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  • So I'm looking at an AP Spec/Data sheet and it states that the maximum conducted transmit power of the AP is 30 dBm with 4 antennas. Does that mean the max transmitter output is 30 dBm and it gets divided evenly between all antennas in use? I understand max EIRP and all that just trying to figure out what my EIRP will be for a couple different antenna configurations.

    1 Antenna - 30 dBm

    2 Antenna's - 15 dBm each

    3 Antenna's - 10 dBm each

    4 Antenna's - 7.5 dBm each

    Thanks

    David King

  • By Howard - edited: November 26

    Probably not.   Was this a US legal AP ?

    The FCC usually limits MIMO radios to less than the maximum(s), for each integer increase in the number of streams-  actually the number of antennas.

    Sorry, I can't give you more specifics.   You may be able to find FCC reports that give you the specifics, or from the manufacturer.

    The manufacturer's Compliance group, or the lab that ran their final compliance test, for sure, has that information.   But you may not be able to dig it up.  I can guarantee that someone has that data.

    If it's a Cisco AP, you'll probably be able to find it.   Look in the technical, not marketing, specs for the AP.

    Good luck. 

  • Hello, The video at the end of this link may be of some assistance - at around 19 minutes 50 seconds (or there about)

    https://www.youtube.com/watch?v=KJfSy8pTns0

    Cheers

  • Thanks for the link Tony.   

    The presentation by Wes gives some concrete numbers in relation to this question.    In fact, that segment of his talk had more concrete numbers than a majority of it did.  

    Glad to see that Wes also specified that he was trying to present technical, as opposed to marketing, answers.   I should have mentioned in my previous response, that the situation is much more complex than what is initially apparent.    Even dedicated compliance engineers disagreed with each other at first.  That is, until the FCC made their pronouncements - and compliance labs had to fall in line. 

    Part of the problem, believe it or not, is the difficulty in actually measuring, or rather defining, the total amount of output power.   The FCC, which is the arbiter of radio licensing in the US,  finalized with what they determined to be a consistent computation.  I'm not saying it's bad, it's just not the most obvious - unless you follow their rules, and then it's obvious (I'm sure there is a philosophical label for this type of definition).

    Obviously different standard's bodies could develop different algorithms, which explains some of the different output power levels available across countries.

    Just looking at Wes's, simplified 2-D only, Beam Forming diagram you can see that different points in the horizontal plane are receiving different power levels.   How does one determine at what point to make a measurement ?   

    Just one more point, is that different manufacturers have different design "philosophies", and that their performance numbers may differ even though they may use the same internal chip sets - which could lead into several more discussions in their own right.

    Thanks again Tony.

  • Thanks for the responses. I was hoping this would be a bit simpler. I should clarify that this design is an outdoor design and the AP in question is the Cisco 1570 series AP and I would be using a single antenna/stream due to the limitations on the client end. The 802.11n client  which is an EW2400MP high power PCI Express Mini Card, transmits at 24 dBm for HT20, MCS0. It also has a 3 dBi gain omni-directional antenna which should give it an EIRP of approximately 27 dBm (minus connector/cable loss). So I want to match this EIRP with my AP/antenna configuration.

    Cisco's data sheet for the 1570 states Maximum Conducted Transmit Power:

    2.4 GHz

    ●  802.11, 802.11b (DSSS, CCK)◦   30 dBm with 4 antennas

    ●  802.11g (non HT20)◦   30 dBm with 4 antennas

    ●  802.11n (HT20)◦   30 dBm with 4 antennas

    FCC 2.4 GHz Band Rules (Point-To-Multipoint) states Max EIRP = 36 dBm at the Intentional Radiator which the FCC defines as the transmitter power of the wireless equipment, such as a wireless access point, router or bridge.

    TX power of 30 dBm can have max antenna gain of 6 dBi for a EIRP of 36 dBm

    TX power of 18 dBm can have max antenna gain of 18 dBi for EIRP of 36 dBm

    TX power of 12 dBm can have max antenna gain of 24 dBi for EIRP of 36 dBm

    So following my own logic I can use a Cisco 1570 AP with TX power set to 30  dBm with one 6 dBi omni-directional antenna or

    1570 set to 23 dBm and a directional antenna with 13 dBi gain.

    So why do I ask the question in the first place? Simply trying to model my setup in Airmagnet Planner to figure out how much equipment I need to meet the coverage area requirements of an outdoor design/build bid. In the software I can set the TX power and the antenna gain.

    It makes sense to me that the transmitter max power would be 30 dBm and if I'm only using 1 antenna port I would have all of the 30 dBm available. If I were using 2 antennas I would only have 15 dBm available at each of the two ports and only 10 dBm if I were using 3 antennas. 

    Best Regards and thanks for the discussion.

    David

  • By Howard - edited: November 28

    I can see from pictures of your client card, that it has 2 U.FL antenna connectors and what looks like 2 native antennas built in.   I assume your using the external connectors, because I can assure you that the built in antennas, are not going to present an especially Omni-directional RF field - nor do they have 3dBi gain.   Hint, be sure the firmware settings for the card, match the physical antenna setup.   

    For outdoor use, you may want to re-evaluate your plan to use single antennas (diversity can work wonders).  Generally speaking, you will find that Receive Sensitivity is more important than Output Power.  

    You didn't give much description for the client's antenna(s), or diversity settings.   I have usually found that it's best to be a little, if not more, suspicious of antenna specs.  I have found it EXTREMELY helpful to dismantle one of the potential antennas, using a Dremel tool with a thin cut-off wheel.   You should be aiming to leave a working, but mis-tuned antenna.   Carefully inspect the active element.  (I have to be careful here lest I go off into antenna fanaticism, so I'll leave the rest for later.)   I will warn you that internal antenna cable routing can be critical to directivity  

    I'm sure you've read about the importance of symmetric, that is "balanced", power levels between AP and Client.   I doubt you will find that 30 dBm output at your AP's will ever be necessary, or for that matter 24 dBm on the clients.    More AP's, with lower output settings will probably work much better.   Twenty dBm, or less, would work fine on your clients for indoor use.  (don't forget that the client card also has recommended minimum distances between the operator and the radio's antenna !)

    Be VERY careful if you are planning on using RRM.   Learn as much about tuning it as you can, and not just from Cisco's perspective.   It can be more of a problem than it's worth.   Search the Forum.   If CWNP hasn't already deleted too many posts (lately), there are plenty of comments, and Links, to help.

    Good luck.

  • Thanks Howard,

    The client end is fixed and already deployed in the field. It's all outdoor on locomotives. The antenna is a roof mounted RFMT-DB-24/58 from STI-CO.

    The coverage areas are all in train yards and I have limited power and network areas to place an AP. I will probably learn a lesson or two on this project. Hope it doesn't prove to be too expensive. 

    Best Regards and thanks to everyone that responded.

    Dave

  • Looking at the antenna specs, it looks very much like a Laird (Antennex) model.   I have only heard positive comments about them.   

    I'm assuming  your AP-antenna mounts will be well above cab level (?).

    Just like any other installation, I hope you are able to run site surveys to look for interference.   Never worked IN a train yard, so I wouldn't know how much to expect.

    My only experience with Train based Wi-Fi was in one location's OTA test range, where the daily commuter run would kill our outdoor tests for about 5 minutes.  We always knew the train was coming, a minute or so,  before we heard their horn.

    Good luck, it sounds like it could be  fun.

    Howard

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