dB to Watt Table and Limits

dBm to Watt Conversion Table 



 01.0 mW1640 mW321.6W
 1 1.3 mW1750 mW332.0W
 21.6 mW1863 mW342.5W
 32.0 mW1979 mW353.2W
 42.5 mW20100 mW364W
 53.2 mW21126 mW  37  5.0W

Usage and Maximum Power Limit Guidelines in the US under FCC regulations:

Before we can go on, first we need to separate the two different classes of users for Spread Spectrum devices that exist and set some guidelines of some of the specs.  

Consumers and IT Professionals Operating Spread Spectrum (DSSS) gear:  

     * Users operate under FCC Part 15 rules and regulations.

     * Frequencies include 902-928 MHz, 2400-2483.5 and 5725-5850 MHz.

     * Maximum Transmitter Power Output (TPO) is 1.0 watt or 30dBm. 

     * The formula for converting antennas from dBi to dBd is dBi-2.2=dBd.

There are two different classifications for operation. You'll commonly hear these modes referred to as Point to Point (PTP) and Point to Multi-point (PTMP).  PTP is when two sites talk only to themselves. PTMP is when many sites talk to a single core site. Each of these modes have different EIRP (Effective Isotropic Radiated Power) limitations.

Point to Multi-point:

The maximum EIRP power allowed is 36dBm (4 watts). 

Maximum transmitter power versus largest antenna table for PTMP:

Transmitter RF Output Power  Antenna Gain  EIRP in watts
30dBm   1W      6dBi3.98
27dBm   500mW9dBi3.98
24dBm   250mW12dBi3.98
20dBm   100mW15dBi3.98
17dBm   50mW18dBi3.98
14dBm   25mW21dBi3.98
10dBm   10mW 24dBi3.98

Losses from the transmitter via cabling, lightning suppression, filtration can be removed from the transmitted power dBm figure. An example here would be say a 30dBm 1 watt amplifier with 100ft of LMR400 (at 6.7dB of loss) brings transmitter power down to 23.3dBm, allowing a 12dBi antenna. 

Point to Point:  

Higher EIRP is allowed if the antennas are directional in nature.

Systems operating in a point-to-point operation may employ transmitting antennas with directional gain greater than 6 dBi provided the maximum output power of the transmitter is reduced by 1 dB for every 3 dB that the directional gain of the antenna that exceeds 6 dBi. Maximum transmitter power versus largest antenna table for PTP: 

Transmitter RF Output Power  Antenna Gain  EIRP in watts
30dBm   1W      6dBi3.98
29dBm   800mW9dBi6.35
28dBm   630mW12dBi10.14
27dBm   500mW15dBi15.81
26dBm   398mW18dBi25.23
25dBm   316mW21dBi40.28
24dBm   250mW 24dBi64.79
23dBm   200mW 

          This information is provided as a guideline. If you are not a professional installer we highly recommend that you read the FCC Part 15 rules and understand them before attempting installations. 

Amateur Radio Operators operating under licensed spectrum:  

     * Users operate under FCC Part 97 rules and regulations. 

     * Frequencies usable from over-the-counter consumer gear include the 33cm 902-928 MHz band, the 13cm 2390-2450 MHz band and the 5cm 5650-5925Mhz band. 

     * In the 13cm band, 802.11b/g channels 1 thru 6 are the only channels in the 2390-2450 MHz bandplan.

     * Maximum Transmitter Power Output (TPO) is 100 watt or 50dBm.

     * You must enable broadcasting of your SSID, which has to include your callsign.

     * Encryption is not currently permitted.

Only authorized licensed operators should be able to access Part 97 installed hardware, so care should be taken to prevent unauthorized users from utilizing said hardware. 

It's highly suggested for Amateurs visit the ARRL website and participate in the HSMM (high speed multimedia) working group. The HSMM group only deals with working on high speed data via Amateur radio. This group is producing proposed rule making changes to be submitted to the FCC that would make operating simpler and allow more reasonable usage of for example encryption. The author of this document, Dave Anderson is a licensed amateur radio operator (KG4YZY) and is on the ARRL HSMM Working group and a founding member of ARBA, the Amateur Radio Broadband Alliance.