One section caught my eye, and I wanted to add some detail:
>In order to efficiently send and receive signals, antennas should be no smaller than half the size of the radio waves they’re dealing with. If cell waves are 6 to 14 inches, their antennas need to be 3-7 inches. Now stop and think about the average height of a mobile phone, and why they never seem to get much smaller.
A common neat trick is to take advantage of the fact that this is only true in air, and the wavelength depends on the material around the antenna. Some modern antennas (especially common in GPS and Bluetooth) are built as a metal foil around a ceramic element, which has a much shorter wavelength and allows them to be shrunk[1].
You can also "fold" the antenna a bit, and hence get away with a quarter wavelength in return for reduced performance in other areas. A common example in phones is the Inverted F antenna [2].
Finally, a large "invisible" component of many antennas is the size of the ground plane attached. Shrinking this can affect antenna performance a lot. Generally, this is an internal copper layer of the PCB that is used by other components too, but it's important to realise its an important part of the RF performance. Therefore, you can't judge how good an antenna is from the visible size of it alone, the form of the whole device matters.