Receive Antenna

6 posts by 2 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: May 30, 2019:
  • Hi all,

    I was told different things so thought I'd confirm here.

    Some people are under the impression that the APs need to have their transmit power limited to the maximum transmit power of the weakest client device, while I understand the concept here my question resolves around the receive antenna on both the client device and the AP.

    Question 1. Does the gain on the antenna apply to the receiver antenna too? I.E If my AP has an antenna gain of 8 dBi and max transmit power of 200 mWatts and my client has an antenna gain of 2dBi with a max transmit power of 100 mWatt, following the above rule my AP should have it's max power level limited at 100 mWatt to match the client device but due to the difference in gain between the AP's antenna and the client's antenna the distance is different even at the same transmit power right?

    Question 2. If my 2dBi gain client shoots a 100mWatt beam at my 100 mWatt AP with an 8 dBi gain at the AP's maximum range, following the above rule the AP would not be able to hear the client due to the AP's EIRP being higher than the client's but if the antenna gain also applies to the receive antenna of the AP then it wouldn't make a difference because the AP's receive antenna also shares the same gain/EIRP?

    Question 3. Does the receive antenna actually shoot out a propagation pattern like the transmit antenna or is it just receiving? It is difficult to understand if antenna gain applies to the receive antenna or not if it does not shoot a propagation pattern.

    Thanks in advance for any clarity on the above questions.

  • By Howard - edited: February 22, 2019

    First, let's start with your premise about power levels.  I suggest you watch Devin Akin's video from the 2018 WiFi trek here:

    And pay close attention to what he calls the LCMI - Least Capable Most Important device.   He also talks about the Rule of 10's and 3's, which is indispensable in conversations like this.

    Starting with your #3 first, the receive pattern of an antenna will match its transmit pattern.   Be aware that this is NOT saying that the transmit and receive signal paths between the two will be the same, in a multi-path environment.   

    Also, antennas have little , if anything, to do with a radio's receive sensitivity.  - although more expensive devices tend to have better sensitivity.

    Your Question #2 is a little hard to follow - "beam" ?.   Generally speaking, increasing the gain of your AP's antenna will also help the clients, allowing you to reduce the  power at the AP, and not require more power at the client.   I hope this answers, indirectly at least, this particular question.

    In Question #1, iIf you mean transmit "range" when you say "difference",  the AP will still have a much greater maximum range than the client.  From your numbers, the AP would be putting out (200/100) = + 3 dB more power..   The antenna gain difference is (8-2) = 6 dB. 

     3 + 6 = +9 dB advantage to the AP.   A difference of 6 dB, is already double the range compared to the client.

    A real problem is that you can't equate most client device antennas with those on an AP.   I can tell you that most client devices are quite directional, and have low gain antennas - 1.2 dBi, or lower is not that uncommon.  Antenna aperture, not just resonant frequency, is an important consideration.  Always test your client devices at different orientations, and if mobile, with thru-body tests.

    Cheaper devices, are more likely to exaggerate their output power levels, the receive sensitivity of their radios, and the gain of their antennas - if they even provide these numbers at all .

    Please select more realistic output power levels in your future examples.  e.g. very few clients are going to develop 20 dBm (100 mW) due to FCC restrictions. 

    Hope this helps.

  • Hi Howard,

    Great reply and insight into my queries, I probably could of asked my questions more accurate/articulately.

    I do have a follow up query however,

    "Starting with your #3 first, the receive pattern of an antenna will match its transmit pattern."

    Does this mean the receive antenna is also transmitting like the transmit antenna as "it matches the transmit pattern of the transmit antenna"?

    I had always thought the receive antenna was like a receiver that does not propagate anything and just waits until an RF beam hits it from a transmit antenna meaning that we only need to know the receive antenna's sensitivity, TX Power and antenna gain of the transmit antenna of the client device and AP to workout what EIRP the AP should be on and adjust the TX Power to match the client's EIRP.

    But if the receive antenna has a transmit pattern then I'm confused because why does the RX antenna need to be transmitting? And if it is then we need to factor in the gain of the RX antenna right? Also what is the RX antenna transmitting to?

  • By Howard - edited: February 22, 2019

    It's not that the receive antenna is transmitting, it's just that its "pattern", i.e. its azimuth and elevation, vectors will look the same in either transmit or receive.

    Another picture might be clearer if you looked at a highly directional antenna's plot - both it's best transmit and receive direction are the same.

    You'll often hear this called the antenna "reciprocity" theorem.   Likewise, you'll also hear that a good absorber, is a good emitter - and a poor absorber is a poor emitter. 

    The EIRP is only a radio's transmit characteristic,  not a receive parameter. 

    However, HOW MUCH of the signal power is received is dependent on the transmitters EIRP, the receivers sensitivity, and both antenna  gains - over the signals route.

    Obviously, in a non-LOS situation with multi-path, the path(s) and calculations are more complicated.

  • Thanks for your explanation Howard, all makes perfect sense now, just hard to picture stuff via words and concepts/ideas gets confused in text, thank you.

  • By Howard - edited: June 1, 2019

    Something I only hinted at before, is that RF links are not necessarily symmetric.  That is, the actual paths taken by transmissions in one direction may not be the same as those in the opposite direction.   This creates additional problems for modelling software when compared to results in the field.

    LOS (line-of-sight) predictions are much easier to make than NLOS (non-LOS) ones. 

    The RF characteristics of Antennas are "reciprocal" as far as gain and directivity go, but their signal's propagation can be different due to the surroundings at each location .   This behavior is well known to Ham Radio operators, especially those working the HF bands.

    I would also like to point out, that based on hundreds of hours of client testing, I can confidently say that receive sensitivity is a better indicator for maximum range than is output power.   A difference of 0.4 dBm sensitivity, can have a much bigger effect on range than say a difference of 2.0 dBm, or more, output power.

    This is often hard for some people to believe, but based on $50k WLAN test instruments, and both indoor and outdoor range testing we repeatedly found it to be true. 

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