Problems on wrt54gs and extern antenne?

Discussion in 'HyperWRT Firmware' started by micheld, Apr 3, 2005.

  1. micheld

    micheld Network Guru Member

    Have anyone also problems with a linksys wrt54g(s) connect to onmi antenne or extern antenne?

    I have a problem with wrt54gs 2.0, on own little antennes no problem but on onmi 7dbi antenne is look like there is no power anymore? I also try 15 dbi antenne also same problems.
    But i test this antenne's on senao NL-3054 and the coax and antenne works very good!
    I check the choice in the software left and right side.

    I want 5 new linksys wrt but i don't whats the best verion for extern antenne.

    Please post here your wrt type and version if you use on a extern antenne! Thanks

  2. 3dogs

    3dogs Network Guru Member

    I do

    I use a 12db omni with Firmware Version: v3.01.3 - HyperWRT 2.0b4 on one unit and the same firmware on a second unit with the stock linksys antenna.
  3. jagboy

    jagboy Network Guru Member

    or you could use a wrt54g v2 with the linksys tnc antennas with dd-wrt22 pre3.2.

    or linksysv2.2 looks stable but i am still testing it. i am testing under heavy bandwidth loads and etc. if you want i can post results
  4. Smoky

    Smoky Network Guru Member

    ive got a 12 dBi omni antenna and a 24 Dbi directional grid antenna and i run hyperwrt and i have no problems at all also what kinda coax are you using and how many feet of it? also those senao cb3's are way better then the wrt54gs and one more thing 12 to 15 dBi antennas are built for medium to long range also those senao's are pretty high power 125.893mW whats the power (transmit power) of your wrt54gs? also try and not to go over 200mW on the wrt54g(s) cause over 200mW the signal gets real dirty and will cause alot of noise
  5. micheld

    micheld Network Guru Member

    Thanks for anwsers!
    I try a new version of dd-wrt and have no problems anymore! Thanks
    I don't now why, i thinks the hardware have a problem but new fimrware there are no problems anymore! Great!!!

  6. Westwind

    Westwind Network Guru Member

    WRT54GS with external antenna

    I had a problem with the wrt54gs v2.0 and a hawking 9dB external omni antenna with 30ft cable. I could not get any signal from the external antenna. what did you do by updating the firmware. I tried hyperWRT and dd-wrt by upping the power out to 200 mw and the signal was still weak.

    any help would be greatly appreciated.
  7. davidsonf

    davidsonf Network Guru Member

    Re: WRT54GS with external antenna

    There is exactly one trick with an external antenna: short feedlines. Unfortunately for your setup, 30 feet is not a short feedline. The results you are getting are almost guaranteed, unless you want to spend some real money on feedline (extremely low loss, and extremely high cost).

    Generally (but not always) the smaller the cable, the higher the loss. Definitely, the longer the cable, the higher the loss. Even a short length of the coax cable used for TV feedline will have a huge loss at the 2400MHz frequencies used by 802.11b/g radios.

    If you must use a length of feedline of more than a foot or two, you'll need to look at something like semi-rigid coax, called heliax, in sizes larger than 1/2" in diameter. But even with that you don't want to do 30 feet of it. Best advice is to keep it less than 15-20 with heliax, and less than 15 feet with regular low loss 1/2" diameter coax. With any of the really thin low loss cables, don't go more than 1-2 feet.

    Here are some raw numbers. I'm using 3 dB, the half power point,as a reference. Note that a 6 dB loss reduces the distance a radio will work over to about half.

    RG-174, 1/10th inch diameter coax has a loss of 3 dB (half of your power) in just 5 feet. RG-58CU 1/4" diameter cable has a loss of 3 dB in 8 feet. RG-142 1/5" diameter cable loses 3 dB in 12 feet. Even low loss premium polyfoam RG-58 will lose 3 dB in 15 feet.

    If we switch to looking at larger sizes of coax, RG-213 low loss 1/2" cable, will lose half the power in 25 feet. Any of LMR400, LM400, or CNT400 (all actually the same) will lose half the power in 40 feet. (You can see where 10-15 feet of this is not bad!)

    Now, if you really have to run 30 feet, think about heliax. LDF4/50A loses 3 dB in 72 feet. LDF5/50A loses 3dB in 116 feet! This is good stuff! Your 30 foot run would have a loss of about 1.3 dB with LDF4/50A and about 0.8 dB with LDF5/50A. (The cable will cost more than the WRT54G...)
  8. davidsonf

    davidsonf Network Guru Member

    The article I'm responding to is old, but since there was a new article posted in this thread, it seems reasonable to go back and clear up other issues raised here, even if for no other reason than to archive this to avoid leaving incorrect impressions for people researching these issues in the future.

    The question about coax is appropriate. The statement about a 12 dBi omni and a 24 dBi grid antenna is ambiguous, but if I'm reading it right, that is asking for trouble. If those two antennas are on the the same WRT54G (as opposed to if he meant one antenna on one unit and the other antenna on a distant unit), there is going to be a problem.

    The WRT54G (nor any other similar 802.11b/g radio that I know of with two antennas) does NOT connect to both antennas as once, but instead switches between them with a software driven algorithm. If there is no signal, they are switched at regular intervals looking for a signal. When a signal is being received, the antennas are switched to determine which has the best received signal. All is well with that, because with two disparate antennas the radio will select the best of the two for received signals.

    The problem is that the next transmit will always take place on the last antenna selected for receive. So lets say that the high gain antenna is pointed off in a due north direction to connect to one particular client that is 2 miles distant, and the omni antenna is meant to be used by all near by clients. If a nearby client to the south is received... and the next transmission is addressed to the distant client, it will be sent on the omni antenna, will not be received at a location 2 miles distant, and the system will sit there waiting for a reply until it times out. This will be seen as very annoying data dropouts, where no traffic is passed for a second or two, and some very very slow data transfers anytime there is mixed traffic between near clients and the client connecting via the high gain antenna. (If the unit is in WDS mode, and repeating to that distant client, it will happen for virtually all traffic to the distant client.)

    The only reasonable way to use two antennas is with identical antennas oriented in exactly the same way.

    To accomplish the near and far client, get a second WRT54G and hang that high gain antenna on the connector nearest the power connector (shortest path inside the radio). It's probably best to lock the unit onto antenna 1, though it won't make a great deal of difference if no antenna is on the 0 connector to confuse the system.

    I'm not familiar with a senao cb3, and won't comment on which is better. The power issue is pretty much meaningless though. The WRT54G has one of the best receivers of any wireless radio, and that makes it a very good unit. Increasing the transmit power does not extend the range if the WRT54G cannot receive a good signal from the distant unit. Hence higher power is totally bogus. Of course the WRT54G can easily be set to higher than 125 mW anyway. But there is probably little reason to go even that high unless the distant radio can also increase power by the same amount.

    I've seen many people repeat that claim. I have yet to see anyone support it with facts though. For that matter, I haven't seen anyone say it who even understood what the significance of the comment is! I really doubt that the output of a WRT54G gets any dirtier at power levels above 200 mW, though I'll grant that it could happen. It is, however, very likely that output that high is going to cause more interference to nearby receivers even if the signal is very clean.

    Hence without supporting numbers telling me what kinds of distortion there is, I don't believe it. Primarily because if that were true the bit rates would take a nose dive at the higher powers. Not the just the bit rates for other nearby units, but for a receiver connecting to the high power transmitter. That is not what is being reported, so I doubt that the transmitter output is actually impaired at all.
  9. Westwind

    Westwind Network Guru Member

    wrt54gs and external hi gain antenna

    Thanks for the info Davidsonf.

    However even if i connect the antenna without the cable to the router. the signal is still very weak. I get around a 5% -10% signal only around 40ft away (with one glass window in the way)
  10. davidsonf

    davidsonf Network Guru Member

    Re: wrt54gs and external hi gain antenna

    Not good, eh?

    If you mean literally you can see the other unit through the window 40 feet away, you may have some bad equipment. But if you mean the radio is through that window and two feet lower than the window, it is not going through the window! Instead the signal is trying to go through the wall, and walls tend to have things like aluminum foil backed insulation in them, and simply do not pass 2.4GHz signals very well at all.

    If you can put the radio right up against the window, that might help. Also it may help to simply reposition one or the other end, so that it really is a direct line through the window.

    If that fails or can't be done physically, the first thing to do would be to verify that things actually do work. Temporarily move one or the other unit to where they are eyeball to eyeball in the same room. If that doesn't work, one of them is not healthy.

    If they are actually working, but you can't locate them in a position that works, that is the ideal reason to obtain a repeater. The WRE54G is not ugly, and can be very easy to mount right in a window. It does have disadvantages though. It doesn't work with WPA and if security is a big concern it should probably be avoided. If that is true, another wrt54g(s) unit with third party firmware will work great. In either case operation as a WDS repeater will cut the throughput in half, so file sharing on a local network will be slower, but since the bitrate is still far faster than most Internet connections it won't much affect web browsing.
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