On Mon, 24 Jul 2006 22:58:11 -0700 Jeff Liebermann wrote: | snipped-for-privacy@ipal.net hath wroth: | |>What are 2 antennas being used for? | | Diversity reception. | |>WRT54G and many others have 2 or even 3 antennas. | | Two antennas are diversity receive. | Three antennas are MIMO which is quite different. | No antennas are beam forming or beam steering.
So only one of the antennae is used for transmitting. Dare I ask which one :) ... ah, answer below.
|>There are a number of things I could envision what |>they might do with this. Anyone know what they actually are doing? | | Well, it would be nice if you would limit the question to a specific | model or type of radio.
It's a generic question because I see a number of radios with 2 or 3 antennas. The answer for one model would not necessarily give me an idea of the scope of what is being done overall. If two different models do different things, I'd like to get an idea of possibilities.
|>Here are some possibilities. | | Wait a minute. You're asking a question and offering multiple choice | answers? I recognize your deductive abilities but wouldn't it be | better if you ask and not guess?
The examples were intended to give the scope and level of answer I am looking for in that question. Were one of those guesses true, it would be an exact answer to my question. It's not really a multiple choice question. If the answer is something else, then all guesses are wrong.
|>1. Split band. For dual band models, one antenna could be attached |> to 2.4 GHz RF circuitry, while the other is attached to 5 GHz RF |> circuitry. This would avoid the need for an internal splitter and |> a dual band antenna. | | That's not the way it's usually done. Most dual band access points | use common antennas for both bands. However, there are chipset that | seperate the bands and it could be done this way. It's very common | with the new 4.9Ghz/2.4Ghz MotoMesh access points, that have seperate | antennas for each band. However, for commidity access points, the | antenna is usually (not always) common for both bands.
And so not likely concurrent operation on both bands at the same time other than simultaneous receive.
|>2. Split RX/TX. One antenna could be used for receive, while the other |> is used for transmit. I don't see any significant advantage to this |> unless the TX power is low enough to still allow RX on a different |> channel for models than can operate on 2 channels at the same time. |> For remotely wired antennae, this might be an advantage. | | That was done on some access points about 5 years ago, where one | antenna was outside, and the other was inside the plastic box. I | vaguely recall the DLink DWL-900AP+ was one of these. The inside | antenna was almost totally useless for transmit, so all transmissions | were from the outside antenna. I'm not sure why they even bothered | with receive diversity. | |>3. Diversity. Two receivers work together in case one antenna position |> would be in a null spot for another device, the 2nd antenna might |> get the signal. Transmit might also select which antenna provides |> the better path to the destination. With extra compnents used to |> realize the receive advantage, this might not be done. | | This is the most common arrangement. What's not obvious is that there | is a MAIN and AUX antennas. The radio sits on the MAIN antenna most | of the time and only tries the AUX antenna if it detects a high error | rate. The main advantage is to reduce frequency selective fadeing in | a reflective environment (such as all indoor systems). See: | |
formatting link
| |
formatting link
| |
formatting link
| for a few articles on the common methods of wireless diversity.
You're right, MAIN vs. AUX was not obvious. I suppose if I have worked on trying to guess every possible combination of how to do things, that would have come up among them.
So for transmit, it would alayws be from the MAIN antenna, and leave it up to the other end to have receive diversity in the event that MAIN on one unit to MAIN on the other unit happened to be a bad RF path.
|>4. Directionality. Similar to diversity, but an RF stage taking signal |> on both antennae at the same time can get up to 3db gain in certain |> directions (and be entirely deaf in others). This could be used to |> enhance certain weaker devices and/or null out interference sources. |> TX could do the same. This would be difficult for the average user |> to manage, and require extra components, so I doubt it would be done. | | Well, with two rubber ducky antennas, you'll never realize enough gain | to make the effort worthwhile. Worse, with the antennas twisting in | random directions, there's no way to aim it. That's not going to | happen.
The distance between them would dictate a beam pattern, if merged at the RF level. But it would be more confusing with wireless networking than it would be trying to adjust rabbit ears on an analog TV (digital TV is going to mess that up, too).
| However, there is a class of access points that use beam streering, | and another that uses beam forming, for directing the signal. Inside | the box is an elaborate antenna array. For beam steering, the idea is | to put the peak at toward the client radio. Beam steering is | considerably more elaborate in that it also aims the peak, but also | detects sources of interference and directs a null at the | interference. | |
formatting link
Much more expensive, no doubt.
| |>5. Placement. Wire a remote antenna on ONE of the connectors to get |> coverage in two distinct places. | | That doesn't work. Read the Cisco article mentioned previous for | details. Another major problem is that traffic through the access | point, from one antenna to the other doesn't get switched fast enough. | If you were to try to use it as a repeater between antenna ports, it | would need to switch antennas on literally every packet. The | diversity algorithm is nowhere near that agressive.
Or basically be 2 radios in one, just on the same band and programmed to not transmit at the same time (unless on different channels, but then, that's a different animal).