FT-991A Duty Cycle

From time to time some concerned FT-991A owner or potential buyer asks about the equipment's Duty Cycle. This information is not depicted in the manual or documentation.

I'll provide my own experiences here - so you can take your own conclusions.

My usage

I have been abusing my FT-991A in almost a daily basis, since Nov/2018. I have tried almost every possible mode with the radio: FT8, RTTY, SSTV, C4FM, FM, SSB, FreeDV - ranging from eventual QSO to Contest mode. And more often than not, using the equipment's full power: 100W in HF/6m, 50W in 2m/70 cm. Some modes, especially SSTV are especially challenging - Differently from FT-8 that transmits a carrier for 14 seconds, a high resolution SSTV takes up to 240 seconds of uninterrupted transmission time as well - which I have done a (good) number of times - at full power. Some RTTY CQs takes between 30-60 minutes, and so forth.

Also add to perspective that I live in a tropical country (Brazil) - where temperatures in the summer are past 30C (and air conditioning is not a staple here).

The radio is fed to a antena with good SWR, and the radio is going strong, as-new, in terms of power and quality - everything nominal.

Thermal design

The FT-991A features an interesting thermal design- The fan speed is dictated by the temperature sensor near by the finals - there's a thermal sensor for both HF and V/U final power PA, increasing the fan speed as the sensor temperature increases.

The finals are bolted to the aluminum chassis. In the below picture you can see the seating pad for the two RD100HHF1 HF Finals, and the RD70HVF1, responsible for the VHF and UHF bands.

The chassis flip side sports a finned curved heatsink, steering the wind flow inside the radio. The air enters from the rear openings, is steered by the curved fins - then it enters the Tuner Unit (orange arrow) and finally exhausted by the fan (red arrow).

That ensures a proper equipment cooling.

Other than that, the thermal sensors that controls the fan speed also sends this information to the equipment's CPU - so the CPU can command a RF power foldback via Reverse ALC (RALC) if it detects that the radio is outside its safe thermal envelope. Read more in this link.

So, I think that it boils to keeping your equipment with unobstructed air access, clean air intakes and good air circulation - so the radio can effectively exchange heat with the environment, avoiding overheating.

Hope that helps; 73 de PY2RAF.

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