Power dissipation of a sealed plastic balun enclosure

Correspondents raise instances of damage to baluns from time to time, and there is a steady stream of reports online.

The thinking is often “my balun is rated at 5kW, how can I damage it with a transmitter that is only 100W?”

This article discusses heat dissipation from a small sealed plastic enclosure. This is not an unusual problem, it exists widely in industry and is solved routinely for enclosures ranging from small ones like these to large switchboard enclosures.

An example balun deployment

Above is a balun enclosure located under the soffit of the building, so outside in free air and where the sun will not shine on it.

This box is ABS, has a surface area of 0.034m^2, and will probably withstand 80° before there is risk of deformation.

Allowing for a maximum ambient temperature of 40° for this location, a temperature rise of 80-40=40°=40K can be accommodated.

If the box surface is of uniform temperature, we can expect that the enclosure can dissipate around 5W/m^2/K from internal sources in still air, so about 7W. (Some manufacturers publish data from which a base figure outside in still air in the range 3 to 8 W/m^2/K can be derived.)

Of course dissipation would be higher in a breeze, lower if subject to external heat (such as insolation), and lower if the box surface temperature is not uniform.

Because of the way the core assembly is clamped to the base of the box, I would rate it at 5W because of less than uniform temperature distribution.

Note that a similar sized PVC box might not be safe above 60° (depending on its formulation), so temperature rise 60-40=20°=20K, leading to maximum  dissipation of 3.5W on the same rationale.

Heat capacity and averaging

Ferrite cores have quite high heat capacity, it may take more than half an hour for the core to reach even half of its final temperature given constant power input. This means that there is thermal averaging and the core temperature will depend on power input averaged over many minutes, perhaps even half and hour, depending on core size.

So, it is the power averaged over minutes, perhaps even as long as 30min that applies to the problem.

Estimating safe transmitter power

The case of a transmitting balun is more challenging than the common electrical enclosure in industry as power dissipated is dependent on many external factors.

Several approaches are in common use:

1. ignore it;
2. copy others, hope for success;
3. depend on manufacturer's power rating;
4. calculate power dissipation from manufacturer's loss specification and guestimate safe dissipation.

None of these properly take account of your own scenario.

Calculation of temperature rise based on manufacturer's power rating or published device loss is fraught with problems.

The simplest method that has some reliability is the progressively increase power whilst observing exterior temperature of the enclosure. A simple temperature test is that if you can touch the thing for 10s comfortably, without getting burnt, it does not exceed 60°. This might not be very convenient if it is hoisted aloft and some other method might need to be devised.

Keep in mind the very long thermal time constant and averaging, and that average to peak ratio is dependent on modulation type and on/off duty cycle. If you have proven that it stands 1kW PEP SSB telephony for 5min, that does not prove it will withstand a RTTY contest hour on hour.

Realise also that the mechanism is one of temperature rise approximately proportional to power dissipated. If you observe a barely acceptable box temperature on a cold day, the temperature rise is high and the same power scenario may be damaging on a hot day.

Enclosures subject to high mechanical forces, like suspending the spans of a dipole from eyebolts fixed to the enclosure, may deform at lower temperatures.

Under fault conditions, dissipation may change for the worse and one of the consequences of a fault in an antenna system is balun damage. If you observe changes in antenna system VSWR, or ATU settings, investigate the cause.

Living dangerously

If a station is capable of peak power of say 1kW, and the balun enclosure is capable of dissipating say 5W, that is 0.5% of PEP. Whilst that might work for a given modulation type, transformer / choke loss profile, antenna impedance / balance… it might not provide much wiggle room if something goes wrong with the antenna system.

Baluns that are marginally rated for your own scenario might just be an expendable component when things change.