What are 2 antennas being used for?

What are 2 antennas being used for? WRT54G and many others have 2 or
even 3 antennas. There are a number of things I could envision what
they might do with this. Anyone know what they actually are doing?

Here are some possibilities.

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.

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.

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.

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.

5. Placement. Wire a remote antenna on ONE of the connectors to get
coverage in two distinct places.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-07-24-(E-Mail Removed) |
|------------------------------------/-------------------------------------|

phil-news-(E-Mail Removed) 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.

>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.

>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?

>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.

>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:
| http://www.commsdesign.com/design_co...cleID=16500279
| http://img.cmpnet.com/commsdesign/cs...eat1-jan03.pdf
| http://www.cisco.com/en/US/tech/tk72...8019f646.shtml
for a few articles on the common methods of wireless diversity.

>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.

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.
| http://www.ruckuswireless.com/technology/beamflex.php

>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.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 24 Jul 2006 19:30:05 GMT, phil-news-(E-Mail Removed) wrote in
<(E-Mail Removed)>:

>What are 2 antennas being used for? ...

Spatial diversity.
<http://en.wikipedia.org/wiki/Diversity_reception>

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_HowTo>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

John Navas <(E-Mail Removed)> hath wroth:

>On 24 Jul 2006 19:30:05 GMT, phil-news-(E-Mail Removed) wrote in
><(E-Mail Removed)>:
>
>>What are 2 antennas being used for? ...
>
>Spatial diversity.
><http://en.wikipedia.org/wiki/Diversity_reception>

Not applicable. The above article is for common techniques used in
telecom wireless. In all cases mentioned, the multiple signals
received are combined in some manner. This is NOT done in any
commodity Wi-Fi routers. It would be too expensive to have two
identical receivers in one box. Instead, the methods of Wi-Fi
diversity are called "switched diversity" or "selection diversity" as
mentioned in this article:
| http://www.commsdesign.com/design_co...cleID=16500279
These requires only one receiver and are admittedly not as effective
as full time diversity (as described in the Wikipedia article).


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Mon, 24 Jul 2006 23:20:39 -0700, Jeff Liebermann
<(E-Mail Removed)> wrote in
<(E-Mail Removed)>:

>John Navas <(E-Mail Removed)> hath wroth:
>
>>On 24 Jul 2006 19:30:05 GMT, phil-news-(E-Mail Removed) wrote in
>><(E-Mail Removed)>:
>>
>>>What are 2 antennas being used for? ...
>>
>>Spatial diversity.
>><http://en.wikipedia.org/wiki/Diversity_reception>
>
>Not applicable.

Close enough for this context.

>The above article is for common techniques used in
>telecom wireless. In all cases mentioned, the multiple signals
>received are combined in some manner. This is NOT done in any
>commodity Wi-Fi routers. It would be too expensive to have two
>identical receivers in one box. Instead, the methods of Wi-Fi
>diversity are called "switched diversity" or "selection diversity" as
>mentioned in this article:
>| http://www.commsdesign.com/design_co...cleID=16500279
>These requires only one receiver and are admittedly not as effective
>as full time diversity (as described in the Wikipedia article).

Update the article.

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_HowTo>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

On Mon, 24 Jul 2006 22:58:11 -0700 Jeff Liebermann <(E-Mail Removed)> wrote:
| phil-news-(E-Mail Removed) 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:
| | http://www.commsdesign.com/design_co...cleID=16500279
| | http://img.cmpnet.com/commsdesign/cs...eat1-jan03.pdf
| | http://www.cisco.com/en/US/tech/tk72...8019f646.shtml
| 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.
| | http://www.ruckuswireless.com/technology/beamflex.php

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).

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-07-25-(E-Mail Removed) |
|------------------------------------/-------------------------------------|

On 25 Jul 2006 15:56:40 GMT, phil-news-(E-Mail Removed) wrote in
<(E-Mail Removed)>:

>... 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.

I think transmit will typically occur on the current antenna; i.e., if
AUX is getting the best receive signal, then it will be used for
transmit as well. Using only MAIN for transmit would be a poor design.

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_HowTo>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>

phil-news-(E-Mail Removed) hath wroth:

>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.

It's easy enough to check. Find a dual trace oscillosope and connect
CH A to the diversity switch IC and CH B to some place that indicates
if the radio is transmitting. See what it's doing. I did this about
3 years ago with a WRT54G in my rather sloppy attempt to reverse
engineer how the diversity switch actually worked (and to figure out
which is are the left/right and main/aux antennas). I also tried to
connect a red/green LED to the diversity switch IC to indicate what it
was doing, but the switching rate was just too fast to be useful.

If I have time, I'll do it again. It's fairly easy, but the local
temperatures have been far to high to do anything useful in the shop.

>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.

It apparently varies with chipset. As I vaguely recall (not sure),
the Broadcom chipset in the WRT54G switches TX along with RX. I
believe (not sure) that some chipsets can be set to work either way.
One thing I do recall is that the switch spent almost all its time on
the MAIN antenna.

>| | http://www.ruckuswireless.com/technology/beamflex.php
>Much more expensive, no doubt.

Not really. See:
http://www.netgear.com/products/details/WPN824.php
as an example. About $90. It's the mechanical parts that add cost.
The smart antenna board eliminates a mess of connectors, cables,
antennas, hardware, assembly labour, found in conventional diversity
access points and is probably a cost savings.

>| 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).

What do you mean by 2 radios in one? I'll guess you mean full duplex,
which is possible but has not commonly been implimented in consumer
wireless. BelAir Networks and some others have dual radio access
points that can act as repeaters. It's used in mesh networks to
eliminate the bandwidth reduction caused by single channel systems. No
diversity of any kind as each antenna is connected to a seperate
radio.
| http://www.belairnetworks.com

There was also Bermai, which had a dual receiver (TwinRX) diversity
system for 802.11a that was targeting consumer wireless multimedia.
With two receivers, they implimented full time diversity reception
with no switching.
| http://www.commsdesign.com/design_co...cleID=16501888
The company closed down in 2004. No idea what happened to the assets
or IP rights.



--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

"John Navas" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On 25 Jul 2006 15:56:40 GMT, phil-news-(E-Mail Removed) wrote in
> <(E-Mail Removed)>:
>
>>... 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.
>
> I think transmit will typically occur on the current antenna; i.e., if
> AUX is getting the best receive signal, then it will be used for
> transmit as well. Using only MAIN for transmit would be a poor design.
>

Or it could just transmit on both antenna at the same time; which would
probably be the most sensible way, as well as the easiest.

"Paul" <(E-Mail Removed)> wrote:
>Or it could just transmit on both antenna at the same time; which would
>probably be the most sensible way, as well as the easiest.

Actually, this could be the worst possible thing to do, depending on
phasing and antenna separation (to say nothing of antenna angle) you
could get some really bizarre antenna patterns. [The thought of
external antennas in this situation makes my brain hurt.]