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In Germany the wifi(5470-5725 MHz-Subband 2) maximum output power is 1000mW. I would like to buy a Airmax5 which has 200mW(23dbm) output power. In the Airmax5 there is 13dbi built in directional antenna according the documentation the maximum distance is 20km.I would like to replace the directional antenna with 12dbi omnidirectional antenna or a 16dbi sector antenna. Also on the coaxial cable there is minimum 2-3db. I would like to establish a connection within 2-3km.

  • The 1000mW is the maximum module output power, or with the antenna?
  • How can I calculate weather do I exceed the maximum output power?
  • Is it possible to estimate the maximum distance between two wifi antenna?
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3 Answers

up vote 2 down vote accepted

Things will be a lot easier to calculate if you convert everything to dBm. The conversion is

P(dBm) = 10 · log10( P(mW) )

So take you maximum output power of 1000mW and you will get 30dBm. I am no expert on Germany law, but my instinct tells me that max power is after accounting for the antenna gain. The reason my instincts tell me this is that for one, it is how it is most places. And secondly, it makes logical sense. The reason for having power limits is for both limiting the interference to neighbors trying to use the same open frequency as well as for health concerns (you don't want consumer pumping dangerous amounts of RF power into places they shouldn't be). With those reasons in mind, the antenna gain affects both the interference caused to neighbors as well as the amount of energy being directed from the antenna. The technical termonolgy for this is EIRP or Equivalent isotropically radiated power. Essentially it just means, if you have a perfect omni-directional antenna, what would be effective power it would have to output to be the same as the directional antenna you are using.

So now going back to what type of antenna you can have. The access point advertises 23dBm radiated power and a 13 dBi antenna (note, the i on the dB stands for isotropic, it relates back to EIRP). What this really means is that the access point is pumping out 23-13=10dBm of power before it gets to the antenna. This means that you have 30-10=20dB of headroom for antenna gain. It is not recommended to go right up to your power ceiling as there are many things that can cause you to go over. The antenna gains and output power from your access points are just estimates (although usually pretty close), but there is no reason to risk legal issues just to get a bit more power.

The maximum distance you can transmit for is a little bit of a difficult thing to estimate and would require a lot of work on my part to explain it. In order to get any estimate at all, we have to make a lot of ideal assumptions, but real world can change things a lot. I would recommend using something like this RF Link Budget Calculator to estimate the distance you can travel. Usually the SNR is a bigger factor then the received power itself. Using a directional antenna can help improve your SNR since you wont be picking up noise from all around, but if you get too directional you may actually hurt your SNR if you don't have it aimed directly at your receiver.

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You need to look at the regulations for Germany to see if the power restriction relates to actual transmitter output power or whether it's ERP (Effective Radiated Power). In the first case it doesn't matter how much antenna gain you have so long as the transmitter output power is within the legal limit, whereas in the second case you need to calculate ERP based on transmitter output power, antenna cable loss, and antenna gain, and keep that ERP within the legal limit.

As for distance, it depends on a lot of factors, but the most important is to get a "line of sight" path between the two antennae. You may also want to consider using a directional antenna at the receiving end, not only to increase received signal level but also to help exclude co-channel interference.

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You should always use directional antenna if it is possible. They give much better signal quality in real world situations. Especially on long-distance applications like here it doesn't make much sense to use omnidirectional antenna, because you can hardly have the line of sight all around in this diameter.

At first you should ask at the regulation office or somewhere what actually are the limits in your country. Then simply multiply by 2 for each 3 dB etc., you probably know this. In Czech Republic the limits are defined including the antenna, so some big parabolic antennas can't be used at all without attenuation of the signal in the device. So I would expect the same in Germany.

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