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gmichels
join:2005-06-01
| [Help Me] Antennas suggestion Hey all
I just finished setting up a linux access point w/ WPA2 using a D-Link DWL-G520 pci card. Everything is ok, but I'd like to increase the signal coverage in my office.
Currently I'm using the standard antenna. Looking at the available antennas, I can see several models with different dBi levels such as 8.5, 12, 18 and so on.
I'd like to know if there's some sort of dBi/length correlation, so I can choose the best model for my needs. Just FYI, I'd like to cover an area of about 8m (26ft) by 25m (82ft). It's a 2 floor building and the main coverage would be on the upper floor. Coverage on the lower floor is desirable but not necessary. The computer with the antenna is located in the middle of the upper floor.
Thanks in advance,
Gustavo | |
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peter_m
Premium
join:2005-07-13
Canada, QC
3 edits | Re: [Help Me] Antennas suggestion For conventional antenna, longer it is, the higher the DB rating (usually). Basically what it does is focus the radio radiation in a horizontal plain perpendicular to the antenna. Imagine sticking the antenna in a vinyl record's center hole and that's what the pattern of radiation looks like. Anything away from the plain, like a floor above or bellow, will have less reception. The stronger the db, the stronger the focus and the stronger the rejection of off plain signals.
For your main floor, the standard 2db antenna might be enough if it is relatively open and don't have too many obstacles. Also it will be best for the other floor, since it will offer the least rejection of off plain signals. But that depends on the floor. Is it wood or concrete?
Just a note on the DWL-G520. It keeps the antenna too close to the CPU case and that might cause a problem for you. What ever antenna you choose, I would make sure it comes with a short cord that can allow you to have it a foot or two away from the CPU case.
Peter | |
| Dan Koerner
join:2000-08-05
Clinton, TN
1 edit | Sounds like you need a collinear array antenna. These have omni-directional patterns with higher gain.
3 dB = ~2x ERP over 1/4 wave stub.
6 dB = ~4x ERP
9 dB = ~8x ERP
Any antenna needs a good ground plane to be effective. Normal wireless antennas don't normally have associated ground planes... at least, not proper ones.
As Peter said, the higher the gain, the less signal to your lower floor.
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Dan | |
| gmichels
join:2005-06-01
| Thanks for your replies 
peter_m, all walls are concrete. On the upper floor, there won't be more than 3 concrete walls separating clients to the antenna. The lower floor is far more open and only one room with special walls (we call them here 'dry-walls', I'm not sure if you know the term). Anyway, wifi coverage on the lower floor is low priority.
Dan Koerner, excuse my ignorance, but I don't know what would be a 'collinear array antenna'. Could you provide more information about them?
Anyway, I was analyzing some omni-directional antennas and it looks like the best ones for my case would have between 5 and 9 dBi. Also, as peter_m suggested, I am looking for antennas with a base and cord, so I can put it over the computer case.
Currently, with the default 2 dBi antenna, signal strength from the AP to my laptop's 2915ABG wifi card (2 walls away, ~10m or 33ft) is around 50-60%, while the rate fluctuates heavily between 11 and 48 mbps.
Do you think a 5-9 dBi omni-directional antenna would help on this case?
Thanks! | |
| | Dan Koerner
join:2000-08-05
Clinton, TN
| Re: [Help Me] Antennas suggestion Any omni-directional antenna with a gain greater than 2 dBi is a collinear array, i.e., it's 1/2 wavelength elements are aligned end-to-end with a 1/2 wavelength phasing device between each element.
A 5 dBi vertical antenna has 2x the ERP (Effective Radiated Power) of a 2 dBi vertical antenna along any radial in their respective ground planes. This works both directions, sending and receiving. I have no idea as to the signal strength indicator calibration of the wireless cards... each manufacturer probably has a different system of measurement.
BTW, I really hate dBi (isotropic gain) references. The 0 dBi reference antenna is a hypothetical antenna that radiates or receives equally in all directions, i.e., it's radiation pattern is equal at all points on a surrounding sphere. dBi numbers are not real... just computer generated number by some unknown algorithm and a theoretical design. See »www.its.bldrdoc.gov/fs-1037/dir-···0317.htm. It is just to easy to hype a number that cannot be measured/proofed.
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Dan | |
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peter_m
Premium
join:2005-07-13
Canada, QC
1 edit | Concrete, ouch! That is a tough one. I would get a 9db on a base/cord to get the most on the main floor and just live with what ever you get on the other floor. Just don't get too much cord. The longer it is, the more signal is wasted since 2.4ghz doesn't like wires. Good luck
Peter | |
| gmichels
join:2005-06-01 | Many thanks for your help, guys. I'll buy a 9 dB omni antenna and hopefully my needs will be fullfilled.
Thanks again | |
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