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OFCOM states that the maximum EIRP within the UK for 2.4 GHz is 100mW. Simply put, what is the point in being able to buy 2.4 GHz high gain antennas which can say do 24dBi, which already puts the output power at 256mW?

Is this a case of OFCOM turning a blind eye to things, considering the USA for example has limits which far exceed the UK?

Source sites:

WiFi Transmit Power Calculations
Max EIRP (1)
Max EIRP (2)
24 dBi antenna

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Not everyone is subject to that EIRP limit. For example, part of the 13 centimeter amateur band overlaps the 2.4 GHz ISM band, and a licensed amateur has a very much higher power limit. Commercial licenses also exist. Regional regulations vary but it's a safe bet there are licensed services available in most jurisdictions with a higher limit.

A higher gain antenna is also more directional, and thus has better receive performance by virtue of receiving less noise from other directions. Even if power is reduced to keep the EIRP at 100 mW, a more directional antenna will have better link quality.

And yes, people install these illegally. I imagine enforcement is very difficult unless there's a specific complaint.

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  • $\begingroup$ Very well put! I'd like to add: In applications like automotive radar, you often have separate transmit and receive antennas. Manufacturers use a low-gain TX antenna – in the end, for radar range, the energy emitted in a single direction counts, and EIRP defines exactly that, so you can't "cheat" to get more power to illuminate the path before the car. Thus, using a lower-gain antenna saves space and cost; in a car, you're not really after the few mW that you could have saved by using a more directive transmitter. The receiver on the other hand is a highly directive antenna (array) in order… $\endgroup$
    – Marcus Müller
    May 3 '17 at 9:26
  • $\begingroup$ … to be able to tell other cars from static targets and especially the humongous radar cross section that things like nearly perfectly corner-reflector-shaped guard railings have. $\endgroup$
    – Marcus Müller
    May 3 '17 at 9:27
  • $\begingroup$ Are you able to offer any sources for the amateur band overlapping the 2.4Ghz and the commercial licenses, I can find no trace of such a thing? $\endgroup$
    – R4D4
    May 3 '17 at 9:27
  • $\begingroup$ @R4D4 where do you live? $\endgroup$
    – Marcus Müller
    May 3 '17 at 9:42
  • $\begingroup$ @R4D4 en.wikipedia.org/wiki/13-centimeter_band $\endgroup$
    – Marcus Müller
    May 3 '17 at 9:44
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A higher gain antenna means you can use lower power for the same EIRP. Or you can use a longer cable between the transmitter and the antenna.

Moreover a higher gain antenna helps you to receive weaker signals.

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Power output limits stated in terms of EIRP can be quite helpful in the case of very low frequencies, where it is difficult to build an efficient antenna and at very high frequencies where feedline and dielectric losses become significant. In these cases, a higher transmitter power than may otherwise be allowed can be deployed to make up for these systemic losses.

A 24 dBi antenna provides a gain of >251 times that of an isotropic antenna. This means that if you put in 100 mW into this antenna, your EIRP will be >25 watts - well over the 100 mW EIRP legal level. To use this antenna legally, the power going into the antenna can be no more than ~0.4 mW. This is probably rarely followed in practice. Do take care as 25 watts at this frequency can be harmful to body tissues at close range.

Assuming the directivity of the antenna favors the application, a high gain antenna offers the following advantages:

  • Increases the received signal by the same gain
  • Reduces receive interference from other (low gain) directions
  • Makes up for feedline loss on transmit and receive
  • Focuses the transmit power in the desired direction reducing interference to other receivers
  • May reduce the odds of reception of transmitted signals for nefarious purposes

The potential disadvantages are:

  • Mechanically more complex
  • A larger antenna profile
  • May not be applicable for a wide area system if gain direction is not favorable to the required coverage area
  • Potentially higher cost

If you require coverage of a larger area and you wish to use high gain antennas you could consider multiple transceivers and multiple antennas aimed in different directions.

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  • $\begingroup$ > if you put in 100 mW ... your EIRP will be >25 watts... take care as 25 watts ... can be harmful to body tissues at close range. Yes and No :) There is no 25 watts. There's only the appearance of 25 watts into an isotropic antenna, and only when you're standing right in the beam, far enough away that you can't tell if it's an isotropic antenna or a dish (perhaps 100 metres away). 100 mW won't hurt you, no matter what the antenna gain. $\endgroup$
    – tomnexus
    Oct 14 '19 at 14:39

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