One antenna VS two antennas in dual-antenna units

Discussion in 'General Discussion' started by rybshik, Aug 23, 2007.

  1. rybshik

    rybshik LI Guru Member


    I’ve read that in dual antennas units (such as, for example, WRT54), the unit is constantly switching the antennas to find the one with better signal reception and that at every time moment only one antenna is active and actually receiving or sending the signal.

    First, I would like to get confirmation that it is indeed the case. That is, I would like to get confirmation that there is no such unit ‘s mode at which one antenna is transmitting and the other is receiving at the same time.

    Second, I would like to make sure that by entirely disabling one antenna and forcing the other for both TX and RX, I will not reduce the functionality of the unit and it’s capability to pick up a weak signal in principle.

    For example, in DD-WRT firmware for WRT54Gver6, I set
    TX Antenna -left
    RX Antenna –left
    So, in such configuration, the right antenna is permanently disabled. Assuming that the left antenna provides the signal reception as good as the right one, is it safe to claim that such my “one-antenna” configuration will be able to pick up a weak signal as good as the standard “dual-antenna“ configuration and I will get as good transmission rate and overal quality.


    From my one-hour playing with my WRT54Gver6 running DD-WRT micro SP2 in a busy wi-fi neighborhood, disabling one antenna, then the other one, I got a (subjective) impression that two still better than one.
  2. scoob8000

    scoob8000 LI Guru Member

    I've always wanted to know the same exact thing.

    There is so much contradictory information floating around about this.
  3. Quijy

    Quijy LI Guru Member

    Interesting question, since I am a RF Engineer so I will be happy to answer. This may be long but here is goes. The WRT use diversity receive (RX). That means it is checking the received signal on both antennas always and using the one with the best signal. All wireless cards (Bluetooth and WiFi) work this way on the RX. This allows the receiver to get the best signal all the time because it changes. What it changes, yep! The RX signal at any receiver changes based on a ton of factors, antenna patterns, antennas gain factors, antenna deficiencies in the design, walls, angles to walls, angles to objects in the walls, relations to angle to the receiver of the transmitted (TX) signal, people moving about between the TX and RX, temp of the air, humidity in the air, absorption of all of these materials of the RF and so on. Everything affects the signal. So to help any receiver, diversity helps with these changes. How you ask, the signal from each TX arrives at each RX antenna at a different time and signal level based on the changes in the environment. A signal to noise ratio comparator or voter checks these signal levels hundreds of time a second and takes the strongest signal. So to answer your question two antennas are better than one, especially in the case of WiFi with the stock or antennas attached to the WRT. So why have the ability to turn one antenna off, like in most third party firmware’s? Well simple; say you have the WRT on an outside wall of your house and you want to have kicking signal in the back yard. First use an external directional antenna like a grid dish antenna and force all the TX and RX to the one antenna in one specific direction. Ok, ok, but I have two antennas and you say two are better than one. Always, by adding another directional grid dish to your directional array you improve the signal both TX and RX. So the long and short of it, turning off one antenna in most cases will NOT improve your signal much on TX or RX unless you have a specific need with an external directional antenna system. As for TX, most wireless cards (Bluetooth and WiFi) use one antenna for TX. In the WRT it is the antenna mounted directly on the board or left antenna. One the wireless card in your laptop you will have a white and black antenna cable; generally the antenna on the black cable is the TX antenna. Hope this answers your question.
  4. scoob8000

    scoob8000 LI Guru Member

    Good information there!

    I thought it had something to do with diversity.

    So I don't want to threadjack here but...

    So, would I be better off with one high gain omni antenna, or the two stock antennae working together?

    I've done some simple comparisons, and can't see much of a difference.
  5. rybshik

    rybshik LI Guru Member

    thanks a lot for reply!

    So, if I get you right, at no time moment one antenna is transmitting and the other antenna is receiving simultaneously, be it stock Linksys firmware or DD-WRT. Correct?

    If you look to WRT from the front, which one is this antenna? And, what it will be in DD-WRT firmware settings: left or right?

    How intelligent that comparator or voter is in case of wireless bridge units? Let’s say, the unit is pre-programmed with SSID “rybshik”, so it can only connect to a network with SSID “rybshik”. Now, when the unit is switching the antennas in its search for better reception, does the unit take into account not only the RX signal power, but also the SSID?

    For example, lets say my unit is in the client bridge mode with SSID “rybshik” and I keep one stock antenna ( call it “A”) and replace the other antenna with the directed greed antenna (call it “B”). Lets say, the total signal power on antenna A is greater than the total signal power on antenna B, but the power of the signal with SSID “rybshik” is greater on antenna B. Which antenna does the unit pick (or switch to)?
  6. Quijy

    Quijy LI Guru Member

    "So, would I be better off with one high gain omni antenna, or the two stock antennae working together?"

    Maybe, depends on the quality of the cable used to the high gain Omni, you could loose all the gain in the antenna line between the antenna and WRT. As for one or two, what works best for in your environment.

    "So, if I get you right, at no time moment one antenna is transmitting and the other antenna is receiving simultaneously, be it stock Linksys firmware or DD-WRT. Correct?"

    Correct, wireless is half duplex, meaning it will TX then RX.

    "How intelligent that comparator or voter"

    Not very at all, very cheap a basic device almost passive. this is a consumer device, not a pro device with a pro radio.

    As for your example, the WRT will listen to to TX signal from the best antenna. The SSID, mac address are all part of the packet being sent between the devices, it does not matter the network config, just the signal strength.

    "If you look to WRT from the front, which one is this antenna?"

    depends on the hardware version but most of the newer versions v2 and up the one on the board is the antenna on the right, looking from the rear it is one on the left. I do not recall how DD-WRT denotes the two but it is in the help files on the DD-WRT site.
  7. rybshik

    rybshik LI Guru Member

    Are you referring to “gain” for TX, RX, or both?

    When installing after-market antennas, do we look for additional gain primarily for RX?

    Does it make sense to do the following:
    1) replace one stock antenna with high-gain one and set it up as RX only; and
    2) to keep the other (stock) antenna and set it up as TX only?
  8. Quijy

    Quijy LI Guru Member

    “Are you referring to “gain†for TX, RX, or both?â€

    Gain affects both, TX and RX. The higher the antenna gain the higher/better the RX signal to the receiver in the WRT. In TX gain will increase the ERP, Effective Radiated Power. ERP is the summation of the antenna system parts. Power output of the TX, loss of the conductor to the antenna (cables and connections), and loss or gain of the antenna equals ERP. In simple terms, 1 watt TX power, loss or minus 3db in the antenna conductor (3db is half of the power lost) to the antenna, times the antenna gain or loss. So 1 watt from the TX, loss of 3db in the antenna conductor equals ½ watt to the antenna times 3db gain on the antenna equals 1 watt ERP. If the antenna was unity gain or zero gain, it would be ½ watt ERP. Every three db double the ERP on TX. Same on RX every 3db gain, increase the RX signal, so if you use a scope an measure the signal with a unity gain antenna and say it is 10mv, change the antenna to 3db gain and it would just almost to 18 to 20mv. Of course this is ideal conditions in a test lab. Real world is a bit different.

    In regards to 1 & 2, I would set a single high gain Omni to RX & TX, using only one antenna.

    As you have learned this is not as simple and use one thing and do this. It is matter of the antenna, cable used between the antenna and WRT, length of that cable, your environment that all of this is in. It is nuts but that is what makes RF fun. The trick is use what works best for you in your environment with the equipment you have. Test and try things till you find what setup works best for you. There is not absolute correct method.
  9. rybshik

    rybshik LI Guru Member

    Speaking of antenna quality and the losses in cables and connectors of non-stock antennas, are brand-name Linksys and D-link any better than Sharperconcepts ones?

    For example, is D-link 2.4GHz Omni-Directional 7dBi Indoor Antenna any better than Sharperconcepts 2.4 GHz 7 dBi Omnidirectional Rubber Duck Wireless LAN Antenna With Magnetic Base
  10. Quijy

    Quijy LI Guru Member

    Honestly the antennas you have listed work and I have used all of them. They are not much more that the stock antennas with a remote base. The coax if you want to call it coax is not much more than shielded audio cable and has high loss factors, what you gain in the antenna you have lost in the cable. These are really meant for just moving the antenna from the back of WRT or PCI card to another location. Like from the WRT in a book case and moving the antenna to the top of the book case because you could not move the WRT to the top of the book case. I would suggest a Diamond or Comet antenna or search the web for antennas, not something you buy at your electronics outlet. There are a ton of antennas out there and better ones sport rigid radomes with n-type connectors, this allows better coax to be used.

    Diamond Wireless LAN antennas

    Comet Wireless LAN Antennas

    WiFi-Link (tons of antennas)

    I guess the question that has not been asked here, what are you trying to accomplish and what are your expectations? I use the 7dbi Linksys whips directly on the back of the WRT. Seems to work the best in my situation, the WRT is on the second floor covering the house. I have tried better antennas, the Comet GP-24-3 and I got better signal but; when you have fours bars everywhere in the house on every laptop no matter where you go, on the 7dbi Linksys whips why do you need all five bars with a larger antenna? It really does not make much difference in my case four to five bars. If you have say 2 bars and keep dropping the signal then maybe you need a little boost and a better antenna may help.
  11. scoob8000

    scoob8000 LI Guru Member


    I have that exact 7db Dlink antenna on my WRT54GS.

    Running netstumbler on my laptop and wandering around my house I can't see much difference between using only it, versus the two stock antennae.

    I'm beginning to wonder what if I add a second one.
  12. Devesh

    Devesh Network Newbie Member


    I have one router with dual antenna of 5dbi

    Can i replace one of them with 9dbi for range extension?
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