quality of the contact. Surfaces that look flat are rough at the microscopic level, and the air in the gaps conducts heat very badly. A thin layer of thermal paste fills the gaps. Use a very thin, even film; too much paste is worse than none, because the paste conducts heat less well than the metal. Clean both surfaces first, fasten the transistor with the screws at the torque recommended, and tighten them alternately to avoid tilting the case.

A flat, burr-free sink surface and a good flange make the difference. If the transistor must be insulated from the sink, as with a package whose case is connected to the collector, use an insulating pad or a mica washer with paste on both sides. Ceramic insulators and the modern silicone pads have a higher resistance than direct metal contact, usually 0.3 to 1 degree per watt, so allow for it. Do not forget the electrical side: a flange with an insulator must also be fitted with insulating bushings for the screws, and the insulation tested with a meter before the power is switched on.

Airflow: The Sink Is Only a Surface

A heat sink moves heat from the transistor to a larger surface, and from that surface to the air. The cooling depends on the air. In still air, warm air near the fins rises, and cool air replaces it, and this natural convection works best when the fins are vertical so that the air can flow upward between them. A sink mounted flat with fins horizontal, or in a closed box, loses much of its power. Forced air from a fan improves the cooling by a factor of two to five, and the fan must blow along the fins, not at a wall of fins, and the air must be able to leave. A radio on a shelf with a wall two centimetres behind it, or a cover on top of the vents, deprives the fan of the air it needs.

Fans are a compromise with noise. Many transceivers control the fan by temperature, so it is quiet during receiving and speeds up during transmission. If your radio’s fan is noisy at a modest power, the best remedy is to improve the airflow, to clean dust from the fins and the grille with a brush and a small vacuum cleaner, and to lower the power a little. Dust forms an insulating blanket on fins and a layer on the blades, and a radio that has run for years may run hotter for that reason alone. The layout advice in the article on ham shack layout includes ventilation space around each device.

Amplifiers and the External Heat Sink

Solid-state amplifiers carry large heat sinks, often with fans, and are usually designed with a margin for SSB but not for long digital transmissions. Tube amplifiers use a blower to push air through the tube sockets and the anode cooler, and the blower has to run after transmission as well, to remove the heat from the glass and the seals. A good amplifier has a timer for this. If you add an external amplifier, think about the room, because 500 watts of heat in a small shack will raise the temperature quickly. The decision to add one is discussed in the guide to HF linear amplifiers.

For a homemade project with a power transistor, size the heat sink with the calculation above, and choose a sink whose catalogue gives its thermal resistance, preferably with a margin of 30 percent. Aluminium extrusions are cheap and good, with black anodised surfaces radiating slightly better than bare. Copper is a better conductor but heavier and more expensive. For the power supply side, the article on the choice of a power supply explains that the supply also heats the room and the shack.

Field and Portable Operation

Outdoors, the radio is exposed to the sun, and a black case in the summer sun can reach 60 degrees before any transmission. Shade the radio, put it on a stand that allows air to pass underneath and use a small USB or 12 volt fan blowing across the heat sink, which costs little and transforms the thermal margin. In winter, the cold helps, but a radio brought from a cold car into a warm room may collect condensation; let it come to room temperature first. For a long digital activation, reduce the power to 30 to 50 percent. The contact rate does not change, and the radio stays cool.

Cooling by Design: Small Habits That Help

Several small habits keep equipment cool for years. Place the radio where the vents are free, with at least a hand’s width of space behind and above, and never stack warm equipment on top of it. Do not cover a transceiver with a cloth or a dust cover while it is on. Keep the amplifier’s exhaust away from the transceiver’s intake, so that the radio does not breathe hot air. In a hot summer shack, a small desk fan across the equipment, or a window opened on the right side, can make the difference between a radio that folds back its power and one that does not.

Power supplies also deserve attention, since a supply running near its limit gets hot and may lower its voltage, which the radio answers with more current and more heat. A supply with a margin of 30 percent or more over the radio’s peak current is cooler and quieter. Check the temperature of connectors and cables after a long transmission: a warm plug is a sign of a poor contact and a voltage drop, and the fix is cheap. The wiring advice in the guide to 12 volt wiring shows how to size the cable for the current.

Signs of Trouble and What to Do

Watch for the symptoms of overheating. A radio that reduces its output after a few minutes is reducing power to protect itself. A fan that runs at full speed all the time, or the heat sink that is too hot to hold a hand against for more than a second, indicate a poor airflow or a high power. Error messages about temperature and sudden resets are other signs. Check the obvious first: ventilation, dust, a blocked fan and the SWR, since a mismatched antenna increases the dissipation in the final stage. A dummy load helps to separate a radio problem from an antenna problem. If the temperature is still high on a dummy load at moderate power, the problem is inside the radio, and the manual or the service department is the next step.

The Bottom Line

A heat sink carries the waste heat of power transistors to the air, and its performance is described by thermal resistance in degrees per watt. Add the resistances of junction to case, case to sink and sink to air, multiply by the dissipated power and add the ambient temperature to find the junction temperature, which must stay well below the limit of 150 to 175 degrees. Use a thin film of thermal paste on flat surfaces, keep the fins vertical or ventilated by a fan, leave space around the radio, clean the dust, reduce the power for continuous digital modes and shade the equipment outdoors.

Frequently Asked Questions

What is thermal resistance?

The temperature rise per watt of heat flow, in degrees Celsius per watt. Lower values mean better cooling, and the values of the parts of the heat path add up.

Why does my radio overheat on FT8 but not on SSB?

FT8 is a continuous mode with a transmission of about 13 seconds out of every 15, so the average dissipation is much higher than on SSB, where speech has gaps and a low average-to-peak ratio.

How much thermal paste should I use?

A thin, even film, just enough to fill the microscopic gaps. A thick layer is worse than a thin one.

Which way should the fins point?

Vertical, so that warm air rises between them. With a fan, the air must flow along the fins.

Is a USB fan under the radio useful?

Yes, especially for portable use or long digital transmissions. A small fan blowing across the heat sink can reduce its temperature rise by half.

What junction temperature is safe?

Data sheets give a maximum of 150 to 175 degrees for silicon power transistors, but for reliability keep it well below, ideally under about 100 degrees in operation.