Does a second antenna help?



  • Just setting up a new alix pfsense machine, and was wondering if having a second antenna hooked up to the mini-pci wireless card helps give a stronger signal or not?
    if it depends on the chipset, i have an atheros abg card, ar5004x.

    thanks



  • bigger one?
    yes!



  • @josey:

    bigger one?
    yes!

    so a second antenna of the same db doesn't help?  than what is the point of having an aux connector?



  • To try an intelligible answer to the question,
    It's closer to a question of less errors than stronger signal. Most mini-pci cards seem to have dual-antenna affinity set in the firmware and manual changes keep reverting to firmware settings. I have observed noticeably more errors running a single antenna. One would think two antennas would be more reliable and slightly faster, due to less re-transmissions, but I have not done any tests.

    PS- Having mixed antenna sizes on your main and aux connectors is a bad idea.

    PPS- This should really be in the Wireless forum…



  • If you're really interrested in this stuff, visit a high frequency class at your next university.
    This is black magic you're dealing with  ;D

    Basically: Two antennas = More "surface area" (even if you have to speak about theoretical surface area).
    If you double the surface you can receive double the energy (power in mW)
    –> you get 3 dBm more energy to the card
    --> 3 dBm less RX sensitivty required on the card itself.
    Although in a normal homenetwork you hardly will notice 3dBi.

    This only applies to the receiving end of a link.
    Since the transmitting power will be divided to the two antennas you can say you have -3dBm per antenna.
    But double the area = +3dBi
    --> 0 dBi gain while sending.
    However if you position the antennas unfortunately, you might end with a less than double the area.
    which would lead to:
    --> Better link from device to AP.
    --> Worse link from AP to device.

    If you position the antennas to near to each other, the near-fields of the antennas might interfer with each other (even if you're sending the same) (try googleing for "near field" and "far field").

    As josey wrote: A bigger antenna will probably bring more in terms of RX sensitivity and less problems with the TX-side.

    @dotdash:

    PS- Having mixed antenna sizes on your main and aux connectors is a bad idea.

    I disagree.
    Depending on the card you can set one connector to RX-only and the other to TX-only.
    Like this you could have one large RX antenna (more surface-area = more RX-dBi's) and "normal" TX antenna (whatever is legal where you are).

    Also i have a few setups where i need wireless around the pfSense and a single wireless access a bit further away.
    I use a single antenna for local access and a directional antenna for the far point.
    granted i loose 3dBm in the local area, but with the appropriate directional antenna i can get a lot better signal in the far place.
    But yeah. If you dont know exactly what you're doing, dont mix different antennas since you might end up with a worse result (you even might with the same antennas :D). ;)

    The other thing with multiple antennas is:
    2.5 Ghz has a wavelength of about ~12 cm.
    If the antenna is in a bad spot where echos cancel the signal …...
    A second antenna might be a few cm next to it and have less distortions.
    802.11n actually extends the idea of that and puts some more hardware behind it. (if you're interrested in that google for "MIMO")



  • as GruensFroeschli explained you to details and long way, i put it in simple 3 word, bigger antenna? yes.

    Take only one antenna with higher gain, 9dbi would be fine for home use, and good quality, not to long pigtail…





  • @josey:

    Take only one antenna with higher gain, 9dbi would be fine for home use, and good quality, not to long pigtail…

    Sorry, this is another: IT DEPENDS.
    Neither can you safely say this nor can you assume the opposite.

    bbzidane didn't even tell us what kind of antenna he's using: omni or directional (the latter with >dozent possible variants).
    Assuming you use an omni, adding gain usually means gain in the horizontal plane but also loss in the vertical. If you have to cover two floors of a building it might be the wrong choice. Therefore: IT DEPENDS!



  • @bbzidane:

    so a second antenna of the same db doesn't help?  than what is the point of having an aux connector?

    With a second antenna you can use diversity if you set your wireless accordingly.
    Usually you gain more stability in reception with moving transmitters.


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