How/When Can WiMax Antennas Actually Degrade Performance instead of Improving It?

We once in a while receive a support ticket themed around

I have attached an outdoor high gain WiMax antenna but my performance is actually worse with the antenna than without it!!

There are several reasons why this might be the case:

1. Wrong antenna or cables used
   Typically the problem is with the cable but we have seen people use 2.4GHz antennas and get surprised that everything is not working as well. Remember antennas are passive and by design wide-band (just how good they perform at different bands than what they were intended for is a different story). So an antenna designed for 2.4GHz will not completely block the signal but the performance will deteriorate significantly outside the bandpass. Same goes for Coaxial cables. The cable attenuation increases with frequency to an attempt to use CATV cable (designed for frequencies < 1GHz) with WiMax (Clear WiMax at center frequency of about 2.6GHz) would lead to large cable losses. We recommend LMR-400 cable for runs less than 100ft and LMR-600 for longer runs. Also need antennas specifically designed for this frequency band: http://www.rfwel.com/4g-data/wimax-a...wimax_antennas
2. Too much cable/connector loss
   Even when using the correct antenna and the correct/recommended cable, if the antenna gain is not too large and the cable is too long your effective gain might actually be smaller than the factory-installed integrated internal antennas of the modem. For example since LMR-400 cable at 2.6GHz has an attenuation of 6.9dB/100ft if you have a 100ft run, are using an 8.5dBi wimax omni plus adding insertion losses from connectors and possibly a lightning surge device your effective antenna gain would be less than the typical 3dBi of integrated antennas.
   
   For very long runs consider using for example a 24dBi Wimax grid antenna or 23dBi WiMax parabolic dish antenna and/or getting lower loss cable such as LMR-600.
3. Loose connections or defective antenna switch in the modem
   Does the pigtail fit snugly into the CPE/USB modem antenna port? The modem with external antenna ports have an antenna switch 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.
   Note: too much pressure or using a wrong pigtail could permanently destroy this.
4. Use of a passive antenna coupler for modems without an external antenna port
   While universal couplers do work for select modems and select scenarios there general use is not recommended. When possible we recommend getting a modem which has an external antenna port and using an appropriate pigtail. See http://www.rfwel.com/4g-data/ for more info on what modems have external antenna ports. The problems with using the passive antenna coupler and which could actually make performance worse include:
   1. Imperfect coupling to internal antenna leading to large coupling losses. Leads to the same effect as having an extremely long cable run or having a lightning surge device or connectors or any other device (eg. splitters, combiners, diplexers, etc) which have a large insertion loss.
   2. Loss of MIMO performance gain. See WiMax Antenna Primer for a discussion on MIMO's importance. Most modems currently in the market include two internal integrated antennas. When you attach the passive coupler you cannot discriminate between the two internal antennas since the two internal antennas both radiate equally to the coupler (so there's no way to attach two external antennas if using the passive coupler).
   3. Location and design of internal antenna. RF shielding from the modems or CPE's case could mean that very little RF radiation would make it from the coupler to the internal antennas and vice-versa. Furthermore the internal antenna design may be such that adding a parasitic radiator in the vicinity of both antennas may distort performance. In fact all current Clear/Sprint CPE modems are NOT compatible with this solution at all. We have seen limited performance benefit of the passive antenna kit with for example the Franklin U300 in areas where the outdoor signal is quite strong.
5. Antenna directivity & orientation
   Radiating into each other when using multiple antennas. Using antenna with too narrow a beamwidth in areas with plenty of multipath propagation (high beamwidth or high directional gain best suited in situations where you have a clear Line-of-sight (LOS) to the Wimax BTS.
6. Using a single antenna vs two antennas
   See WiMax Antenna Primer for a discussion on importance of MIMO's multiantenna technology.
   
   EXPERIMENT to verify if a 2nd antenna is needed. If possible take the modem outside at location where you would mount the 2nd antenna. Leave the primary external antenna connected and observe performance. Now disconnect the primary antenna and observe performance again (i.e with no external antennas connected).
   
   CONCLUSION
   - If having modem outdoors with cable/antenna hooked up to primary performs better than modem indoors with same cable/antenna then you need a 2nd antenna.
   - If having modem outdoors with cable/antenna hooked up to primary performs significantly worse than modem outdoors at the same location without any external antenna then there could be something going on with your antenna e.g wrong antenna, defective antenna, too long of cable, wrong cable etc
7. Too much antenna gain
   Contrary to what might seem intuitively obvious too much antenna gain is not good and can be just as bad as too little. The high gain 24dBi Wimax grid antenna or 23dBi WiMax parabolic dish antenna should really only be used to compensate for long cable runs or where outdoor signal is quite weak.
   
   Too much input power causes the input low-noise-amplifiers (LNA's) to operate on the extreme ends of their spec'd dynamic range which leads to non-linearity and signal clipping. This in turns leads to exacerbation of such effects as intermodulation distortion (IMD) which all leads to a worse bit-error-rate. Remember since WiMax uses adaptive modulation and coding, the BTS would actually throttle down the bitrate by moving to a lower modulation mode which simple means your speed would throttle down. Not good!
8. Defective or terrible antennas
   Of course the antenna(s) could be defective which is possible but not too probable. The antennas could also simply be not of good quality. For example could have higher VSWR or lower gain linearity vs what is quoted.