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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?

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  • $\begingroup$ For the record - I've contacted both manufacturers with this question. $\endgroup$ – Aleksander Alekseev - R2AUK Apr 30 '19 at 23:08
  • $\begingroup$ I think this may have something to do with Foldback SWR Protection $\endgroup$ – Chris K8NVH May 2 '19 at 22:31
  • $\begingroup$ @ChrisK8NVH yep, it looks like the transceiver reduces the power depending on SWR when it knows that FC-50 or some compatible external tuner is used. $\endgroup$ – Aleksander Alekseev - R2AUK May 3 '19 at 15:02
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    $\begingroup$ "That's not a bug, that's a feature!" $\endgroup$ – Chris K8NVH May 3 '19 at 15:55
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Common practice is to tune with low power. The FT-891 can do that automatically with SOME tuners. My DIY-tuner does not, so I have to reduce power from the FT-891 down to 5 watt, switch to FM for a constant envelope/carrier signal and match the antenna to lowest SWR-reading in the display, or in my (manual) tuner SWR-readout. When I try to tune WITHOUT power reduction, I see the message "high SWR" in the FT-891 and, as already mentioned in the thread, the power fold-back protection is/becomes active. That is rather close to your observation, I think. And when tuning is succesfull, switch to SSB and adjust the power to 100 Watt or what you need for the QSO.

The conversion tables for SWR and loss in dB as found on the internet and in documents are ONLY VALID FOR 50 Ohm source and load systems! An SWR of 1:2 results from 25 Ohm or from 100 Ohm load impedance (or from other complex mismatch). SWR is RELATIVE mismatch. In transmitter systems there is no power loss as with mismatched 50 Ohm-intended laboratory systems. My FT-891 can deliver more than 100 Watt with output load 40 Ohm, but that is close to the survival-limit of the power amplifier..... Compare it with a mains outlet: for a certain power you need a certain resistive load. The system is designed for use with that load. The output impedance of a transmitter can range from zero (voltage source) to infinite (current source). Mismatch will lead to a high current or a high voltage and destroy your power amplifier. That is why there is an SWR-protection that reduces power at mismatch. PA0FSB

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