Question - Omni Antenna Gain/Coverage

Sturmbahn

I understand that using high gain antennas with flatten the 'doughnut' of wireless coverage, but is the amount of flattening dependent on the gain of the antenna or the total EIRP of the system.

For example, would a 30 mW access point have the same 'doughnut' shape as a 100 mW access point using the same 6 dBi antenna? Would the 100 mW access point just have a stronger signal with the same shape of coverage or would it have a flatter shape due to the higher power radiated from the antenna?

JDMurray

The shape of the donut changes with the output power of the antenna, as measured in dBi.

As the gain (dBi) of an omnidirectional antenna is increased, the "donut" gets wider and flatter. A 2.5dBi omni would be a near-perfect isotropic radiator, but with dimples at its North and South poles. An 8dBi would cover a wider area and would be more of a donut or bagel shape. A 12dBi would be even larger and flatter, with a very wide coverage to the horizon, but no signal in the North and South directions.

Look at this page under Antenna gain:

http://channels.lockergnome.com/it/archives/20050216_troubleshooting_antenna_issues_on_wlans.phtml

Sturmbahn

Thanks, jdmurray, that helped.

So if a 32 mW IR with a 5 dBi omni-antenna would give a (comparatively) weak signal over a vertically wide and horizontally short range, changing antennas to a 16 dBi omni-antenna would put the same weak signal over a vertically tiny and horizontally long range and would only "strengthen" the signal by virtue of compressing the signal into a smaller vertical space.

Boosting the 32 mW IR with an amplifier and using the same 5 dBi omni-antenna would not flatten/widen the "doughnut" coverage as would the 16 dBi omni-antenna, but would instead "inflate" the coverage, covering more space vertically and horizontally with the same basic shape as the 32 mW IR.

Cool. Thanks again.

JDMurray

32 mW? Are you playing around with a Linksys access point?

Sturmbahn

Dlink Premiere A/G 108 Mbps Router with the matching PCI card. A little upset that it's power is fairly weak, that MIMO came out shortly after I bought them and that AirSnort/AiroPeek doesn't support the card. I'll just buy an amp from www.fab-corp.com some day and play around with that.

JDMurray

Linksys' APs are fixed at 32mW output, so that's why I wondered.

I subscribed to and downloaded the firmware from www.sveasoft.com and it allows my Linksys AP to output up to 100mW. My five 2.4GHz cordless phones used to beat up my WLAN, but now it's the other way around!

JDMurray

How to Pick the Right Antenna: http://www.odessaoffice.com/wireless/antenna/how_to_pick_the_right_antenna.htm

Sturmbahn

A good read, thanks. If only I read this before the test, I might have had an idea about the question concerning lobes of a yagi antenna. Oh well, an 89% is good enough. Thanks, again.