30 miles is a really long distance for a PCS phone that was designed to work with cell sites much less than 1 mile away. It's amazing that you get reliable service to begin with. I got a free airave femtocell device from Sprint to use at my mom's home, where there are Sprint towers 1.1-1.75 miles away. You might want to consider that since you're both Sprint customers.
When I was testing a product prototype that relied on a Sprint EVDO card for wireless data out in the boonies, I found that a lot of the time the best data service I could get was 1xRTT ("2G"), which is roughly as fast as an ISDN data line, for those who remember them. I'm sure that the cell sites themselves were the limiting factor because at the time Sprint was getting EVDO R.0 to the Chicago city limits, and not an inch beyond. I'm sure that a lot of rural cell sites are still running 2G, although IIRC all the old (D)AMPS 800MHz frequencies should be transitioned to 1900MHz roughly right now.
With everybody but T-Mobile rushing to the 700MHz band and LTS for data right now, I'm sure that rural cell towers get the least attention because they generate the least bang for the buck. So I'm guessing that in areas where the nearest tower is 30 miles away, you're not likely to ever see EVDO or WiMAX come to that tower if it's not already there. If Sprint's transition to LTE is as slow as it was to EVDO, or as failed as it is with WiMAX, you might be better off with another carrier there for a few years.
Depending on who's operating grandma's cell tower that you're roaming off of, your problem could be anything from the kind of dropouts that you can expect when relying on a competitor to connect to your own network, to outages related to service at the one and only tower that you're relying on, to the physical limitations of radio technology at those distances and power levels. Take your pick.
Another thought: if grandma doesn't have high speed Internet (Cable modem or DSL) service wired to the home, which is needed for the airave, you might consider investing in a cellphone signal booster for grandma's house. These things use linear amplifiers to boost the signal to/from a rooftop antenna (preferably). The one I have is designed to connect to an antenna port on a phone or data card, but some are made to work with an indoor antenna (they have to be separated so they don't "walk over" each other) so you can use your cellphones normally.
Hope that helps...or that grandma gets a service upgrade soon.
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