SDRplay Community Forum

Firstly, just to put things in perspective, the equipment currently in use is an RSP2, using SDRuno and antenna port A, with the MW/FM filter selected.
Lacking the highly desirable loop antenna, that currently in use is an Ukrainian marketed active mini-whip, mounted in the clear on an 8M carbon-fiber pole, earthed via a 4' copper rod.
The Hi-Z port is currently dedicated to an end-fed longwire antenna, not appropriate for this purpose.
With my somewhat less than encyclopedic knowledge of radio, I would appreciate any answers or clarification as to what may be "going on" with regard to the following:
1.The Lerida (LRD) and Merville (MRV) NDB's are both published as being on 404Khz. However, whilst I receive MRV on 404Khz in AM as an SSB transmission, LRD is only and clearly audible as a CW transmission on 403Khz.
2.Another example of an anomaly is The Coruna NDB (C) received as a CW transmission on 409Khz, whilst the published frequency is 410Khz.
Is this something that has an answer in radio theory/practice, or an RSP/uno/SDR "quirk?"
3. Not believed to be an anomaly, but for which clarification would be appreciated is that there seem to be a few NDB' s, such as ST Brieuc, which transmits on 353Khz in CW.
Is it the case that AM SSB and CW modes are both used and if so why. What are the advantages and disadvantages.

AFAIK, NDBs are radiated only as double-sideband-with-carrier signals, that is AM. If reception is clear, copy should be easy in AM mode using a filter wide enough to fit both sidebands (upper and lower) with the carrier sitting at mid-point.
Under difficult conditions (very weak signal or noise on channel) a narrow filter may be better, but this means you should select USB or LSB as you cannot include them both within the reduced bandwidth. If you use CW mode, an offset will be automatically selected and this may be what is causing the apparent frequency discrepancy.

There you go!
Weakness easily exposed - I've been looking at the AM signals and processing in AM - looking again - notwithstanding, thanks to previous hints I've found a pretty fair few.
353Khz is pretty clearly CW only though...
Thanks again Andy.