“The technical explanation for the variation in signal strength found when closing the iPhone’s case gaps with one’s fingers is, that the antennas are being “loaded”. The loading from fingers, a material with a high relative permittivity of ~50 at 2 GHz, changes the input impedance and resonant frequency of the antenna resulting in loss.
If I had to take an educated guess, I suspect pressing one’s fingers across the gap increases the gap capacitance and changes the feed input impedance and electrical length of the antenna. It is analogous to holding on to a piano wire with two fingers while it is being struck. Its resonant frequency will shift and its quality factor will drop, resulting in a sound that is off key and muffled.
A solution often employed in the field is to preload an antenna. Preloading is including a large permittivity in the design, so when a human puts it close to their body, there isn’t a significant change. The down side of this is that it often reduces bandwidth and efficiency. So you can either have a great antenna that takes a hit when your fingers are near it or have one that is not quite as good but never changes. Isn’t it like Apple to choose the former and like the masses to cry for the latter? 😉
Note that if the antennas were designed to be inside the case as opposed to being part of it, they would likely have a loss somewhere between the best (unloaded) and worst (loaded) performance of the existing antenna. It’s all an engineering tradeoff.
The best solution is to not touch the antennas at the gap (Steve is right) or to buy a phone cover that keeps your high-permittivity sausages away from the RF sensitive areas. Apple made the trade off for you and in my opinion it is a novel and good one, provided it can survive the bad PR generated by the griping gap grippers.”