In a year where the 5G radio will be the new iPhone’s spotlight feature, Apple has decided to design the phone’s antenna itself.

Apple balked at the QTM 525 millimeter-wave antenna module offered to it by Qualcomm because it doesn’t fit into the sleek industrial design Apple wants for the new phone, a source with knowledge of Apple’s plans told Fast Company. Qualcomm will provide the 5G modem chip–its Snapdragon X55, also used in Samsung’s new Galaxy S20 phones–for the newest iPhone, which will likely be announced in the fall.

However, Apple typically designs on several tracks, and it’s concurrently working on another design that uses both the Qualcomm modem and antenna. It could default to this option later this year, our source said. But that would require Apple to settle for a slightly thicker iPhone than it wants. Qualcomm has said that its QTM 525 antenna module will “support 5G smartphone designs sleeker than 8 millimeters thick.”

For Apple, this is a tricky situation in multiple ways:

Apple isn’t great at designing antennas. The company has designed its own antennas before, with less than perfect results. Most famously, its antenna design in the iPhone 4 was blamed for dropped calls when you held the phone in a certain way. Another more recent Apple antenna design required twice as much power as comparable antennas to produce the same amount of radio signal, our source said.

5G antennas are hard. The antennas required for millimeter-wave (mmwave) 5G devices are harder to design than other kinds of antennas. Because these antennas send and receive higher frequency signals than earlier generations, our source said, there’s less room for error in their design and manufacture. A slight imperfection in an antenna coming off the production line might lead to connection problems later on.

The 5G iPhone will use a “phased array” antenna with two parts that work together to form a beam of radio signal. The beam can be electronically steered in different directions without the antenna moving. The modem chip and the antenna module work closely together to make this work properly, our source said. Having the two parts made by different companies may introduce some uncertainty and bump up the difficulty level of the overall design.