Hi,

I have a few questions about 802.11 wireless connections.

1) If I purchase a 21dbi antenna, I assume the transmission power from the wireless card, needs to be high. However how do I calculate what transmission power is required for a particular dbi. Or is dbi completely irrelevant from wireless card transmission power?

2) I assume to get a 50m line of site range from one antenna to the next, I need both antenna's to get the ideal dbi. If that is the case, why do people war drive with a high gain antenna? Doesn't the dbi of the antenna your detected need to be high enough, to transmit to your receiving war driving antenna?

As I hear that the higher the dbi, the more networks you can detect. If that is true then why do wireless links have two large antennas pointing at each other.

I hope someone can enlighten me on this. Thx.

The dbi is not related to the wireless card power. What it specifies is the gain in the antenna's best direction. The extra power is at the expense of power in other directions, so the higher the gain, the more directional the antenna; an antenna is effectively a focusing device. In other words, the 21dbi 'gain' means 21dbi better than an antenna that radiated equally in all directions.

To get maximal range, both antennas need to be directional. However, even if one antenna is not particularly high gain, it still helps to have a high gain one at the other end. The gains are combined to give the overall gain/loss.

It also helps to have higher power at the transmission end. A typical wireless network card puts out about 100mW, but it is legal to transmit up to 1W in that band in many countries. However, it is a lot easier to get the same gain by selecting an appropriate antenna (and less antisocial). Increasing the power by 10 times is equivalent to only 10dB more, whereas a 21dBi antenna provides around 100 times the power at the receiver end (and only affects people in the line of sight).