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3 Radio power & other legal stuff·Can I legally modify my wireless network equipment?
·Maximum power for 802.11a
·What are the useful RF units of gain and power for the Wireless ISP?
·How does one convert Watts to dBm?
·What is the maximum output power allowed for wireless Ethernet.
·How is antenna gain (dBi) measured and used?
·What other power losses are there?
·How does output power relate to distance?
·How can the above be used to calculate output power?
·How is output power calculated when an external amplifier is added?
·Can splitters be used? What happens to output power with a splitter?
·What are the FCC equipment certification requirements?
·What are the power limits in Australia?
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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.
In the meantime, the following reference will hopefully prevent any further confusion and answer "what's legal and what's not" questions:
http://www.arrl.org/tis/info/part15.html
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by korym  |
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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)
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by antennitis edited by seagreen
last modified: 2006-08-30 23:23:52 |
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Watts and dBm are units of power.
To calculate total output power, the dimensionless units dB and dBi are used.
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by StudMuffin edited by korym |
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Use a calculator, conversion table (pdf) or equation to convert the power units. Here is a short conversion table: | dBm | Watts | | 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.
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by StudMuffin edited by korym |
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The maximum output power allowed for wireless Ethernet (802.11, 802.11b, etc.) by the FCC at 2.4Ghz is 36dBm or 4Watts.
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by StudMuffin edited by korym |
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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: quote:
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...
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by StudMuffin edited by seagreen
last modified: 2008-03-23 10:24:45 |
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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.
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by StudMuffin edited by korym |
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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.)
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by StudMuffin edited by korym |
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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
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by StudMuffin edited by korym |
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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
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by StudMuffin edited by korym |
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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
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by StudMuffin edited by korym |
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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:
DISCLAIMER: THIS INFORMATION IS INTENDED AS A GUIDE AND FOR DISCUSSION PURPOSES ONLY. THIS IS ONLY MY OPINION AND ANY INTERPRETATION IS ULTIMATELY UP TO THE FCC ONLY!
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:
•Have a modular certification for the radio inside AND have a Declaration of Conformity to comply with Subparts C and B respectively – OR
•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:
»https://gullfoss2.fcc.gov/prod/oet/cf/kd···sult.cfm
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:
»https://gullfoss2.fcc.gov/prod/oet/cf/ea···arch.cfm
I hope this information helps to give you a better idea of the FCC certification requirements!
-Harold Bledsoe
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by dongato17 edited by seagreen
last modified: 2006-07-26 14:20:44 |
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Please see pkhooper 's excellent explanation here:
»Re: Power Limits in Australia?
Associated Documents: Radiocommunications (Low Interference Potential Devices) Class Licence 2000.pdf
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by seagreen
last modified: 2008-07-19 09:18:22 |
