I've been battling with intermittent signal problems in my house for some time. I have an Apple Time Capsule base station, which supports dual band 802.11 g/n on 2.4Ghz and 5Ghz bands. And my iPad and Macbook Air both support 802.11n on both bands. My iPhone only supports the 2.4Ghz band, as I recently discovered.
At the other end of the house, opposite the base station, is my bedroom, where the WiFi issues occur. Initially, I added a WiFi extender, in the form of an Apple Airport Express. I've tried two models, and both gave me great signal strength, but the connectivity would intermittently stop (as it did without the extender), sometimes requiring a restart of the extender. I eventually got rid of the extender, and decided to research a different solution.
Upon thinking about the problem further, I realized that the iPhone rarely, if ever, had network issues. It's always the larger devices. What gives? And then it dawned on me. I remember having to use an older 900Mhz wireless house phone to get better signal at one point because of all the walls and microwave in between. The way it works is, the lower the frequency, the better it is at going through barriers, but the shorter the distance it can travel. The higher the bandwidth, the farther it can travel, but the fewer barriers it can pass through before the signal degrades.
So I went into the Airport configuration utility, and set the base station to segregate the frequencies onto separate WiFi networks, so I see two different WiFi network names when browsing for a network. I then make sure that I "forget" the 5Ghz network, so my device cannot automatically connect to it, and then I explicitly connect to the 2.4Ghz.
So far, the problem seems to have been alleviated. I see a slightly higher signal strength than before, and although it sometimes drops very low, I always have connectivity. Prior, it would drop in strength and I'd lose network connectivity. My belief is that the devices were trying to stay on the faster 802.11n network at 5Ghz, which would drop out sometimes, and then it would switch to the 802.11g network and recover. Then every so often, it would reconnect to the 802.11n network again, repeating the cycle.
The other recommendation I would make, and one I will try if my connectivity doesn't remain constant, is to just move the base station to limit the barriers between it and the farthest part of the house. I just figured this made more sense, and is cheaper than buying a bunch of really long ethernet cables.