antenna-theory.com

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Found this article, it is somewhat interesting:
https://www.macrumors.com/2020/02/14/apple-5g-iphone-antenna-module-custom/

Basically, the 5G modules with integrated antennas operate at 28 or 39GHz, so the antennas are small enough to intergrate directly on top of the chip (IC). This then forces the device manufacturer to have a certain amount of metallic keepout in their product, and apple doesn't like anyone telling them where they can put metal, so they are designing their own antennas.

This part is kind of funny:
As Fast Company points out, Apple has run into problems with antennas designed in-house before. The ?iPhone? 4, for example, had an antenna design that resulted in dropped calls and other problems when the ?iPhone? was held in a way that covered the antennas. Fast Company's source says that another recent Apple antenna design "required twice as much power as comparable antennas to produce the same amount of radio signal."

If an antenna requires twice the power as comparable antennas for the same radiated power, they are basically saying the efficiency is 3 dB lower than typical.

More comments:
Creating 5G antennas for mmWave networks is harder than creating other kinds of antennas because they send and receive higher frequency signals, leaving less room for error. 5G performance is also reliant on the antenna design.

To be fair, 2G, 3G, ... performance is reliant on the antenna design. How else is it wireless? 5G tends to be more difficult to design because you must be more exact and equipment is much higher, but the easier part is the antenna is more localized in the device, whereas for 800MHz the whole device acts as part of the antenna.

Overall I would expect Apple to design their own antennas because it is not that hard, and they are generally willing to give up a couple dB of antenna performance for a device that looks better to consumers.