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I am trying to understand how long range Wi-Fi works. As far as I know, Wi-Fi consists of Tx and Rx. When a laptop is connected to an access point (AP), the laptop is able to receive data from the AP (Rx) and also transmit data back to the AP (Tx).

Let's say I want to build a long range Wi-Fi to cover a large area. I connect a high-gain omni-directional antenna like this to the AP.

Let's say the original AP's Wi-Fi signal radius is 250 m. By using the high-gain antenna, the radius become 1000 m.

At 1000 m away from the AP, by using a normal laptop (without any high gain antenna), I try to connect to the AP.

The Tx signal from the AP is able to reach the laptop. But, the Tx signal from the laptop should not be able to reach the AP. Under this condition, can the laptop connect to the AP?

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4 Answers 4

up vote 21 down vote accepted

The antenna changes the shape of the transmission. The electrical signal doesn't get more powerful, but less is wasted in transmission in directions which aren't useful (e.g., up and down).

Similarly with reception, the signals are received from a more narrow field, this strengthens reception and reduces interference.

It's similar to talking through a cone, then listening through the cone for a response. the person at the other end doesn't need any special equipment, but you've increased your range and sensitivity.

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The analogy of talking through a cone is elegant, but the person using the cone is often required by law to keep his voice down, so that the advantage in range is limited to reception. –  Marcks Thomas Aug 12 at 19:45

"High gain antennas" provide gain on both transmit and receive.

So, with such an antenna at only one end, you'll get more range than with a standard antenna at each end, but less than with a high gain antenna at both ends.

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1  
Sure? How? You can send data with higher dBm and probably receive with lower dB because of higher antenna gain. But it still be not that much. Regular Wifi has a natural borders of reachability, dependent on the damping of the air. –  Watsche Aug 1 at 14:25
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@Watsche Choice of antenna is just as important as choice of radio. Your statement is completely wrong. Check out this 125 mile link with a 300mW radio on one end, and a 30mW radio on the other. youtube.com/watch?v=cT6H9IqJOBI The difference here is a very high gain antenna. –  Brad Aug 1 at 15:16
    
but the question said "high-gain omni-directional antenna" not beam directional type. –  Psycogeek Aug 1 at 15:29
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+1. You might also mention "antenna reciprocity" -- if the directional antenna can transmit 4 times as far as the standard antenna, then the same directional antenna can receive from 4 times as far as the standard antenna. –  David Cary Aug 2 at 15:31
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@Psycogeek Any passive antenna gain is achieved by focusing the radiation pattern. 0dB would be a theoretical isotropic source where the antenna radiates in all directions equally... up, down, sides, it doesn't matter... a perfect sphere of radiation. Omnidirectional antennas with gain focus these sphere into a donut shape, allowing communication on all sides at the expense of vertical coverage. –  Brad Aug 3 at 16:23

Some things to consider: The wireless access point (WAP) receive side has one obstacle the transmit does not, and that is receive noise. The received signal will have a certain signal to noise ratio, i.e. how much above the noise the signal strength will be in db. The "gain" antenna can pick up more noise, especially if there is a "noise source" in the path between the two access points. Signal-to-noise ratio will directly affect the Bit Error Rate (BER) and throughput of the system. But, generally, focusing the transmit and receive signal in one direction with the antenna will increase the distance while maintaining the same BER, within limits.

The omni-directional antenna achieves "gain" by flattening the "donut" pattern so more signal is radiated in a straight lines, like extended wheel spokes, vice wasted with upward direction. This is achieved with the physical construction of the antenna.

Positioning two omni-directional antennas (whips) next to each other with proper "feed", at a calculated distance can also increase the "gain" bi-directionally.

Reflection of the signal from the ground, buildings and other objects may cause the direct signal and the reflected signal to "subtract" as they arrive at the wap receiver (fading). This is exacerbated with an omni-directional antenna.

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I was not able to comment, but you might want to look at Radio Antenna Design and their Signal Radiation Patterns. It will show variety, shape, direction and intensity of ENERGY LOBES that are formed by various antenna. Wish I had more time to share a full answer, but I care that you head in the right direction versus points.

PS: I tried to comment but was not able to.

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