[SCCC] FT991A 160M Question

[Steve]

Sorry to ask this on the eve of the Stew Perry; but I seriously doubt 
the run-of-the-mill FT991A user groups or the facebook user group 
will have the least bit of experience with 160M DXing, let alone 
contesting. I'll ask the question there, too, but won't hold my 
breath. Don't know where else to ask this; the guys at HRO sure 
wouldn't have any idea what I'm talking about, and the Yaesu depot in 
Bueno Park doesn't seem to have any outdoor antennas, so I don't know 
how I could demonstrate this problem to them...

While trying to resolve a local noise problem tonight that shows as 
low as 1836 MHz, I happened to tune from 1800.00 to 1799.99. And LO 
AND BEHOLD: the RX noise level and signals jump UP by 10 dB or so as 
soon as it gets below 1800!!! This can also be seen on the S-meter as 
well as the panadapter scope. Neither the internal attenuator nor the 
IPO (receiver preamps) show any change. My TS-680S doesn't do this. 
It's only apparent when the FT991A is connected to an external 
antenna; when you listen to the receiver's background noise without 
an antenna, you cannot hear the noise floor change.

Anybody ever run across this in any modern DSP Yaesu (or other) radio 
before?? It's highly annoying as I've always felt this radio was 
lacking in RF gain on both 1.8 and 3.5 MHz and hey el presto:  maybe 
that gain is being purposely reduced for some reason, perhaps a 
setting somewhere?? I don't see a mention of it in the manual.

Thanks,
SteveH K0XP