Well 2 things really. Yeah the hardware is a bit garbage as it only goes up to 802.11n only on 2.4 GHz and it's also a 1x1 with a poor antenna. The big thing though is it requires you use the same channel which is disastrous for repeater performance.
Ideally you'd have 802.11ac/ax with 2 separate PHYs each with 4x4 (even if the clients are 1x1 or 2x2 the additional beamforming helps) set to two separate channels.
That's not to say it won't function well enough in a pinch but I wouldn't seek this out as a permanent answer if I were buying new.
"That's not to say it won't function well enough in a pinch but I wouldn't seek this out as a permanent answer if I were buying new."
Like I said this functions fine, as is evident by it having been posted, but the question was why it wasn't a very good repeater not was it a functional use of an existing pi 0 laying around.
It's also not the best you could do buying new if that is what you had to do as ~$16 will get you a 2x2 802.11n repeater that functions much better, has external antennas, and has an ethernet port too. Not $10 but it's also enclosed and doesn't need a power supply or uSD. ~$30 will get you decent AC repeaters you could use outside of a singular IoT project. Less for either if you're willing to go used. The ideal 4x4 ax double phy wireless repeater is indeed going to be more than a pi 0 but it was an example of what makes a repeater good (to the question) not what should have been purchased for this project.
Again nothing wrong with the solution used here, that's not where my comment was pointed.
I'd guess the hardware is the problem. Most antennas I've seen on access points are larger than the entire Pi Zero W. While I'm no radio expert, I'd expect APs to use the smaller antennas if they were capable of being as good as bigger antennas.
Ah, that reminds me of an article someone here linked to a few years ago: [1]. That claims the efficiency is -3.5 dB compared to a typical -1.25 dB dipole in dedicated wireless equipment. Not bad; the difference is only a factor of ~2 in power for x8 reduction in size.
