The antenna connections is what I did when I had a known poor signal - just pointing that out. In your case I'd not fool with that just yet as it's a WMC issue.
I too have had to come over to the world of WMC as I'm preparing to dump DirecTV for a MediaCom CableCard + HDHomeRun Prime which only WMC & MythTV (Linix-based) currently support. I was trying to address this rescan issue last night (1:45 am) and again this morning.
Finally this morning I got it ALL worked out! I'd found in the Settings/TV there's a place under Guide to ADD missing channels - you can go there and add 48 1 and assign any characters to the Name and Save... then there's a place to Edit Channels - go there, scroll down and highlight YOUR new channel and you can do a Preview (shows as a ghosted background image) from the left-hand column. This will show you (assuming your signal strength is as it was on Tuesday or better) that you can tune YOUR new (missing) channel (RF-48) that you added! (Confidence builder! **Yea**) But this is just a teaser...
I found that doing a RESCAN, even if you DELETE the existing channels (on that menu page) doesn't clear out what WMC already believes are your current stations (possibly because they are linked to GUIDE information) and will not erase them automatically. You must do this yourself ahead of the rescan. So.... go back to where we were, in Settings, TV, GUIDE, EDIT channels and do a CLEAR ALL (left hand column). All the channels will be UNchecked (meaning they will no longer show up in your GUIDE). GREAT.... Now BACK up to ADD MISSING channels and DELETE your channel and save (you don't need that one lying around dirtying up the place). Now go to TV SIGNAL which this time it either knows of no channels or delete them here (I can't recall). It's at this point your WMC is completely free of any channels/guide relationships it once knew and is ready for you to do the START SCAN and this time you'll have a prestine scan!
I found this through trial and error. I wrote this instruction/post leaving in the step about adding the missing channel because it's good to know that WMC has a way to just "direct enter" a channel's RF and try to tune it! Sure, you'll have to get it back out but it's a quick verification that you'll get there! Apparently, the CLEAR ALL under Edit Channels is a key operation in preparing for the rescan -- I can't say how that would effect your scheduled programming and you may want try this once without the CLEAR ALL by simply unchecking the 48-1, 48-2 & 48-3 at that Edit Channels menu to minimize impacting your entire recording schedule. I can't say if such a "partial" clearing would work the same. It seems WMC needs to have the guide disassociated from the channels before it can reassociate "new" channels found during a scan with guide information - almost a circular reference but it's not.
On my system now I have all the correct channels/frequencies, guide data and no remnants lying around in my ADD MISSING CHANNELS buffer.
I hope this was clear enough, specific enough to help you... let us know :-)