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It Is Possible to Use one antenna to 2 wireless routers by a RF splitter adapter ?

Discussion in 'DD-WRT Firmware' started by qubo, Oct 26, 2008.

  1. qubo

    qubo Addicted to LI Member

    It Is Possible to Use one antenna to 2 wireless routers by a RF splitter adapter ?
    anyone has made this ? I have seen some splitters that are T one N-type male 2 n-type female
    so with this and 2 little pigtails ntype male to rp-tnc male can be connected to each router sharing one antenna.

    why am I interesting in this ??? well. I plan to have 2 routers in client mode. to take signal AP for each. then conect each router linksys WRT54GL to a dual WAN router to get the 2 connections in same network
    I plan to select different channel for each router so does not conflict each other
    so my question is if this is is possible, pros cons. and any experience if someone has made this ?
    and second question

    It Is Possible to Use one antenna to 2 wireless routers by a RF splitter adapter ?
    anyone has made this ?
    what Antenna is better in your experience with linksys router OMNI or Directional
    if would be needed for multi SSID ?
  2. HennieM

    HennieM Network Guru Member

    The splitter works. It has very little to do with what devices the splitter is connected to. However, you lose, in my experience, a lot of signal through a splitter (like 8dB if you're lucky - that's nearly a 3-fold power loss - every 3dB is a doubling of power). On the Tx side, you can make up for that with increased (triple or more) Tx power; on the Rx side however, the only remedy is to have a rather strong antenna.

    Omni vs. directional: It's not about "which is best"; it's about "where do you want your signal". If you have an AP, and wireless clients all around that AP, you cannot use a directional antenna, as a directional antenna "directs" the signal to one side. (That "one side" could be a very small pizza slice of say 5deg., or a rather big slice like 180deg.) An omni spews signal all around the antenna (omni means "all directions in the horizontal").

    If you want to connect one AP to another AP a mile away, use directional antennas to direct the signals of the 2 APs towards each other.

    The SSID - one or multiple - has nothing to do with the antenna. There's only one radio in most wireless devices, so all traffic happens on one frequency only, no matter how many SSIDs you define.

    If you have a device with 2 (or more) radios, it's a diffent ball game.
  3. qubo

    qubo Addicted to LI Member

    very nice answer. but where did you get the info of 8 db of loss ? is there a table?
    I plan to put a omni directional antenna 19 dbi with a pigtail 3 meters RG174 an was planning to put 2 routers WRT54GL with diferent channels each. the routers would work as client mode. it pourpouse is to get signal ap from around less than 500 mts
    then the 1 router ddwrt would have 2 ssid connections at same time and other is running tomato with 1 conection. then both routers would be connected to a dual wan router to send 3 connections in one network.
    i am very interesting to know why is so much loss in the splitter. as i told you is the are table
    if I only use the pigtail 2 meters without splitter is there a loss too ?
  4. HennieM

    HennieM Network Guru Member

    Got the splitter's loss from the splitter's spec sheet. Can't remember details no more as I have not got that splitter no more, but I remember it was around 8dB loss. I guess you get better splitters, but check that before you buy.

    You lose signal through any cable. You also lose signal through connectors. The question is how much. There are tables indicating how much is lost for different cable types. Some of the lowest I saw is 0.2dB/m for LMR400, around 0.8dB/m for RG-58, and 2dB/m for RG-174, all at 2440Mhz (channel 6 is around 2.44 Ghz). Calc yourself at http://www.ocarc.ca/coax.htm

    That means, with RG-174, if you have 3m of cable, you lose a whopping 6db, that means only 1/4 of your AP's power transmitted is reaching the antenna. Alternatively, only a 1/4 of the signal coming in through your antenna, is reaching your AP.
  5. qubo

    qubo Addicted to LI Member

    thanks so much.
    one question. I have seen a omni antenna +22dbi. I was wondering if the router with ddwrt have enought power to use full 22.dbi of antenna. or what power level should have ?
  6. HennieM

    HennieM Network Guru Member

    The power of the router has very little to do with the antenna strength - you don't need more power to drive a stronger antenna.

    In fact, the stronger your antenna, the less power your router needs to put out. To such an extend, that if you live in an ETSI country (most of Europe and Africa), you need to limit your Effective Isotropic Radiated Power (EIRP) [google for EIRP and for ERP] may not exceed 20dBi (100mW).

    EIRP = AP_Tx_power_in_dB + Antenna_gain - cable_losses - other_losses

    AP_Tx_power_in_dB = 10 log(mW Tx power)

    In FCC land (USA), there's a more lenient way of looking at the max ERP, depending on whether you use a directional antenna or not.

    So, with a 22dBi antenna in ETSI land, you'll have to lose 2dB in your cable etc., and then set your AP's Tx power to 1mW [10 log(1 mW) = 0 dB] to be compliant.

    If you lose say 6dB in your cables, connectors, etc., you can set your AP's Tx Power to:

    AP_Tx_power_in_dB = EIRPmax - Antenna_gain + (cable_losses + other_losses)
    AP_Tx_power_in_dB = 20dBi - 22dBi + 6dBi = 4dBi

    AP_Tx_power_in_mW = 10^(dB/10) = 10^(4/10) = 2.5mW

    The thing about an antenna (which nothing else can beat) is that you have the antenna gain on both transmission and receiving, so incoming signals gets boosted by the antenna gain before it gets to the AP.
  7. qubo

    qubo Addicted to LI Member

    thanks so much for your time helping me.
    very apprecitated

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