Link Budget and EIRP - per antenna or per antenna cluster?

Discussion in 'Wireless & RF Design' started by Michael_Hansson, Feb 22, 2016.

  1. Michael_Hansson

    Thread Starter New Member

    Feb 5, 2016
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    First post - I hope I have found the right subforum.

    After some years working in the field (antenna / radio link installation) I am finally studying for my degree.

    In one of the courses we are to make a calculated model of the RF powerdensities encountered in a urban setting with a number of BTS (Base Transceiver Station = cell phone antenna masts) nearby.

    The required calculations are rather easy, as the models only are to include free space loss and no attenuation from topological features or buildings.

    We are to base the calculations upon EIRP-values from the link budgets described in:

    Holma, H. & Toskala. A. 2009. LTE for UMTS: OFDMA and SC-FDMA based radio access. John Wiley & Sons
    Holma, H. & Toskala. A. 2010. WCDMA for UMTS: HSPA Evolution and LTE. John Wiley & Sons

    (As it probably would be a copyright infringement to include a scan of the relevant pages I have found a website which references most of the link budget values from the two books. The URL is:
    https://sites.google.com/site/lteen...ng-and-rf-planning/lte-link-budget-comparison )


    However, one of the other students in my project group has a very different understanding of EIRP than I have. He is fully convinced that the EIRP-values in the link budgets are given per antenna cluster, not per antenna.

    (Most BTS-installations (at least around here) are made with tree antennas, each covering 120 degrees. This means that each antenna cluster (consisting of three antennas) cover 360 degrees.)

    In all of the antenna installations I have made (albeit radio links, not BTS) EIRP was always understood to be calculated/declared per antenna, not per system or per antenna cluster.

    I am pretty sure that my understanding of the EIRP concept is correct - that EIRP always is per antenna, but I cannot (despite a couple of days searching) find any sources (online or in the library) which clearly settles the issue.

    The professor teaching the course is on holiday, so I cannot get any help there.

    Any help is greatly appreciated! Especially a reference to some authoritative material which clearly settles the issue.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,155
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    I don't agree. Effective Isotropic Radiated Power needs to be istropic, that is must be the same in all directions at a fixed distance from the radiator. If we had a gain versus azimuth plot for a tree antenna we could see if the isotropic radiator correctly models such an antenna. I believe that the goal of the tree-antenna is to do that, I'm just not sure how pronounced the lobes are.
     
  3. Michael_Hansson

    Thread Starter New Member

    Feb 5, 2016
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    0
    Thank you for your reply!

    Gain/azimuth-plots have not been mentioned in the class in relation to the assignment. I agree that the modelling would be vastly more realistic if the values were taken from actual antenna specifications with gain/azimuth-plots instead of generic textbook examples.

    Both in Balanis "Antenna Theory" and in the books referenced above EIRP is calculated as transmitter power * antenna gain. Do you mean that the available transmitter power (44.5-46 dBm) actually is shared between the three 120 degree (18dBi gain) antennas in the antenna cluster?

    Thank you!
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,155
    1,795
    No not exactly. I think what is happening is that the three antennas together, with some sleight of hand called a phasing harness, match the transmission line and the transmitter output impedance of 50 Ohms. You would have to know exactly how they were connected to be sure.

    VHF operators use the phasing harness on stacked yagis all the time.
     
  5. Michael_Hansson

    Thread Starter New Member

    Feb 5, 2016
    3
    0
    Thank You!

    I didn't know phasing harnesses could be used with mobile telephony systems!

    I will try to find some definitive source using that search term!

    Thank you!
     
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