Why you should avoid RG-58
Many, if not most of us live in houses with lots of modern electronics, switching PSUs in wall warts, computers and controllers on all kinds of appliances and so on, all creating wideband noise that wasn't around in ye olde days. On top of that we play "radio" with a computer and usually other electronics in the same room, with the receiver only a USB cable length away from the computer. Then some of us have neighbors with the same amount of wideband noise stuff per household, adding up to the "noise cloud" in which we live. Quite often, the coax runs for considerable lengths through this noise, sometimes even in close proximity to the noise sources.
Even when your receiver is sitting on the window sill and the coax leaves the house after a few inches, it's still running through the "noise halo" that surrounds buildings. Old sources claim that said halo can extend "5m/15ft" beyond the walls, but given the considerably higher amount of noise sources today I'd say that distance should be doubled. So we can assume that even under ideal circumstances, the coax would be running for at least 5m/15ft through noise fields.
So what does that mean? I said above that the simple shielding of RG-58 gives you an attenuation of 45dB at best. The graph above indicates that it's even worse but let's ignore that for now. Now even if your noise doesn't exceed -70dBm anywhere on the length of the coax, the best you can hope for with RG-58 is getting this down to -115dB. IIRC the internal noise floor of the RSPs is around -130dBm, so there's still 15dB of added noise and not matter how good your antenna is/could be, a whole layer of otherwise clearly discernible signals will be covered by that noise. The remaining stations will suffer from a decreased signal-to-noise ratio and fading will have more impact on intelligibility, because - after finding the best receiver and antenna your money could buy - you skimped on the coax.
Of course, unless your QTH is in the ITU "rural/remote" category, your antenna picks up considerable amounts of QRM too and maybe you're more interested in listening to rather strong broadcasting stations, so this may not matter that much for you but if you're after DX and want to get the most out of your receiver, use proper coax! H155 is not cheap, but its shield is good for 80dB of noise suppression and its 5mm diameter fits SMA connectors nicely.
Some graphics in this blog post illustrate the problem a bit:
http://newsroom.bonito.net/why-even-goo ... e/?lang=en