Access point with multiple channels

amer99 · Nov 17, 2013

Hi all,

This is a very basic question but i am totally new to wireless. Is it possible for a single antenna to transmit both at 2.4 GHz and 5Ghz at the same time. I know about dual bands that support both these bands but i am talking about transmitting both frequencies at the same time.

I think its not possible since a radio (antenna) and only radiate at one frequency.

RealBeast · Nov 17, 2013

Each frequency uses a different radio and antenna set.

amer99 · Nov 17, 2013

RealBeast :

So in case of simultaneous dual band antenna, is it one antenna or pair of antennas ?

RealBeast · Nov 17, 2013

It depends on the model, but each radio would have at least one antenna and with N/AC units there are multiple antennae for each radio to take advantage of MIMO and allow greater speed.

amer99 · Nov 17, 2013

RealBeast :

This is what is confusing for me. Can you please confirm that an antenna can only work with 1 frequency at a time. I am drawing this conclusion since an antenna can only radiate for 1 frequency right ?

amer99 · Nov 17, 2013

RealBeast :

Basically what i want to ask is that, is it possible for a single antenna to work on 2 or more frequencies at the same time ?

RealBeast · Nov 17, 2013

amer99 :

Probably not in a functional way, I expect that the results would be extremely poor as antennae are optimized for their frequency and beyond their operational band they would reject signals.

Take a look at THIS to see the construction of a very simple 2.4GHz antenna.

What are you trying to accomplish?

bill001g · Nov 17, 2013

I suspect what they do is put antenna on that are midpoint between the optimum for 2.4g and 5g. There is only a fraction of a inch difference in length on 3db antenna.

I would have to go ask my RF engineering buddies how exactly you can connect 2 signals to the same antenna and why it does not cause a issue but I know the data is actually sent and received over the same antenna simultaneously from both radios. I suspect it is because the frequencies are far enough apart the 2 radios do not interfere with each other.

On a receptions side it is obvious the antenna can receive multiple signals at the same time look at how tv antenna works all the channels all come in at the same time.

amer99 · Nov 17, 2013

RealBeast :

Basically i have started my wireless studies (ccna wireless) and during my research i came across this question. I have seen that simulataneous dual band antenna exist, i just wanted to know is it a single antenna or pair or antennas ?

Also, the main question that confused me is, is it possible for a single antenna to work on 2 or more channels in 2.4Ghz range ? i mean not channel bonding which is merging of adjacent channels but rather transmitting 2 channels independtly. Is it possible

bill001g · Nov 18, 2013

The problem with certification tests is there is the answer that is correct and then there is the answer that is correct when you consider only cisco equipment.
If you look 2 models they sell the ap-3600 and the ap-3500.

The 3500 has a total of 6 antenna. It uses 2 to transmit and receive 2.4g, 2 to transmit and receive 5g and 1 each to receive only 2.4 and 5. The 3600 has 4 antenna they transmit and receive both 2.4 and 5 on 3 and receive only on 1.

Now on top of that some cisco AP have multiple radios in each band in affect multiple AP in a single box. This lets you use say channels 1,5,11 in a single device rather than mounting 3 ap at the same location. Again some of these devices have more and less antenna. So in some devices they do run 2 radios on the same antenna and on others they do not. I have not used cisco AP in many years so I do not know the model numbers.

So the answer is you can do it but cisco may be asking if you SHOULD do it which I do not know what the disadvantage would be to sharing antenna either in the same frequency band or even in the same band on different channels.

In general we know we can do this because years ago when tv used to be analog you could get RF video converters that would transmit on channel 3 or channel 4. You could hook 2 up one on each channel and they worked fine and if you then hooked the common cable to a antenna you could (illegally of course) transmit the data a short distance.

amer99 · Nov 18, 2013

bill001g :

The problem with certification tests is there is the answer that is correct and then there is the answer that is correct when you consider only cisco equipment.
If you look 2 models they sell the ap-3600 and the ap-3500.

The 3500 has a total of 6 antenna. It uses 2 to transmit and receive 2.4g, 2 to transmit and receive 5g and 1 each to receive only 2.4 and 5. The 3600 has 4 antenna they transmit and receive both 2.4 and 5 on 3 and receive only on 1.

Now on top of that some cisco AP have multiple radios in each band in affect multiple AP in a single box. This lets you use say channels 1,5,11 in a single device rather than mounting 3 ap at the same location. Again some of these devices have more and less antenna. So in some devices they do run 2 radios on the same antenna and on others they do not. I have not used cisco AP in many years so I do not know the model numbers.

So the answer is you can do it but cisco may be asking if you SHOULD do it which I do not know what the disadvantage would be to sharing antenna either in the same frequency band or even in the same band on different channels.

In general we know we can do this because years ago when tv used to be analog you could get RF video converters that would transmit on channel 3 or channel 4. You could hook 2 up one on each channel and they worked fine and if you then hooked the common cable to a antenna you could (illegally of course) transmit the data a short distance.

I really appreciate your response. My query is not at all specific to cisco. I am asking in general that is it possible for a single antenna (or radio. a single entity) to transmit at 2 different channels within a band simultaneously

amer99 · Nov 18, 2013

bill001g :

The problem with certification tests is there is the answer that is correct and then there is the answer that is correct when you consider only cisco equipment.
If you look 2 models they sell the ap-3600 and the ap-3500.

The 3500 has a total of 6 antenna. It uses 2 to transmit and receive 2.4g, 2 to transmit and receive 5g and 1 each to receive only 2.4 and 5. The 3600 has 4 antenna they transmit and receive both 2.4 and 5 on 3 and receive only on 1.

Now on top of that some cisco AP have multiple radios in each band in affect multiple AP in a single box. This lets you use say channels 1,5,11 in a single device rather than mounting 3 ap at the same location. Again some of these devices have more and less antenna. So in some devices they do run 2 radios on the same antenna and on others they do not. I have not used cisco AP in many years so I do not know the model numbers.

So the answer is you can do it but cisco may be asking if you SHOULD do it which I do not know what the disadvantage would be to sharing antenna either in the same frequency band or even in the same band on different channels.

In general we know we can do this because years ago when tv used to be analog you could get RF video converters that would transmit on channel 3 or channel 4. You could hook 2 up one on each channel and they worked fine and if you then hooked the common cable to a antenna you could (illegally of course) transmit the data a short distance.

Basically all the replies that i am getting (which i really appreciate) are talking about 2 or more antennas. My query is very very specific to one antenna (one radio)

bill001g · Nov 18, 2013

You are now asking a different question. The antenna and the radio are not the same thing. A antenna is purely a passive amplifier. The radio on the other hand is a active device. If we ignore all the discussions of side bands etc etc the radio cannot transmit 2 signals at the exact same instant. But 2 radios can transmit at the same instant and as long as they do not interfere with each other can be attached to a single antenna .

RealBeast · Nov 18, 2013

First, my comments are limited to radio used with wireless network devices.

Keep clear in your mind that the radio and antenna are different elements.

You seem to be struggling with the difference of whether two different radio signals can be multiplexed to use one antenna and whether one radio can operate at two distinct frequencies (i.e. 2.4GHz and 5GHz). One radio cannot broadcast at two frequencies simultaneously, there is a separate radio for each frequency, and AFAIK each radio is limited to its frequency range.

While it is possible to use a single antenna for both, I've not seen it in APs as the antenna would not be optimal for one frequency, the other, or either of them, depending on the antenna design (in which I have no expertise at all).

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Access point with multiple channels

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