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How much transmit power can I use?

Discussion in 'HyperWRT Firmware' started by mw1001, Apr 8, 2006.

  1. mw1001

    mw1001 Network Guru Member


    I have 2 WRT54GS v2.0 running with Hyperwrt 2.1b1 + Thibor14, both configured as AP+WDS.
    My connection lost between the 2 WDS bridged WRT only stops with Transmit Power at 67% (53 mW) or more.

    So, I must ask: how much power is safe to use in these routers, without any danger?

    Thanks in advance,
  2. vincentfox

    vincentfox Network Guru Member

    It's usually better to pursue using directional antenna, especially when you are using 2 WRT units for a bridge. Even a simple reflector you can make yourself like shown at http://www.freeantennas.com/ will do great things.
  3. mw1001

    mw1001 Network Guru Member

    Hi vincenfox.

    First of all, thanks for the quick answerer! Man, that was fast :)

    I also agree that an upgrade of antenna is the best option, before trying any increase of transmit power. But I have 2 problems:
    My setup is indoor and my routers act as AP too, so I must use an omni directional antenna. With is not the biggest problem. The biggest problem is: I live in Brazil, and good antennas are very hard to find.
    It´s an absurd: the only store I founded selling the 7db Linksys´s antenna, is charging for it 50% more than that my routers has cost.
    And, to complete, I´m not very good with DIY… :(

    I will keep searching for a good antenna solution, but until I found, I want to know if I´m making something danger…

    Thanks again,
  4. Toxic

    Toxic Administrator Staff Member

    The ETSI in Europe has set standard 54g power to max out at 100mW on most european wireless devices. going higher than this in enclosed spaces could affect your health I guess, giving you headaches and feeling nauseated, if the antenna is away from you ie top of roof etc then higher output would be ok to use, however maybe illegal in your country.

    freeantennas do so good templates that you could use for deflecting the signal to where you want it to go.
  5. mw1001

    mw1001 Network Guru Member

    So, using below 100% (84mW with this firmware) of transmit power isn’t dangerous to health or to the equipment?
    I’m using 53mW for now. May I conclude that is a safe amount of power for indoor use?

    I will take a good look at freeantennas.

    Best regards,
  6. Toxic

    Toxic Administrator Staff Member

    yeah well safe as can be... if the ETSI says 100mW is safe then i guess 53mW is fine
  7. triggernum5

    triggernum5 LI Guru Member

    Heres what I know from a friend who works in the wifi industry..

    Linksys routers often suffer from heat issues when boosting to absurd amounts (84mW isn't really absurd in most cases, 251mW is..) I actually have a fan mounted on mine..
    Physical harm isn't the reason for limits at all.. I believe the fcc imposes a 28mW limit (could be off) on devices of this class in the USA, (other countries will differ).. This is to prevent flooding the air with conflicting signals, but obviously you won't get caught for the miniscule wattages we can use, I know ppl with 3KW CB boosters in the boonies)..
    Heres the kicker though, my friend in the industry owns some pretty hardcore Signal Analysis tools.. When we had Sveasoft alchemy on the router we played with it pretty extensively, (I'm new to hyperwrt though so perhaps this has been fixed, but I think its more intrinsic to the hardware than the firmware, and perhaps this is why hyperwrt imposes the limit which would be very cool of them).. In anycase, our results showed that boosting the power output on a WRT54G v2.2.. Did increase the desired signal strength slightly/moderately, but the effect was much more weighted towards increasing the signal bandwidth.. At 100mW centred on channel 6, we were flooding channels 4.5-7.5 roughlywith a considerable signal.. At 251mW you might as well name your router 'local_a-hole' because in an apartment you'd overwhelm quite a bit of area (especially if you broadcast B-signal which I don't advise unless you need it since its range is much higher..
    Antennas really are the way to go.. A simple parabolic can work wonders.. Or you could make a homemade yagi and shotgun the signal whereever you want it, or wherever you want it from..
  8. vincentfox

    vincentfox Network Guru Member

    A reflector is something you can easily make yourself. It only takes a few minutes with some cardboard and aluminum foil. See the website http://www.freeantennas.com/

    I really don't think these are difficult to make. If you are that clumsy with scissors and tape perhaps you can get someone to help.

    Also it is a mistake to assume that you cannot get ANY signal in other directions. A reflector is only able to redirect some energy, not all of it. As an extreme example I can sit 20 feet behind and below a large 24 dBi grid-dish, and still get good signal.

    Try it yourself, you will see.
  9. furyoo

    furyoo Network Guru Member

    I'm sorry to bring this up, but I'm a bit confused.


    On the above page, it states that "Equipment complying with thiw standard can be operated at 100 mW eirp in the band 2 400 MHz to 2 454 MHz, or at 10 mW in the band 2 400 to 2 483,5 MHz throughout the European Community".

    Now eirp as I understand works by the formula:

    EIRP(dBm) = (Power of Transmitter (dBm)) – (Losses in transmission line (dB)) + (Antenna Gain(dBi))

    Basically I used the parabolic template from freeantennas, which they state on the website as having 10 to 12 db gain (http://www.freeantennas.com/projects/template/index.html), and my WRT54G is set to 42mW.

    If my calculation is correct, and assuming no loss in transmission and 12 db gain, would that not make my EIRP 681.5 mW???

    Is there a better way to calculate? It is of concern to me because I tend to spend quite a bit of time right in front of my router.
  10. BeHappy

    BeHappy Network Guru Member

    Power of Transmitter (dBm) calculates by this formula
    100mW = 10 x log(100) = 10x2 = 20dB, so at 100 mW as 20 dBm
    So theoretically you can gain EIRP 681.5 mW, but you must only transmit max. 100 mW in private use.

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