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Measuring the RF output power of an FT-991A

I bought an FT-991A transceiver and measured the output power.

Bought in Switzerland in November 2025, SN 4F910844, Europa Version

I updated the firmware of the FT-991A:

I connect the output of the FT-991A to a 30 dB attenuator, followed by a 20 dB attenuator. The power is then measured with an AliExpress power meter using an AD8307 up to 30 MHz. At higher frequencies I use another power meter from AliExpress.

Based on my own measurements and datasheets, I estimate the error of these components as follows:
Component Error
Attenuator 30 dB ± 0.3 dB
Attenuator 20 dB ± 0.4 dB
Power meter A with AD8307 up to 30 MHz ± 0.5 dB
Power meter B above 50 MHz ± 1 dB, not checked
attenuator 30 dB, 100 watt Click: full size
attenuator 20 dB Click: full size
FT-991A with connected attenuators and power meter A. Click: full size
Click: full size
Example test at 432 MHz with 50 W target power. SWR clean at 1.0 Click: full size
Click: full size
VDD 12.3 V? Dear Yaesu developers, this scale is anything but easy to read. One tick mark per volt on the lower scale would not be a big luxury. Where I see 13.8 V labeled, it looks more like 13 V if I use the tick marks of the upper S-meter scale. A direct measurement with a multimeter at the power connector of the transceiver reads 12.93 V. Click: full size
IDD at 9.5 A is very low. According to the datasheet, I would expect about 15 A here. Click: full size
From the manual (FT-991A_OM_ENG_EH067M205_2111A-KS-1_manual) I read: 13.8 V ± 15% : 11.73 V to 15.87 V. The 12.3 V I read would therefore still be well within the allowed range. Click: full size
The display shows a transmit power of 50 W. If my audio drive were too low, or the supply voltage too low, the displayed transmit power would also be lower. Click: full size

For the measurement I use WSJT-X in tune mode and observe that the AGC slightly deflects. I adjust "RF PWR" on the transmitter. For a few measurements, I switched to FM and transmitted using the hand microphone as a cross-check. The power was the same.

I measured the voltage directly at the connector of the FT-991A: at 432 MHz, 13.46 V idle and 12.93 V while transmitting. At the lower frequencies: 13.46 V idle and 12.76 V while transmitting.

The power can be selected from 5 W to 100 W in 1 W steps on the HF bands, and from 5 W to 50 W in 1 W steps on the higher bands.

I measured at several frequencies and at 5 different power levels.

Frequency (MHz) Set (W) Measured (W) Ratio Power Meter
1.8 100 91.2 91% A
1.8 50 45.7 91% A
1.8 25 20.9 84% A
1.8 10 9.1 91% A
1.8 5 4.6 91% A
3.6 100 95.5 95% A
3.6 50 46.8 94% A
3.6 25 22.4 90% A
3.6 10 9.3 93% A
3.6 5 4.8 96% A
7.1 100 87.1 87% A
7.1 50 42.7 85% A
7.1 25 20.4 82% A
7.1 10 8.5 85% A
7.1 5 4.5 89% A
10 100 87.1 87% A
10 50 45.7 91% A
10 25 20.9 84% A
10 10 8.9 89% A
10 5 4.8 96% A
14 100 81.3 81% A
14 50 43.7 87% A
14 25 20.9 84% A
14 10 8.5 85% A
14 5 4.8 96% A
28 100 81.3 81% A
28 50 42.7 85% A
28 25 20.0 80% A
28 10 8.5 85% A
28 5 4.8 96% A
50 100 89.1 89% B
144 50 60.3 121% B
432 50 61.7 123% B

As an alternative measurement method without a power meter, I also measure the heating of the 30 dB attenuator after 90 s of transmission. Click: full size
1.8 MHz 100W heating 18.6°C
432 MHz 50W heating 10°C
Yes, the ratio of the heating matches quite nicely with the ratio of the expected power. 		Click: full size

The transmit power appears to match the specifications very nicely.