Hi Clint, good afternoon! Based om my somewhat limited usage of the MFJ tuners, I will have to say that the MFJ product is by far the weak link there. That said, I too have a "300 watt" MFJ tuner and I've been able to arc it using only 100 watts. It really depends on frequency as you've already found, and the mismatch that the tuner is being asked to tune. 160 Meters ( aka Top Band) is usually harder to match than other bands. I have a couple of kilowatt plus (supposedly) tuners that cannot match high power on 160 without some drama, or no match at all. I'm currently using a full wave 80 meter loop. My Palstar AT-4K will match it, barely and the Nye-Viking won't find a match at all on 160. Both of those tuners are well made, with big high-voltage and current capacitors and inductors. So, I guess what I am saying is that whet you are seeing is, sadly, about what you can expect from a piece of equipment like the MFJ tuner. It's not that the tuner can't load an antenna, it's just, as you point out, that the specifications for the tuner might be a bit optimistic. To say the least!
Thanks for your comments - I agree with you in regard to the MFJ almost certainly being the weak link here - and you're quite right that top band is harder to match. 73
Antenna tuner power ratings are poorly defined and subject to a lot of interpretation. Here's a section from the Ameritron ATR-30 tuner manual that speaks to just that: Different manufacturers handle this in different ways but it's not unusual to see tuners arc over well below their advertised power ratings especially on 160m.
