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3 Radio power & other legal stuff

This is tricky question to answer simply because the FCC hasn't been very specific about what constitutes a legal or illegal modification of FCC-certified wireless networking gear.

Manufacturers generally have to submit their products to the FCC to earn the "FCC-certified" bill of health prior to selling the equipment. Many consumers can now purchase wireless networking gear from stores such as Circuit City, BestBuy, CompUSA, etc. These units already have antennas built into them that obviously legally work with other accessories within the equipment line (i.e. same manufacturer APs work with their related PCMCIA cards, USB adapters, etc.)

With the recent articles being posted about how to conduct "war driving" (i.e. build a wireless radio which "sniffs" out wireless networks, etc.), extend wireless network ranges by adding more powerful antennas, etc., it's becoming apparent that the FCC will need to act soon.

Feedback received on this FAQ entry:
  • "what's legal and what's not" link at the end of the article is dead.

    2012-01-06 08:50:00 (xiarcel See Profile)

by korym See Profile edited by seagreen See Profile
last modified: 2012-01-06 16:43:12

FCC limits for new 802.11a standard are different for 3 bands, as follows:

5.150-5.250GHz Indoor 50mW (17dBm)
5.250-5.350GHz Indoor/Outdoor 250 mW (24dBm)
5.725-5.825GHz Outdoor 1 W (30dBm)

An article in WNN WiFi Net News says this:
In the 2.4 GHz band and parts of 5 GHz, the maximum power from the radio is 1 watt (W), and the effective power (EIRP) is 4 W on an omnidirectional antenna. (You can push far more power if you narrow the antennas beam. And parts of the 5 GHz band restrict radio power below 1 W.

by antennitis See Profile edited by KeysCapt See Profile
last modified: 2008-08-31 10:35:43

Watts and dBm are units of power.
To calculate total output power, the dimensionless units dB and dBi are used.

by StudMuffin See Profile edited by korym See Profile

Use a calculator, conversion table (pdf) or equation to convert the power units. Here is a short conversion table:



0 .001
15 .032
24 .250
27 .500
30 1.000
36 4.000

The equation is:

dBm = 10 * (log (1000 * P))

P = Power in Watts

1000mW = 1 Watt

Note, doubling the wattage increases dBm by 3. Here is a more comprehensive tutorial on Using and Understanding Decibels.

by StudMuffin See Profile edited by korym See Profile

The maximum output power allowed for wireless Ethernet in the U.S. (802.11, 802.11b, etc.) by the FCC at 2.4Ghz is 36dBm or 4Watts.

From FCC Basics of Unlicensed Transmitters October 2007 (PDF File)

WLAN WISP interpretations

    • Fixed Remote station can operate as a point to point system even if the base station operates as a point to multipoint system.
    • Equivalent Antenna changes allowed without additional filings. See Section 15.204 antenna change polices.
    • Adding additional amplifiers only allowed for 15.247 and 15.407 devices. Amplifier must be Certified with the transmitter per Section 15.204(d)1.
    • All Sectorized systems are point to multipoint subject to 4 Watt EIRP limit except those systems that qualify as a Smart Antenna System (SAS). See SAS guidelines.
    • For SAS system only, Prohibited Broadcasting does not include occasional broadcast management signals or
    non-permanent multi-casting. See SAS guidelines.

by StudMuffin See Profile edited by KeysCapt See Profile
last modified: 2008-08-31 11:08:00

The unit of antenna gain is dBi. dbI means "Isotropic", a perfect POINT SOURCE, which radiates in a spherical manner. A perfect dipole radiates with a donut pattern, broadside to the long dimension of the dipole. It is a relative measurement to an ideal dipole that radiates in a perfect sphere. To achieve higher gains, antennas are constructed such that they radiate more in one direction than another. An omni directional antenna radiates uniformly in the horizontal plane and radiates very little up or down. Panel, sector, yagi, and parabolic grid antennas radiate in cones of various widths. The higher the gain, the smaller the horizontal and vertical angles. Jerm gave the sprinkler head analogy: for a given amount of water, the distance the water shoots can be increased by focusing the spray; for a given amount of of microwave energy, distance can be increased by focusing the beam.

Antennas angles are specified by their half power point (3 dbi less than the specified max output).

For example, one '14 dBi' directional antenna has 14dBi gain straight ahead but only 11 dBi gain 32 degrees horizontally and 31 degrees vertically; one '24 dBi' parabolic grid also has 24 dBi gain straight ahead but only 21 dBi gain 6.5 degrees horizontally and 10 degrees vertically.

Feedback Question:
please provide with the formula to calculate the gain of the antenna in dBi .....

A: No simple answer - see the following forum thread for more information: »Forum FAQ - pending feedback...

Feedback received on this FAQ entry:
  • dbi? & dBm? tutorial ... Email ... telrad.ec@gmail.com

    2013-02-26 05:52:27

  • ^^^ an isotropic antenna has 0 dBi

    2011-12-30 13:53:09

  • I want to know much more detail about antenna. microwave and radio wave

    2009-03-17 08:14:17

  • So is an isotropic antenna considered a gain of 0 dBi or 1 dBi?

    2009-02-05 20:04:43

by StudMuffin See Profile edited by seagreen See Profile
last modified: 2008-03-23 10:24:45

Connector losses can be estimated at 0.5 dB per connection. Cable losses are a function of cable type and length. LMR400 is nominally 6.6dB loss per 100 feet.

Feedback received on this FAQ entry:
  • excellent tutor

    2010-02-06 00:53:04

by StudMuffin See Profile edited by korym See Profile

The rule of thumb is that increasing output power by 6dB doubles distance. (However, I can't find a document-able source for this rule.)

Feedback received on this FAQ entry:
  • If strength of received EMR varies by the square of the distance from the source, then doubling the distance should quarter the intensity. Since doubling power adds 3db, then quadrupling power will add 6db, and quartering it will subtract the same amount.

    2010-11-29 12:31:28 (clarknova See Profile)

by StudMuffin See Profile edited by korym See Profile

Example 1: PC Card alone

Output power for a Lucent/Orinoco/Agere/Avaya 802.11b PC card is 15 dBm. If there is no external antenna then that's the answer. 15dbm.

Example 2: PC Card with 14 dBi external antenna and 50 ft of LMR400

Assumed total lost for a pigtail adapter is 0.5 dB. Loss for each connector on LMR400 is 0.5 dB.

+ PC card - pigtail adapter - connector - 50 ft LMR400 - connector + 14 dBi antenna = ? Total output power

Substitute the six items above with their 'db-counterparts' with + for things that add power and - for losses

15 dBm - 0.5 dB - 0.5dB - 3.3 dB -0.5 dB + 14 dBi = 24.7 dBm

Example 3: PC Card with 24 dBi external antenna and 50 ft of LMR400

15 dBm - 0.5 dB - 0.5dB - 3.3 dB -0.5 dB + 24 dBi = 34.7 dBm

by StudMuffin See Profile edited by korym See Profile

Most amplifiers used for Wireless ISP purposes (including 2.4Ghz flavors) are rated for output power. Therefore, input power and losses prior to the amplifier can be ignored. Typically, the amplifier is mounted very close to the external amplifier, without any extension cables or extra connectors.

Example 1: PC Card with 0.5 Watt amplifier and 8 dBi external antenna.

Note, 0.5 Watt = 27 dBm.

+ amplfier output + 8 dBi antenna = ? Total output power

27 dBm + 8 dBi = 35 dBm

Example 2: PC Card with 1 Watt amplifier and 6 dBi external antenna.

30 dBm + 6 dBi = 36 dBm

by StudMuffin See Profile edited by korym See Profile

For a given input power, a two-way splitter will typically decreases output by 3.5 db, three-way by 5.3 db and four-way by 7.5 db.

Example 1: PC Card with 1 Watt amplifier, three way splitter and three 11 dBi external antennas.

Each tap off the splitter goes to one sector antenna, the output power is the same for each antenna:

+ amplifier - splitter + antenna = ? Total output power

30 dBm - 5.3 dB + 11 dBi = 35.7 dBm

by StudMuffin See Profile edited by korym See Profile

Clarification on FCC Requirements

We have been investigating the exact requirements (as of now) for the Part 15 radios used in WISP deployments. To try to clear things up some, I decided to try to summarize what we have learned over the past year or so and try to help others to understand exactly what is required and expected. That said, I am not a lawyer and my general disclaimer would be to use this information as a guide and not as the final word. The final word, as always, resides with the FCC in the USA and other regulatory organizations in other countries.
To spell it out:
All that said, FCC certification can really be broken down into two parts: Subpart B and Subpart C certifications.
Let's take a look at what each of those is:

  • Subpart B:
    This is for unintentional radiators, meaning that any electronic devices that have signals will put out interference. Because of this, the FCC requires this interference to be measured to make sure it complies with the guidelines that have been set by the FCC.

  • Subpart C:
    This is for intentional radiators, meaning that the device is intentionally transmitting RF (AKA a transmitter). The FCC has set certain rules for these transmitters based on band, modulation, etc.

Now that we know what the two parts of certification, we can talk about what the requirements for each are. There are two types of devices on the market today (that I'm aware of at least): complete systems and certified modules.

  • Complete systems are those systems that are certified as a whole; meaning that the certification includes everything from the power supply, to the enclosure, the radio card / PCBA, pigtail, etc. It was tested as a complete system and is therefore marketed as a complete system. Any changes to the complete system (enclosure, antennas, etc.) void the certification and must be re-certified. There are exceptions to this which are covered later. Complete systems carry an FCC ID that covers Subpart C requirements and a Declaration of Conformity that covers Subpart B requirements.

  • Certified modules, on the other hand, have been tested to meet the standards as a stand-alone device. For example, a mini PCI card may have been tested without any housing, etc. There are certain requirements that must be met in order to be certified as a module, but once it is certified, all Subpart C requirements are met automatically when the module is integrated into a final system as long as the stipulations written on the grant are met. In this case, the module carries the FCC ID label to cover Subpart C requirements. In order to meet Subpart B requirements, however, the final system MUST STILL BE TESTED and covered under a Declaration of Conformity. The final system must also have a label on the outside that says: Contains FCC ID: xxxxxxxxx. The grant notes section of the FCC grant must say Modular Approval.

There are some exceptions to these rules. For antennas, any antenna of the same type and lesser gain may be used with either a certified system or a certified module. By same type and lesser gain, it means that if the certification has a 12dbi Omni, other omnis that are <=12dbi may be used by the installer. Also, different lengths of coax can be used with certified systems and modules.

So what does this really mean for the WISP? It ultimately means that the device you are using must either:
  1. Have a modular certification for the radio inside AND have a Declaration of Conformity to comply with Subparts C and B respectively OR
  2. Have a system certification for the complete system (covers Subparts C and B both)

The easiest way for a WISP to tell if a system is FCC certified is by the required label on the device. For #1 above, there should be a label on the outside of the device that says Contains FCC ID: XXXXX and have the Declaration of Conformity on the label as well or in the manual. For #2 above, there should be a label on the outside of the devices that says FCC ID: XXXXX and have the Declaration of Conformity on the label as well or in the manual.
My final comments are on changes and what it means for the certification. I have created a matrix that covers complete systems and certified modules as well as some common changes:

As I said before, this is to be used as a guideline. If you have specific questions or concerns, you can ask the FCC lab directly at this link:
This site contains the FAQs for most of the items covered above as well as a link to Submit An Inquiry on the left nav. I encourage all of you to read through this information and ask questions if you have them.
To search for FCC grants, you can use this site:
I hope this information helps to give you a better idea of the FCC certification requirements!
-Harold Bledsoe

by dongato17 See Profile edited by seagreen See Profile
last modified: 2006-07-26 14:20:44

Please see pkhooper See Profile's excellent explanation here:

»Re: Power Limits in Australia?

Associated Documents: Radiocommunications (Low Interference Potential Devices) Class Licence 2000.pdf

by seagreen See Profile
last modified: 2008-07-19 09:18:22