How to calcualte antenna separation?

[homy1]

I have two radio cards (24dbm) operating at channel 1 and channel 6 with ACR(adjacent channel rejection) of 35dbm.
How far apart do I place these antennas?
How to calculate it?

Thanks,

 

[Higgler]

To calculate loss between two antennas, and to set that loss greater than a certain dB level, you need the frequency. What frequency are your units working at?

So called Space loss (or antenna to antenna isolation) is defined as how much two zero dBi gain antennas couple to each other when they are spaced X feet apart. It changes with frequency. It's a simple formula, Google "space loss" or "space loss attenuation".

Those space loss numbers are in free space too. If your hardware is indoors, or around things that reflect energy (ground, people, cars, etc.) then the isolation between the two antennas can be much less than you'd think.

kch

 

[homy1]

I use 802.11b at 2.4 Ghz
channel 1: from 2.401 to 2.423
Channel 6: from 2.426 to 2.448
I'm not in electrical major so I don't understand much.
Suppose the isolation between two antennas is 36dbm, what is it related to adjacent channel rejection(ACR)?
How do I calculate an appropriate distance between the two antennas with respect to ACR?

Thanks,

 

[VEBill]

An isolation (pathloss) between two antennas might be xx dB, but it is not "36dBm" (which is an absolute power level: 36dBm = 4 watts). The 'm' means one milliwatt and dB is a log ratio. The B should be capitalized.

Your question is simple enough, but giving an accurate estimate would be complicated. You'd need to know the spectral purity characteristics of the trasmitter, the selectivity characteristics of the receiver, SNR of the end-to-end system, etc., etc. Even such deep concepts such as 'phase noise' can be a deciding factor. You almost certainly do not have access to sufficient information (detailed RF specifications) to make an accurate estimate.

Another approach is to try it and see. Reality takes precedence over anyone's calculated estimate. Calculations are based on models. Models are based on simplified human understanding. The simplifed human understanding is, well, simplified and very, very human.

In summary, try about 3 feet (~1m).

 

[Higgler]

For most antennas, separating them slightly gets you more than 20 dB isolation. If you orient them differently, say one up, one sideways, with not too much in between them, your isolation will jump to 40 dB. If you're indoors, it's hard to get more than 60 dB isolation.

kch

 

[watchjohn]

.

Sounds like you have 2 W-lan cards connected to a PC
and they have 2 external antennas.

"How far apart"...you ask

To achieve what ?


a)

To not interfere ?...meaning 1 TX troubeling the other RX ?

...theoretically as much as possible distance.
...in practice as long as the cables allow.


b)

To get radiation patterns which are not influenced by the
other antenna ?

...30 cm´s should be fine.



Maybe you could try below
and test the interference using
a program called "Netstumbler"
it´s free software...and will show
you the quality of received signals.

http://www.netstumbler.com/downloads/

a)

Tune one of the cards to receive a low or medium-strength
network...from someone else around you.
Best to find one on a channel as close as possible to
the channel of your other card.


b)

Use "Netstumbler" to see the signal-quality
of that neighbouring station/network.

Netstumbler will show you :

Signalstrength.
Signal-Noise ratio.
...etc

c)


Then switch on your other card...and make shure
it transmits (as much as possible).


d)

Observe what happens to the first channel that you
receive from your neighbourhood.

If the levels go worse..then obviously your second card
influences the first card.


Above is just an idea...and you need to play
around a little.



WatchJohn