Why a repeated signal carries less

A wireless radio is half-duplex: it can transmit or receive, not both at once. A repeater that uses one radio for the link to the router and the link to the phone has to hold each frame and send it again on the same channel. Every frame therefore costs two turns of airtime instead of one, and the most the extender can deliver is roughly half of what reaches it.

That is the best case. The signal quality on the link from the router usually takes a further share, because that link is the weaker of the two.

A wireless channel is shared, and only one device can usefully transmit on it at a time. Capacity is therefore measured in time on the air. A frame sent directly from router to phone occupies the channel once. The same frame sent through a single-radio extender occupies it twice: once from router to extender, once from extender to phone, in sequence, because the extender’s one radio cannot do both together.

The halving is not a design flaw in any particular product. It is what relaying on one channel costs, and every single-radio repeater of every make pays it.

Wireless equipment adjusts how densely it packs data into each transmission according to how clean the signal is. A strong, clean link uses a dense scheme and moves data quickly; a weak or noisy one falls back to a sparser scheme that moves data slowly but survives errors. This adjustment happens continuously on every link.

An extender has two links, and its throughput is bounded by the slower of them. The link from the router is normally the slower, because the extender is placed some distance from the router by definition. If that link is poor, the extender receives slowly, and no amount of proximity between the phone and the extender can restore what was lost before the relay. This is the reason placement decides so much: it sets the quality of the link that everything else depends on.

Consumer wireless equipment works in two frequency bands, around 2.4 GHz and around 5 GHz, and a dual-band unit has a radio for each. That opens an arrangement a single radio cannot offer: keep the router on one band and the devices on the other, so that receiving and transmitting happen on different radios at the same moment. The channel is no longer occupied twice, and most of the penalty disappears.

Manufacturers market this arrangement under various names. Whatever it is called, the trade is the same: the band reserved for the router link is no longer available to devices, so a device with only a 2.4 GHz radio cannot use an extender that has given that band to the router. Tri-band units add a third radio, reserved for the link to the router, so that both ordinary bands stay available to devices.

A class figure such as AC1200 or AX1800 is the sum of the theoretical peak rates of every band the unit carries, added together. No device ever receives it. It is not a promise of speed; it is a way of ranking products within one maker’s range, and it says nothing about what the unit will deliver from where it is plugged in.

The halving is measured against the wireless rate, not the internet rate, and the two are often far apart. A link that receives at 120 megabits per second and relays at 60 still comfortably exceeds a 40-megabit internet line, and a reader on that line notices nothing. The same relay behind a 500-megabit line is the bottleneck for everything beyond it.

Traffic that stays inside the house is different. Copying files between two computers, or streaming from a local media server, runs at wireless rates and feels every halving. For that kind of use the wired access-point mode described in what an extender does, or a different approach altogether, usually serves better.