Interpreting FCC regulations for the 2.4GHz ISM band
The FCC has two classifications of ISM band links: PointToMultipoint and PointToPoint.
PointToMultipoint
PointToMultipoint involves any number of radios. PointToPoint involves only two radios, and no more.
The FCC uses something called EIRP to measure the amount of power a station transmits. This is a combination of both the radio output power and the antenna gain. Say we have an 8dBi antenna and a radio that emits 15dBm of power. Add these two up and this is your EIRP. The EIRP of this configuration is 23dBm.
In a PointToMultipoint configuration, the FCC limits EIRP emissions to 36dBm (4 watts). Our above example is within FCC specifications.
But what about a 12dBi omni with a 1 watt (30dBm) amplifier? This results in 42dBm and violates FCC regulations.
The following table contains the absolute maximum antenna gain for a particular input power level
Radio Output |
Maximum Allowable Antenna Gain |
Example Radios |
Example Legal Antennas |
15dBm (30mW) |
21dBi |
Orinoco, |
Omnis, Sectors, Panels, Yagis, Low end parabolics |
20dBm (100mW) |
16dBi |
Cisco, |
Omnis, some sectors, some yagis, some panels |
23dBm (200mW) |
13dBi |
Senao, |
Most omnis, some panels |
27dBm (500mW) |
9dBi |
1/2 watt Amp |
Some omnis, some patches |
30dBm (1 watt) |
6dBi |
1 watt Amp |
Some omnis |
PointToPoint
PointToPoint can only involve two radios. Any more than this is considered a PointToMultipoint link. You can't twist the rules to get around this one.
Recently, the FCC ruled that a phased array antenna system (a Vivato product) could operate under the PointToPoint rules. This is because the phased-array antenna system is considered an electronic array of multiple point-to-point links, since each steerable beam is very narrow and high gain.
The reason why the FCC created these two sets of rules was to avoid the flooding/spamming of the 2.4GHz unlicensed band. This essentially allows a high density of narrow point-to-point links, and a high density of low powered point-to-multipoint links.
In an effort to keep emissions within a tight beamwidth, reducing interference, the FCC decided to give us a bonus when we configure fixed point to point links.
Here's where legal combinations of antenna gain and input power can get confusing. Starting from an antenna of 6dBi, we can put 30dBm of power into the antenna. For every 3dB over the 6dBi antenna, we have to subtract just 1dB of input power.
dBi |
dBm |
EIRP in dBm |
EIRP in watts |
6 |
30 |
36 |
4 |
9 |
29 |
38 |
6.3 |
12 |
28 |
40 |
10 |
15 |
27 |
42 |
15.8 |
18 |
26 |
44 |
25.1 |
21 |
25 |
46 |
39.8 |
24 |
24 |
48 |
63.1 |
27 |
23 |
50 |
100 |
30 |
22 |
52 |
158.5 |
33 |
21 |
54 |
251.2 |
36 |
20 |
56 |
398.1 |
39 |
19 |
58 |
631 |
42 |
18 |
60 |
1000 |
Example: 24dBi antenna with a Prism 2.5 card of 200mW / 23dBm output power:
dBi |
dBm |
EIRP in dBm |
EIRP in watts |
24 |
23 |
47 |
50.1 |
Do you really need a lot of power for a 2.4GHz 802.11 link?
No, you will run into SIFS/DIFS timing issues or run out of earth before you will run out of power. With 24dBi parabolics on both ends, and 23dBm of input power, even a 1000km (or 621 miles) shot will provide -88dBm receive strength.
Check out the "Simple Wireless Network Link Analysis" at http://www.ecommwireless.com/calculations.html if you want to calculate SNR margin yourself.
