I noticed a weird behavior of my Yaesu FT-891 transceiver. I'm using it with an external autotuner mAT-30 (120W SSB/CW, 30W PSK31), which is compatible with Yaesu FC-50.
When I connect a 100W 50 Ohm dummy load, switch to SSB mode and say "aaaaah" to the microphone the internal meter (MTR) shows SWR 1:1 and 100W transmit power, as expected. However, if instead of dummy load I connect a real antenna through mAT-30 tuner and enable the tuner by pressing TNR in the transceiver menu, the transmit power (according to MTR) drops to 50W while the SWR is about 1.5:1. I've found a few tables online (the first one, the second one) that say that SWR 1.5 correspond to only 4% power loss, thus I'm a little surprised.
At first I thought this might be some sort of bug in the firmware, thus I decided to re-check the readings using an external analog SWR/power meter. I have a second (manual) tuner MFJ-971, which has a build-in SWR/power meter. I bypassed the capacitors and the inductance of MFJ-971 using an alligator clip, which turned the device into just a SWR/power meter. Surprisingly it showed the same SWR 1.5:1 and only 50W of transmit power.
Then I decided to disconnect mAT-30 and tune the antenna using only MFJ-971. Now the transceiver works as expected - the transmit power is 100W despite the fact that SWR is about 1.5:1.
I see two possible explanations. The first one is that FT-891 reduces the power when it sees an SWR higher then 1:1 and TNR is ON. Another possibility is that the protocol between the autotuner and the transceiver (over 8pin mini-DIN socket) supports some sort of back pressure ("TRX DE ATU PSE DROP PWR"), and the tuner is one to blame. Though I'm not familiar with this protocol and thus I don't know whether the tuner could be responsible.
Did you ever encounter a similar behavior in other Yaesu products, or products by any other manufacturers? How would you suggest to bypass this limitation and transmit full 100W with my autotuner?