AP tx power and antenna

[Adrian4]

Hello,

I recently raised a ticket with meraki because I have a MR74 that seemed to be stuck on a low TX power (well below 20).

They explained that its because the regulatory rules say the max power is 20 (or 23, somewhere around there) - and with the antenna gain, it comes to the max. It made sense so i closed the ticket but now I stop to think....it doesn't make sense at all lol.

If the regulations on max power include the radio power and antenna gain combined, then what is the point in directional antenna? Since any boost you get from narrowing the beam is lost by having to reduce the radio power proportionally. 

[alemabrahao]

This is regulated by the IEEE, see what is said for 802.11g for example:

IEEE 802.11g (2.4 GHz Band)

Table 2 indicates the maximum power levels and antenna gains allowed for the 2.4 GHz radios in most regulatory domains. We recommend that you check your local regulations with the appropriate agencies.

Table 2 Maximum Power Levels Per Antenna Gain for IEEE 802.11g https://www.cisco.com/c/en/us/td/docs/routers/access/wireless/software/guide/RadioTransmitPower.html...

[Adrian4]

thanks for the reply - that confirms the idea, but I still dont understand it. 

If having higher antenna gain means having to reduce the radio power, then what is the point? Other than saving power use.

Is the only point in external AP models like the 76 that they are weather proof? If so, why do they have external antenna?

[cmr]

The directional antennas focus the available power into a smaller area.  Therefore the signal travels further in that specific direction.  Think of the total power being the same but all going where you need it, as opposed to omni-directional antennas that send a bit in every direction.

[Adrian4]

But isnt the focused power represented by the antenna gain? The narrower the focus, the higher the gain in dB's. A gain that must be deducted from the total max tx power allowed.

The rule seems to be that you can only have 30dBm (or whatever it is) total between the antenna gain and the radio tx.

Example -
Radio tx is 23 dB and omni-directional antenna has a gain of 7. (30dB total). 

Then you swap the omni antenna for a directional one with 12dB gain. (35dB total).

But 35 is over the allowed limit of 30, so you have to drop the Radio tx to 18.
and your back to 30dB total.

So now you have a directional antenna with the same range as the omni directional antenna?!?