Let's see if I can clear-up some confusion. Let's go back to basics...
XM and Sirius work on S-Band frequencies. These frequencies don't require exact pointing, which is why you can have a small antenna on your car and not require the antenna to point in one direction. These frequencies are in the 2.2-2.3 GHz range.
DBS (small satellite) uses Ku-Band and Ka-Band (DirecTV only) frequencies which are in the 12+ GHz range. The LNBs convert the signal coming from the dish to a lower frequency in the L-Band range (1-2GHz) before the signal comes in the coax into your house. At no time (unless you get Internet over your small dish) does the small satellite TV dish transmit any signal and the frequencies with the small dish do not overlap any frequencies used by satellite radio. So, unless your are running the antenna wire right through a small dish or even parallel to the small dish'es coax, this is not going to interfere with your satellite radio signal.
There are terrestrial repeaters used in the S-Band as well. As part of satellite radio's agreements with terrestrial broadcasters (AM / FM), satelliite radio agreed to limit the number of terrestrial repeaters used. The two (now one) satellite companies didn't always follow these rules and the terrestrial broadcasters complained. The FCC proposed rules and the number of repeaters was reduced. Here's one of the rule changes being proposed back in 2007,http://www.orbitcast.com/archives/fc...n-tuesday.html
So, a reduction in the number of repeaters near you is a possibility for a true signal strength reduction. You would now be picking up a repeater that is further away.
However, I'm guessing you are using signal strength in a generic way. Basically what you are getting are more drop-outs. If so, there is a second explanation.
Satellite radios all had 10 second buffers when the service was first offered. Go under an overpass and the radio's buffer would already have the next XX seconds of programming and your radio kept playing without interruption. It's great except that meant the signal had to contain 10 seconds of programming at any given time but that was the only way to overcome blockages while driving where repeaters were not present. I borrowed this explanation from another forum from 2005 on how the XM buffers work (Sirius is different but similar),
"Each satellite puts out a direct feed and a delayed feed. Your receiver samples up to five feeds, a direct and a delayed feed from each of the two active birds plus a terrestrial repeater and while each of the five signals are coming and going, fading in and out, derives an audio output from the five received signals that is close to 100% solid."
Over the years as the number of channels have proliferated but the bandwidth has remained constant, satellite radio has had to find more ways of cramming more channels into the signal. One way has been less bits per channel, but another way has been to reduce this buffer down. I'm hearing about a 2 second buffer now. So passing under an overpass for more than 2 seconds results in loss of signal momentarily. This would also be true with moist tree leaves (really good blockers for S-Band) but not for rain which S-Band goes right through. I suspect the reason for the lower amount of buffer time is mostly to save bits when compressing since the current and delayed signals would be more similar but I don't have any technical documentation to back that up.
So combine some wet leaves with a reduced amount of buffer, and you would get more dropouts.
The only way to stop this would be if Sirius/XM starts adding more buffer back to their signal. We used to be able to drive under any overpass without a repeater and not lose signal. Now about half of the time the signal drops for a second or two. One effect of the merger was that both Sirius and XM broadcasts had to add a large number of channels to each channel lineup and this means less bandwidth per channel, including the buffer size.
A third possibility is that the antennas have gotten smaller and smaller. This is done by using more and more power to the antenna to boost the signal. This is another situation where the lack of buffer combined with a greater chance of a signal drop-out due to the smaller antenna makes for more interrupted channels. A smaller antenna may mean more of a chance of programming loss.
And, yes, placing your antenna outside facing south is the best way to get maximum signal strength. S-Band is omnidirectional, so exact pointing does not increase signal strength. For XM, these are geostationary satellites (unlike Sirius) which is why south facing is important.
Also make sure when calling Sirius/XM customer service that you ask where the customer service rep is located. I've found that when you get the Phillipeans that your odds of getting a correct answer have gone way way down. Don't assume their customer service reps have ever even heard a Sirius/XM broadcast.