RFID tags rely on two different forms of signal transmission to communicate with the tag.
Inductive coupling
This is the transmission method of choice for LF and HF tags. The antenna used is a simple tuned coil (ie tuned for the technology operating frequency), which is said to operate in the near field .
In the LF range, building such antenna requires a large number of loops, and as the size goes down (as it is often the case with injectable glass encased tags) a ferromagnetic core is required.
In the HF range, the number of loops required for the tag to operate is much lower, and this allows for the tag to be created on a paper substrate.
The transmission range is directly linked to the transmission frequency (the distance must be considerably smaller than the wavelength), topping at around 1.5 metres for HF (even if LF could in theory support much longer ranges). The antennae size determines the efficiency of the power transmission between the reader and the tag - so, larger antennae, increased range. Also, to maximise the concatenated flux (amount of magnetic field "trapped" by the tag antenna), the planes where the reader coil and the antenna coil lay must be parallel.
The data transmission between tag and reader is obtained by the tag changing (modulating) the load the reader sees on its coil.
Backscatter
This transmission method is used prevalently in the UHF and microwave range of frequency.
The antennae used are folded dipole antenna (even if for the reader other types such as the patch antenna are often used), usually a half wavelength long (roughly 17 centimetres for 868MHz UHF) operating in the far field (where the distance between the antennae is considerably greater then the wavelength).
As proper signal propagation is used, the transmission range is mostly linked to the maximum distance the reader manages to power the tag. The reader "spreads" the power it is transmitting on the surrounding space, and only a small fraction is picked up by the tag's antenna. The tag to communicate reflects this signal back to the reader, but this is "spread" as well on the surrounding space - so the reader picks only an extremely small fraction of the power it originally transmitted. This also explains why BAP tags reach ranges in the order of 100m just by integrating a battery to power the chip (being power transmission to the chip the biggest limiting factor in the UHF range).
For this transmission to work efficiently, the lines where the reader and tag antennae are laying must be parallel (see polarisation).
The data transmission between tag and reader is obtained through the tag changing (modulating) its radar cross section.
