35 feet of RG6 is fine for a scanner. Also, can you fit a Tram 1410 discone antenna in your attic? This model does not have the top whip, so it will probably fit. You will also probably have to put it together in the attic, as it may not fit through the access door once together. I have a feeling it will perform significantly better than those magnet mount antennas. If you are determined to use one of those magnet mount antennas, you will need a reasonably sized metal ground plane under them. (Do not buy those magnet mounts. Those are intended to use on large metal ground planes like the roof of a vehicle.)
You may need an FM broadcast band filter inline as well. With an improved antenna, RF interference comes along as well.
Hmm...if you have insulation that has a foil back in your attic forget about putting it up there. Instead, search for a post by Jonwienke on these forums about using that Tram 1410 discone antenna indoors on a speaker stand. If it is a short run, perhaps use LMR-400 or LMR-240UF as either of those should not be too costly if scanner and antenna are in the same room. Definitely the LMR-400 over the 240, but even the 240UF is better than RG6 for several reasons.
