Electronic – Does Wifi signal strength affect link speed

First, a caveat - I'm primarily a CE, so this is just my understanding of the matter:

1. Generally, the worst reception is when the antenna is upside-down, or nearly so, and the best is when it is vertical. However, horizontal orientations are nearly as good as vertical; the radiation pattern is usually something like this, with "up" in the picture corresponding to the positive Z axis (defects overemphasized):

2. Again, your antenna manufacturer will likely have a reference design for this. Sarantel publishes sample gerbers for many of its antennas. If the antenna is not directly coupled with a ground plane, near-field radiation will still be helpful.

3. No idea. I doubt that it would matter as long as both antennas had decent signal strength. Back-to-back might not be ideal, but everywhere else should work fine.

4. No, it's hardly directional at all.The reason you use a helical antenna is because you want near-omnidirectional performance. If orientation is controllable, use a different antenna.

Signals from other channels are picked up as noise, lowering your signal-to-noise ratio (SNR). In principle, it could be picked up as data, but will almost always fail some very basic checks (correct preamble, destination MAC address, ...).

The channel mapping is built so 802.11b channels have sufficient distance to not influence each other. With 802.11g, the transmission bandwidth is wider, but the channel mapping was retained for compatibility purposes.

Interfering transmissions cause information to be lost, which is generally corrected for by a retransmission, decreasing efficiency of the system.

If any access points using the g protocol are around, it is generally best if everyone uses channels 1, 6 and 11 exclusively, as this avoids interference between adjacent channels, leaving only conflicts on the same channel, which the protocol is built to avoid even between different networks.