The faster Wi-Fi band is the one that cannot get out of the room

Most explanations of radio frequency propagation stop at the point where it starts to matter. This one carries on.

Higher frequency, shorter reach

Radio waves lose energy as they spread out, and the loss over a given distance rises with frequency purely from the geometry of reception. A higher frequency also means a shorter wavelength, so obstacles that a long wave bends around become solid barriers to a short one.

The upper bands therefore deliver much higher throughput inside one room and much less of it through a floor or two walls. The lower band travels further and penetrates better, but it is narrow, crowded and shared with a great deal of other equipment. Neither band is better in the abstract; they are different compromises between reach and capacity, and most homes need both at once.

What actually absorbs a signal

Water absorbs microwave energy strongly, which is why people, houseplants and full water tanks attenuate a signal more than empty space. A room full of people is measurably worse for reception than the same room empty, which surprises anyone who only tests at night.

In the datasheet, plaster and timber pass signal reasonably well, while brick, concrete and anything backed by steel mesh are considerably more difficult. Foil-backed insulation and metallised glazing are close to opaque, so a modern well-insulated house is harder to cover than a draughty old one. Mirrors, radiators and large appliances reflect rather than absorb, creating strong and weak patches only a metre or so apart.

Wider channels are a trade, not a free upgrade

Wireless networks gain speed partly by using wider channels, which carry more data in the same time by occupying more spectrum. A wider channel collects proportionally more background noise, so at the edge of coverage it can perform worse than a narrow one. Wide channels also leave fewer non-overlapping choices available, which forces neighbouring networks to share the same stretch of spectrum.

In practice, in a dense block of flats, a narrower channel sitting on a quiet frequency frequently beats the widest setting the equipment offers. Defaults are chosen to produce impressive figures on an empty band, which is not the situation most homes are actually operating in.

The client decides which band, and decides badly

An access point can advertise the same network name on several bands, but the choice of which one to join belongs to the client device. Clients are conservative and tend to hold an existing connection until it becomes genuinely poor, not merely worse than the alternative available.

This is why a laptop carried upstairs clings to a weak connection on the fast band instead of moving to the band that reaches. Access points can nudge a client by declining to answer on one band or by sending a steering request, which clients are free to ignore.

Switching the device radio off and on forces a fresh decision, which is why that particular ritual so often appears to work.

The far room is better served by the slower band

Throughput on a weak connection to the fast band can fall below what a solid connection on the lower band would deliver comfortably. As signal quality falls, the radio steps down to slower and more robust encoding, and each step roughly halves the achievable data rate. A device clinging to a distant fast band therefore occupies airtime for a long stretch in order to move very little data.

In practice, because airtime is shared, that one slow device delays everything else attached to the same access point while it is transmitting. Splitting the bands into separate network names is crude, but it hands you direct control over which devices sit where.

This is the general case; a specific device may behave differently by design.

Placement is the cheapest fix available

An access point radiates roughly sideways from its antennas, so a unit shut in a cupboard at floor level wastes most of its coverage. Height and openness matter more than finding the exact centre of the house, because the first obstruction does most of the damage.

Keeping a metre between the unit and any large metal surface avoids reflections that cancel the signal in nearby spots entirely. Where the incoming line happens to arrive is almost never the right place for the access point, and a short cable run fixes that. Moving a unit a metre and measuring again is more productive than changing settings, because indoor propagation is dominated by geometry.

The takeaway

Pick the band for the distance, not for the number printed on the box.

Once you know what it is trading away, the design stops looking arbitrary.