WRT54G antenna configuration

Discussion in 'Cisco/Linksys Wireless Routers' started by scriv, Aug 17, 2005.

  1. scriv

    scriv Network Guru Member

    Unit is a WRT54Gv2.2 with DD-WRTv22final.
    Antennas used are Linksys HGA7T.
    Idea was to play with the config in advanced wireless settings for the best result.
    I found that increasing power settings to around 50mw with standard 2dbi antennas produced very little improvement in range and building penetration.
    With both HGA7T's if you leave the tx and rx on auto you actually get reduced performance. I assume this has something to do with interference, would welcome any expert opinions? I'm just fixing the antennas to the back of the unit with the standard spacer.
    If you leave the tx on right, from the front, and the rx on auto you get a big improvement.
    If you just put only one of the antennas on with tx and rx on right it gets even better. What I don't know, because the tests were basically subjective, is how much better and again I'd welcome informed input?
    I'm asuming that if you have both tx and rx on the right there's some slowdown in data throughput but it doesn't seem significant?
    I also assume if you have tx fixed on right and rx on left it will again improve?
    The reason I'm interested is that these antennas are not cheap and if I can get away with using 1 on each unit great :)
    I'd welcome any input from people who've tried similiar exercises and I'd also welcome any contributions regarding outdoor antennas, sources of supply and antenna config. I'm assuming with an outdoor that you have tx and rx on the right? However if you also leave the standad 2dbi on the left and tx and rx on auto what happens? Presumably it'll service indoor and outdoor clients at the same time?
    Thanks for any input / scriv
  2. 4Access

    4Access Network Guru Member

    Couple pointers.

    The Right & Left antenna are determined from the perspective of looking at the RJ45 ports. (Front of the router facing away from you.)

    Wifi radios are half-duplex. (They can transmit & receive but only one at a time. Think walkie talkie or CB radio etc.) Both antennas are connected to the same radio. Therefore there is no performance advantage to configuring one antenna for RX and the other for TX since the router cannot TX & RX at the same time. (The main reason for 2 antennas is to help combat multipath issues. More info)

    You could use just a single antenna on the router and configure it for both TX & RX to save a little $$ but you will loose the multipath advantages of dual antennas. (Then again some routers only come with 1 antenna to begin with so if you're happy with the way the network performs with only a single antenna it may be a viable option.)

    For me the only time I would consider changing "auto" setting for which antenna the router uses for TX & RX would be if I needed to setup a long range link and was hooking a single high-gain directional antenna to the router. In which case I'd obviously only want the router to use that antenna.

    Regarding the use of both an outdoor/directional antenna & a stock omnidirectional antenna at the same time, you'll want to do some searching for "antenna diversity" both here and on the BroadbandReports.com forums. You'll find that this is generally not recommended although some people do configure things this way.
  3. scriv

    scriv Network Guru Member

    Hi 4Access, thanks for the information, some good reading! Couple of quick points.
    Not sure where you're based, I'm in the UK, but can you point me at any good outdoor antenna suppliers?
    The issue of cost with the HGA7T's is quite important when you're doing things commercially. The cheapest I can get them wholesale in the UK is around £29 per pair, so if I can use 1 per unit with no significant drop in performance it works for me :)
    The last thing, the thorny old question of which is the right and left antenna on a WRT54G, I've seen this topic a lot on this forum! I'm sure in general terms you are correct, however when using the dd-wrt firmware the right and left are definitely looking at the front [LED] of the unit. I actually just checked again after reading your post as you sounded very confident, however if you want to check for yourself upload dd-wrt, take 1 of the aerials off the back and switch between rx/tx on right and rx/tx on left. There's a significant drop in performance that proves the issue.
    Thanks again for your information / scriv
  4. Frog Prince

    Frog Prince LI Guru Member

    How to set up for singely antenna

    How does one configure the WRT54G (or any of the Linksys hardware) to operate on one antenna?

    My suspision is the the Linksys methodolgy for 'diversity' is to use Rice Clicks as a detection menthod. If so it is bascially useless in a real world application.
  5. HennieM

    HennieM Network Guru Member

    Scriv, agree with L and R antennas as selected in firmware - both dd-wrt micro and Thibor15c on WRT54GS/GL respectively - look from the front.

    Frog Prince, you can't choose L or R with stock firmware. Custom firmware such as those mentioned allow you to do it. Dunno about the food though....;-) but I bet many people would be interested to know exactly what the diversity algorithm entails.
  6. Frog Prince

    Frog Prince LI Guru Member

    Diversity antanna Rice Clicks

    It's been years (20+) but the technology is based on a Japanese profession by the name of Rice (no joke) in any case he discoved the multpath fading was accomplished by detectable noise burst (later called Rice Clicks) that could be used with a flip flop to change which antenna was used for the best reception.

    Worked in the lab but was a total no show in the real world. E.F. Johnson (Waseca MN) use it on their early cell phones and some 800 MHz SMR units but like I said not effective in the field.

    The only thing going for it was low cost. I suspect this is the method used in the MIMO diversitey sysem for WIFI but as I don't have diagrams or access to a lab I can's say for sure.
  7. HennieM

    HennieM Network Guru Member

    I doubt that "true MIMO" (Airgo) will use such ancient technology, as, as I understand, it entails a sort of full duplex thing, where signals from 2 or more antennas are processed simultaneously and the signals are combined in silicon - not 1 at a time.

    Diversity, however, as found in current WRT54 routers is not termed "MIMO" - once again, as I understand, it uses a sort of analog switch to switch from one antenna to the other, and may very well be (Rice) clicking away.

    Thanks for the info though.
  8. Frog Prince

    Frog Prince LI Guru Member

    Linksys MIMO

    I've looked at the circuit board (but not the circuit diagram) and from my experianc it is unlikely that the system used in the Linksys hardware is much more that a simple antenna switching system. FWIW true (read effective) diversity systems are typicallymuch more complex and as such expensive.

    I'm making a SWAG (super wild a**ed guess) and seeking input as Linksys will not provide any information. (suspect as if the hardware were that great I would expect someone in marketing would be making a point of exactly how good)
  9. HennieM

    HennieM Network Guru Member

    Somebody IS making a (marketing) point of true MIMO - just see http://www.airgonetworks.com/

    What I'm trying to tell you, is that "Linksys hardware" is too general a description. You should look at the standard 802.11 a/b/g stuff, which uses diversity switching and one radio, such as the Linksys W??54 series, seperately from true MIMO stuff such as the Linksys SRX400, which uses http://www.airgonetworks.com/mimo/how/ , i.e. effectively several radios. We see this in some of the Draft-n or Pre-n products from Linksys and other OEMs.

    A third one, often referred to in the negative by proponents of "true MIMO", apparently uses a sort of hybrid between diversity switching and true MIMO.

    If you are searching for circuit diagrams for the diversity switch based stuff, it may be better to look at http://www.broadcom.com/ which makes most of the chipsets used in Linksys a/b/g devices. Most a/b/g devices out there, not only Linksys's, is apparently based on (or even a copy of) a reference design from Broadcom.
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