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9917 Views 11 Replies Latest reply: Mar 7, 2010 4:51 PM by John RSS

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Transmit Power vs Antenna gain

Mar 7, 2010 2:11 PM

eehinesee 484 posts since
Nov 12, 2008

This is beyond the scope of a CCNA Wireless exam, but I'm curious, anyway, as part of my EHCEP.

 

Neglecting cable loss for the sake of this discussion, total power out is a function of transmit power and antenna gain.  The FCC's rule for P2P transmissions allows the nominal max EIRP of 36dBm (in the 2.4 GHz regime, anyway) to be exceeded through the mechanism of reducing transmit power and then increasing antenna gain by three times that transmit power reduction.

 

What is gained by increasing antenna gain vs increasing transmit power?  Either way results in an increase in EIRP, other things held constant.

 

Eric Hines

  • John 2,289 posts since
    Jan 17, 2009
    Currently Being Moderated
    1. Mar 7, 2010 2:26 PM (in response to eehinesee)
    Re: Transmit Power vs Antenna gain

    Hi

     

    The RF is lots of wavelets that spread out/fan out as they travel, this gets bigger and bigger as it travels so by the time they get to the other end they could be too weak to be received well. This then can loses packets and give an intermittent fading signal.

     

    By adding gain it will focus the beam/wavelets and bring them in tighter, making them stronger if you like. On some links i designed, the quality of the link could be improved by just changing the dish to one with a better gain on it. Same dish size, same frequency, same tower, same direction, same height but now it can reach the other end and with quality.

     

    This is why you see you have more gain to play with on the point to point links because you need it more on those links.

     

    I don't see in the 802.11 wireless world why you would use more gain to focus the beam on a point to multipoint. I would imagine if you have a large warehouse that is narrow you don't need the wavelets spread out bouncing off the walls, you want them more focused so they can reach the end of the warehouse and still cover the inside well.

     

     

     

     

     

    Regards

     

     

    John

  • Currently Being Moderated
    2. Mar 7, 2010 2:48 PM (in response to John)
    Re: Transmit Power vs Antenna gain

    Hi John,

     

    We had  "warehouse" type  building, shipping docks with antenna mounted from the I-beams in the roof. I do see your point as well, it would appear to me that with yours, you can get the signal out or to the other end of the building - but for the replies (on the floor/workers mid building) to get back it would be a lot more difficult. I am thinking that I am missing something. Or is this just based on sending the signal from one end to the other with no coverage for the "floor" area?

     

    JCB

  • John 2,289 posts since
    Jan 17, 2009
    Currently Being Moderated
    4. Mar 7, 2010 2:56 PM (in response to JC.Bogard)
    Re: Transmit Power vs Antenna gain

    Hi

     

     

    You are getting mixed up with

     

    point to point (no users on each end just equipment) to link up a network. Someone with more 802.11 correct me if i am wrong.

     

    Point to multipoint (with users at the multipoint end).

     

    So for the warehouse part it would be point to multipoint and you are right, if it was too far they would see the signal but they would not have enough power to send back to the AP. That is where your survey comes into play.

     

    You can send an RF signal 40, 50, 60 kms but as a client you won't be on the other end of that broadcast. You may connect to it via the equipment that receives it.

     

     

     

     

    John

  • John 2,289 posts since
    Jan 17, 2009
    Currently Being Moderated
    6. Mar 7, 2010 3:08 PM (in response to eehinesee)
    Re: Transmit Power vs Antenna gain

    You have your power plus the gain but you lose dB's on connectors, cable lengths etc once all that is calculated you then have your EIRP left.

     

    Too much power can also be a bad thing on a point to point. You could shoot over the far end causing your link to be affected and then affecting other links behind it.

     

    Reflections on the path can also cause your link to drop out, if the phase is out 180 degrees when it reach's the other end it cancels it out. This would be caused by the wavelets bouncing off a smooth surface like water or even a flat concrete base.

     

    If the Fresnel zone hits a building or trees on the path and you can lose 50% of your power, trying to find obstructions on a 50km link is not easy unless you have the right software/maps.

  • Currently Being Moderated
    7. Mar 7, 2010 3:52 PM (in response to John)
    Re: Transmit Power vs Antenna gain

    John & Crew,

     

    I see that I missed the key point there of point to point, the further explanations were very good and help clear up a few other things in my fog bank.

     

    Thanks,

     

    JCB

  • Eric A. Nygren 253 posts since
    Aug 11, 2008
    Currently Being Moderated
    8. Mar 7, 2010 4:16 PM (in response to eehinesee)
    Re: Transmit Power vs Antenna gain

    EIRP, Effective Isotropic Radiated Power, is the combined system measurement that relates to how powerful our signal is as compared to how power the signal would be coming from a transmitter and a"perfect isotropic antennae".  In essence, having a transmit power of 20dBm, a 5 dB cable/connector lose and a 12 dBi antennae gain (EIRP = 27dBm) means that at any point along the main transmit beam you will receive the same quality of signal as you would if that signal had come from a transmitter with power 27dBm connected directly to a 0 dBi isotropic antennae.

     

    So, read the word "effective" as "effectively the same as", or "Effective Isotropic Radiated Power = effectively the same power as would be radiated from an Isotropic antennae."

     

    -Eric N

  • John 2,289 posts since
    Jan 17, 2009
    Currently Being Moderated
    9. Mar 7, 2010 4:39 PM (in response to Eric A. Nygren)
    Re: Transmit Power vs Antenna gain

    Hi

     

    Some people also call it ERP and get mixed up when gain is expressed in dBd.

     

    EIRP

    Effective Isotropic Radiated Power is actual power transmitted in the main lobe after taking in account all cable losses and antenna gain. Based on a theoretical isotropic antenna.

    So if an antenna is specified as having a gain of 15dBd that = 17.15dBi. For exposure calculations we use dBi’s.

    =

    EIRP or ERP

    Another abbreviation we come across when dealing with antennas is EIRP or ERP as some people call it.

    EIRP/ERP = the power fed to the antenna in watts multiplied by the numeric gain of the antenna. It is normally expressed in watts.

    EIRP is effective isotropic radiated power,

    ERP is effective radiated power.

    By convention EIRP is calculated using dBi’s and ERP using dBd’s.

    Regards

    John

  • John 2,289 posts since
    Jan 17, 2009
    Currently Being Moderated
    10. Mar 7, 2010 4:51 PM (in response to Eric A. Nygren)
    Re: Transmit Power vs Antenna gain

    Hi

     

    Here is what i posted a while back.

    =

     

    EIRP

    Effective Isotropic Radiated Power is actual power transmitted in the main lobe after taking in account all cable losses and antenna gain. Based on a theoretical isotropic antenna.

    So if an antenna is specified as having a gain of 15dBd that = 17.15dBi. For exposure calculations we use dBi’s.

    =

    EIRP or ERP

    Another abbreviation we come across when dealing with antennas is EIRP or ERP.

    EIRP/ERP = the power fed to the antenna in watts multiplied by the numeric gain of the antenna. It is normally expressed in watts.

    EIRP is effective isotropic radiated power,

    ERP is effective radiated power.

    By convention EIRP is calculated using dBi’s and ERP using dBd’s.

    =

    Antenna performance

    Antenna performance is measured in

    dBi (the antennas gain/loss over a theoretical isotropic antenna)

    dBd (the antennas gain/loss over a dipole antenna)

      =

    dBi = dBd + 2.15

    dBd = dBi – 2.15

    =

    Gain or Loss

    When referring to:

    Gain or Loss it’s “dB”.

    Power it’s “dBm” or “dBW”.

    Voltage it’s “dBµV” or “dBV”.

    Antennas it’s “dbi” or “dBd”

    =

    Conversion

    To convert from dBm to dBW subtract 30

    e.g. 40dBm = 10dBW.

    =

    Antenna Gain

    When antenna gain is referred to in dBs, it can be dBi or dBd. Whether “i” or “d” is used depends on what the antenna is referenced to.

    If its “i” then the ratio is against a theoretical Isotropic radiator. A point source that radiates equally in all directions, it cannot be manufactured.

    If it’s “d” then the ratio is against a half wave dipole. The difference between the two is that a dipole has a gain over the theoretical Isotropic

    radiator of 2.15 dBs. I.e. a numeric gain of 1.64.

    =

    Types of antenna

    There are 3 types of antennas used with mobile wireless, omnidirectional, dish and panel antennas.

    Omnidirectional radiate equally in all directions

    Dishes are very directional

    Panels are not as directional as Dishes.

    =

    =

    For the exam Eric N mentioned

    Cisco for this level exam expects the following antennea terms: Onmi-directional, Directional (Patch, Yagi, Parabolic Dish).

    =

    Regards

    John

  • Eric A. Nygren 253 posts since
    Aug 11, 2008
    Currently Being Moderated
    11. Mar 7, 2010 4:47 PM (in response to John)
    Re: Transmit Power vs Antenna gain

    John, good summery post.

    Just note that Cisco for this level exam expects the following antennea terms: Onmi-directional, Directional (Patch, Yagi, Parabolic Dish).

     

    -Eric N

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