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12dBi wimax omni antenna not working as expected

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  • 12dBi wimax omni antenna not working as expected

    I recently purchased your 2.3 - 2.7 GHz WiMax 12 dBi Omni Antenna and a pigtail adapter for a Clear USB modem. When I plug the antenna into my clear modem (either connection on the modem) the reception actually gets worse than with no antenna plugged in. I checked the resistance between the center contact of the antenna and the outside ground, and there appears to be a dead short between the two contacts. Is this normal, or could the antenna be defective?

    I have verified that my cabling is good (also tested with a meter to ensure the continuity was proper from end/end and that there were no shorts in the cable). I tried two different cables just to be sure.

  • #2
    First if you only have one antenna you should use the Primary antenna port NOT the secondary diversity antenna. The primary is “ANT1” in image here: http://www.rfwel.com/images/PX1900_img1.jpg

    Yes it’s normal to see the short from a multimeter reading. The antenna is simply a radiator – simplified form of a monopole (or half-wave dipole). The center conductor is the feedline for the omni but it’s directly connected to the whip. Simply just think of a wire as the antenna.

    This is quite odd to see the performance degrade and we hardly have any returns on these or other antennas in general since they are quite simple devices that rarely are defective. There are however some but rare scenarios that might explain this:

    1) Does the pigtail fit snugly into the USB antenna port? The Clear 4G USB modem has a diplexer that selects between internal and external antenna. When you plug in the external antenna pigtail it “switches” off the internal antenna path. If this doesn’t click in perfectly you might still have the internal antenna being used but with additional losses from the loading of the pigtail.

    2) Are you testing this in-building or do you have it mounted outdoor. In-door propagation is quite tricky to understand. Multipath propagation from reflections arising from obstructions is a big problem for in-building coverage. Sometimes adding a high gain antenna makes the situation worse. However if you moved the location of the antenna this goes away. Is this what you are seeing?

    3) Did you buy the cable from us? If not be sure that you are using 50 Ohm cables and not the CATV 75 Ohm. Also be sure that the cable has a low attenuation. We ship LMR-400 cable which has a loss of 6.9dB/100ft @ 2.6GHz. If you use very lossy cable such as LMR-195 or RG-58 etc you are looking at >20dB/100ft!! so even for short runs your effective gain would be less than that of the internal antennas.

    See article How/When Can External WiMax Antennas Actually Degrade Performance instead of Improving It? for more ideas.
    KF7RCQ

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    • #3
      Thanks for the quick response. The antenna is outdoors, actually on a 50' fiberglass boat. The antenna is mounted high up, on the radar arch of the boat with no obstructions. I didn't buy the cable from you as I had to pull the cable through a conduit and then add N-male ends. When at first the antenna didn't work with a 25' run, I made up two shorter cables (less than 6' each) for testing. The cable is 50 ohm RF58 A/U, and I have verified proper wiring of the cable ends with a meter. I have also verified that I am connecting to the proper ANT 1 jack as per your specs. I will procure another cable and two more n-male connectors for further testing.

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      • #4
        RG58 cable is NOT spec’d up to 2.7GHz which is the upper end of the WiMax band used by Sprint/Clearwire. For example a 50ft RG58 cable has a loss greater than the antenna gain (~20dB). Even short runs are very lossy since attenuation rolls off sharply with frequency for such cables. For any run >20ft at any frequency >2GHz we always recommend the thicker LMR-400, LMR-600 for best performance.
        KF7RCQ

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