antennawifi
As far as I understand, metallic covers of devices like MacBook Air should shield the interior from electromagnetic waves rather effectively.
Even if a wifi antenna is right behind the screen, the other side of the display cover is metallic, so reception would be quite directional.
What do metal-clad consumer devices (laptops, phones) do to pick up radio waves?
I would consider a patch antenna. With a thick enough substrate you can cover the entire Wi-Fi 2.4 GHz band. It has a pretty decent front to back ratio (>10 dB) and an almost onmi hemispherical pattern.
While it is impossible to say for sure from you description, it doesn't sound like the antenna is the problem. If it were, you would have had a lot more problems in Tx than Rx (Tx is usually at least 50 dB above Rx). The fact that you are having problems in Rx makes me suspect some sort of coupling between your LNA and analog circuitry. When you put absorbers, you are lowering the signal level in the LNA (or further down the receive chain) and decrease the coupling.
Andy's comment is correct- this is a microstrip patch antenna, which has a hemispherical radiation pattern.
The construction is similar to a capacitor with a ground plane on the back, a dielectric (possibly ceramic in this case), and a rectangular patch on the top. It's designed to resonate at the desired frequency (1.575 GHz in this case).
The round bit is the inner feed to the patch, which is impedance matched to the receiver input through a network (a network which in microwave-world is just a pattern on the substrate).
Here is a calculator to find the dimensions, given \$\epsilon_r\$, dielectric thickness, and desired resonant frequency, and here is some info from Rogers (laminate maker) on designing your own.
Best Answer
They put them where ever they can find an opening and try to do a good job tuning them. Form comes first at apple ;) Take a look at this, they've mounted one antenna just inside the opening for the optical drive on a recent macbook pro.
Also they have a patent for a logo antenna
I once did a tear down and comparison of a bunch of apple products antennas and they performed really well compared to others in the field. Although there was that little problem with the iphone ;) So I think form comes first and then it's up to the EE team to be creative and figure out how they can get it to work.
On other cheaper laptops I think you'll find they just hide them behind plastic inserts.