In reply to LeeWood:
Your phone does not have a fixed transmitting power. It is calculating all the time how much power it needs to transmit to stay on the network and which part of the network it should engage with. If the base station is 10m away then it transmits next to damn all and if it can barely reach it then the transmitter is full on. That's why your battery dies quickly in not-spots: transmit power is high and repeated attempts are made to re-register on the network.
The antenna at the base station is usually a large plate antenna these days (or a high gain tuned antenna at older base stations) compared to the very tiny one on your phone and that plate is backed up by an extremely sensitive receiver that has a range of tricks up its sleeve to make sense of the incoming traffic. Everything in a digital transmission stream is error-checked, so everything works until at the limits, when a number of packets repeatedly fail, the entire call or message fails.
The base station transmits in sections, of usually one third of a circle par antenna and then the signal quickly degrades in accordance with steradian geometry.
Your phone probably wants to receive at least 150 femtoWatts for a decent signal but can still operate when receiving far less. Maybe even 5 to 10 fW for some services. Once you are into the microWatt range then that's a really excellent signal.
A fW is a a quadrillionth of a Watt (thousandth of a million-millionth or 10 to the minus 15th power).
With a typical wifi link using two antennae, it is the same as the microwave links used between some base stations or between oil platforms. The two antennae are to a greater or lesser extent focussed in the direction of each other. You are avoiding the losses of the steradian geometry of normal radio transmission by trying to create a straight 'pipe' of radio energy aimed at the opposite antenna. That way you can transmit considerable distances with far smaller losses of power density due to the geometry.
It might be that you can use one wifi antenna that is less focussed and transmits across the whole building. At 100m, it probably depends what the kit is designed for whether you use one antenna, or use two and bring the signal into the building then retransmit inside. The stuff I have looked at is for 3 to 10km. You need to ask any proposed supplier specific questions about their equipment.
Post edited at 01:43