Apple is unwilling to accept the QTM 525 millimeter-wave (mmWave) antenna module offered by Qualcomm because it doesn’t fit in the industrial design that Apple is exploring designing its own 5G antenna rather than relying on Qualcomm’s existing hardware, a source with knowledge of Apple’s plans, Fast Company reported.

The issue is apparently the size of Qualcomm’s QTM525 antenna, specifically for accessing the faster mmWave version of 5G. Apple will be using the Qualcomm X55 modem on its 5G iPhones and it’ll design its own antenna for it.

Apple is developing an in-house antenna system that would better fit the intended iPhone design. Antennas are important for mmWave technology in particular, given the relatively poor range of the ultra-fast wireless technology, with 5G phones generally requiring multiple mmWave antennas to ensure good connectivity.

Apple has designed in-house antennas before, with less than perfect results for the iPhone4, this phone had an antenna design that resulted in dropped calls and other problems when the ‌iPhone‌ was gripped in a way that extended over the antennas. Fast Company’s source says that another recent Apple antenna design “required twice the maximum amount of power as comparable antennas to provide the identical amount of radio wave.”

Creating 5G antennas for millimeter-wave networks is harder than creating other forms of antennas because they send and receive higher frequency signals, leaving less room for error. 5G performance is additionally reliant on the antenna design.