WiFi Transmit Power Calculations Made Simples

One of the most important things you need to know when it comes to wireless, is how to calculate transmit power (also known as Equivalent Isotropically Radiated Power or EIRP).  This can be important for a number of reasons including.

- Keeping Ofcom happy
- Being a friendly neighbour (ensuring your wireless does not interfere with the networks of people around you)
- Passing wireless exams
- Impressing girls (ok this one’s a long shot… but you never know!)

WiFi Meerkat

When calculating output power the calculation is as follows:

Radio Transmit Power (dBm) – Loss from Cables & Connectors (dB) + Antenna Gain (dBi) = Output Power (dBm/W/mW)

Lets cover each part of this calculation in turn:

Radio Transmit Power

This is the trickiest of the 3 parts mainly due to the fact that it can be expressed in Watts, Milliwatts  or dBm. Ideally you want to convert this to a dBm value if not already expressed this way. At this point it is worth noting the following:

1mW = 0.001W
1W = 1000mW

Effectively 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.

Converting 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:

0dBm = 1mW

Rule 1: The Rule of 3

If you raise the dBm value by “3″ you double the mW value:

0dBm = 1mW
3dBm = 2mW
6dBm = 4mW
9dBm = 8mW
12dBm = 16mW
15dBm = 32mW
18dBm = 64mW
21dBm = 128mW
24dBm = 256mW
27dBm = 512mW
30dBm = 1024mW (5GHz Band B legal limit)
33dBm = 2048mW
36dBm = 4096mW (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 “10″ you multiply the mW value by 10:

0dBm = 1mW
10dBm = 10mW
20dBm = 100mW
30dBm = 1000mW (5GHz Band B legal limit)
40dBm = 10000mW (Over the 5GHz Band C legal limit)

Yup you guessed it, if you lower the dBm value by 10 you divide the mW value by 10.

The above calculations aren’t exact, but they are typically accurate enough to get by on.

Loss from Cables & Connectors

You will need to take into account the loss caused by cables and connectors as each component will add an element of loss. 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. Also keep any antenna cable lengths as short as possible. Normally loss values will be expressed as dB loss per 100ft and you can typically find this value on the spec sheet. Connectors will also add a small amount of loss
Antenna Gain

Antenna Gain is measured in dBi. This one is easy, simply look up the dBi value of the antenna and apply it to the calculation. 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

The calculation above will give you a value in dB. This will typically be converted to a Watts (W) or Milliwatts (mW) value as per the tables above when expressing the output power.

Link Budget

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 includes everything discussed so far (at both ends) and also in addition the loss caused by the wireless traveling through the air. This requires more detail than this post goes into, but the calculations above form a vital part of calculating this.

Practical Application

Here is an example of a scenario in which you may need to use your new found skills:

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. This means you need to stay within the Band B limit of 1W. 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. 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. Again by applying the rule we can see that 10dBm is 10mW. This means that you need to limit the output power of your radio to 10mW in order to stay legal.

As the meerkats on TV would say - "simples!"


Article categories: Indoor Wireless, Outdoor Wireless


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