Again by applying the rule we can see that 10dBm is 10mW. Baring in mind your antenna is already giving you 20dBm and if we assume the loss from cables and connectors is negligible this leaves only 10dBm for the radio. By looking at the above table or doing the maths working up from 0dBm using the rule of “3″ or “10″ we can establish that this means that your total output can not exceed 30dBm. This means you need to stay within the Band B limit of 1W. You are operating in 5GHz and you have a 20dBm antenna but you don't want to give Ofcom any of your hard earned cash. Here is an example of a scenario in which you may need to use your new found skills:
This requires more detail than this post goes into, but the calculations above form a vital part of calculating this. This includes everything discussed so far (at both ends) and also in addition the loss caused by the wireless traveling through the air. The link budget is a fancy term for the result you get when you add up and subtract all the various factors that effect the wireless transmission between 2 radios. This will typically be converted to a Watts (W) or Milliwatts (mW) value as per the tables above when expressing the output power. The calculation above will give you a value in dB. Some Antennas may be measured in dBd in which case simply add 2.14 to its value to get the dBi value (This is quite rare though and I’ll most likely explain why in a later post) Output Power This one is easy, simply look up the dBi value of the antenna and apply it to the calculation. Connectors will also add a small amount of lossĪntenna Gain is measured in dBi. Normally loss values will be expressed as dB loss per 100ft and you can typically find this value on the spec sheet. Also keep any antenna cable lengths as short as possible. You can limit this loss by using high grade cable such as LMR400 or LMR600 instead of LMR200 and using good connectors with well formed joints. You will need to take into account the loss caused by cables and connectors as each component will add an element of loss. The above calculations aren’t exact, but they are typically accurate enough to get by on. Yup you guessed it, if you lower the dBm value by 10 you divide the mW value by 10. If you raise the dBm value by “10″ you multiply the mW value by 10:Ĥ0dBm = 10000mW (Over the 5GHz Band C legal limit) This also mean that if you lower the dBm value by “3″ you halve the mW value Rule 2: The Rule of 10 If you raise the dBm value by “3″ you double the mW value: At this point it is worth noting the following:Įffectively to get Watts (W) you just divide the number of Milliwatts (mW) by 1000 and to get Milliwatts (mW) you just multiple the number of Watts (W) by 1000.Ĭonverting Watts and Milliwatts to dBm is slightly more tricky… You are now entering the wonderful world of logarithmic calculations! Have no fear though, it’s easy if you remember 2 simple rules and remember that:> Ideally you want to convert this to a dBm value if not already expressed this way. This is the trickiest of the 3 parts mainly due to the fact that it can be expressed in Watts, Milliwatts or dBm. Lets cover each part of this calculation in turn: Radio Transmit Power Radio Transmit Power (dBm) – Loss from Cables & Connectors (dB) + Antenna Gain (dBi) = Output Power (dBm/W/mW) When calculating output power the calculation is as follows: Impressing girls (ok this one’s a long shot… but you never know!) Being a friendly neighbour (ensuring your wireless does not interfere with the networks of people around you)
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