OK, so, here are some answers to your questions:
Line of sight-
I am about 7 miles due south of the approach end of the 24 runways, basically along Sepulveda Blvd. Unfortunately, there is a house due north of me (my next door neighbors), and it just barely blocks my line to the eastern half of the airport. The western half, however, is a clear shot, except for a row of trees and probably some 1 story homes that line the southern end of the airport along the Imperial Highway.
As soon as the aircraft on the 25's leave the ground, I can see them, usually well enough to make out the aircraft type. I cannot directly see any airport structures, and do not know where the airport signals are transmitted from (it's not usually the tower itself, is it?) I see a tall building and it might be the tower, but i'd think i could tell the shape from far away, and don't have binoculars handy. It's probably something else.
As far as I can tell, there are no ridges or hills blocking me, since I'm mounting this thing from the balcony railing of the top of a three story townhouse perched on a hill, but my immediate community results in a few homes to block my northeasterly views.
The cable I'm using comes straight down from the mast that extends about 6 feet above a balcony railing, which is directly above my room. It's not well secured yet, and comes right through my window to a scanner, which will be under my bed. I think at most, it's a 25 foot run, but I'm using the 50' cable, so the excess is just bundled. No huge EM fields being created in that short distance, except for a couple of outlets in maybe a 5 foot radius. Nothing to cause crazy distortion i don't think.
Other EM sources-
I live in the LA area, so the airwaves are basically saturated. No large radio stations between the airport and I, but definitely some within a 20 mile radius, and many many within a 30 mile radius. There are also some police and fire dispatch centers nearby. I thought the other day I might have heard music carrying on the ATIS frequency, so perhaps a filter will clean up the feed a little.
I'll definitely try grounding the thing. And yes, Peter's antenna is not tunable, and it worked fine for him at KSYR, as he mentioned. Weather wise it's been overcast the past couple of days but nothing ridiculous.
Keep the tips coming guys. Thank you, and I'll look into what you've said so far.