Xohm Router Model MAX206M2 by ZyXEL

[wayne21224]

Hello there. I was looking at several antennae you carry and was curious which would be the better for my application. I am less than 1/2 mile from the Xohm / Clearwire tower and only receive 1 -2 bars of service at best. The only thing I can imagine is that the tall buildings around here are blocking my signal ( Although I thought Wimax was Non LOS ). I Know where the tower site is around my location and can easily affix an external antenna on the roof pointing in that direction, however all of the "Xohm" antenna kits im seeing only come with a 10 foot cable which isnt nearly long enough to reach.

I was looking at this outdoor model here http://www.rfwel.com/shop/?target=pr...cat=179&page=1 and was wondering If I could use this and get a 20 foot cable, and if so which cable would I need?
Do I need any other accessories to go along with this ?

What would be my other options?


Thank you very much


Sincerely,
Wayne Nestor Jr.

[thuor]

Hi Wayne,

Yes you are right that WiMax is NLOS and utlizes MIMO which should combat multipath fading. However the nature of RF propagation is so complex that it's very hard to predict ubiquitous coverage even with all the technological advances of the 802.16e protocol. Furthermore RF signals at 2.5GHz don't penetrate buildings very well as does for example cellular frequencies at 800MHz or even 1900 PCS.

So yes if can add a roofmount antenna oriented in optical line of sight to a xohm/clearwire tower then this would definitely help. Note that optical line of sight doesnt necessarily mean RF line of sight but this should still be fine. Only thing we have to worry about however is to play fair and not boost our modems to too high of an EIRP to drown out adjacent signals. Provider wouldn't like that

As for your question on cable lengths we have a 20Ft kit here (use the drop down to select 20FT). You can also custom build your kit using the antenna, enclosure, low-loss cable & radio connectors (to convert cable to SMA-male since the Zyxel modem and the antenna are both SMA-female). If you are also mounting this on a rooftop above any other lightning arrestors we recommend a lightning surge device as well. The key though is to realize that the longer the cable the longer the attenuation (lower the performance benefit). Also adding lightning surge devices leads to an insertion loss and reduces the signal power at the modem.

Also realize that since RF over-the-air propagation in an urban environment is quite complex if you have any multipath effects (strong non-line-of-sight components) it's hard to predict outcome and we have seen cases where performance has deteriorated for specific antenna placements or orientations. In this case we recommend utlizing the MIMO capabilty of the modem and of wimax by adding a 2nd antenna. In addition to an addition of channel capacity (hence throughput), MIMO also gives you spatial redundancy in a rich multipath environment.

[wayne21224]

Thank You

Do you think adding a second antenna is necessary? If the OEM built in antennae are rated at 5dbi ( 3dbd ) , wouldnt adding a 16dbi rooftop be sufficient ( as it should still equal near 14 dbd ) which should more than double the antenna power currently?


If so, would I have to place it aiming at the same direction or would I want to cross the 30 degree paths ? I know there are two tower locations within a 1/2 mile range, but I also know that I can only connect to one of the two at any given time even though the router does support mino ( or am I mistaken there? ) I wouldnt want to intermittently connect to the further tower right?

Do you offer any kind of a discount when purchasing more than one ?


Thank you very much

[thuor]

Very good question Wayne. We get this often so let me try and answer it generally for anyone else in a similar scenario:

If you have a good RF line of sight then the 2nd antenna usually doesn't do much and as such is not needed. It all depends on your resulting SNR at the modem input (and by SNR here we mean Signal to Noise plus Interference ratio). If you do have a 2nd antenna then you would want to point it in a different direction to capture a signal that is uncorrelated to the first one. The more uncorrelated the two antennas are the better MIMO works ... in fact one of the most surprising results from MIMO research 10-odd years ago was this very thing that the more multipath a channel had the better!!! (of course LOS is still king )

At the risk of being too technical and being careful not to butcher the underlying signal processing details which can be quite intricate ...
MIMO in the simplest sense creates multiple parallel RF paths from the Transmit antenna to the Receive antenna. The idea is that you can carry independent data in these paths hence adding channel capacity -- more throughput. In actuality however it creates a matrix channel. For example a 2x2 MIMO system with 2 transmit antennas and 2 receive antennas has 4 paths (picture a 2x2 Matrix).

....TX1 --- RX1....
........ \...../........
.......... \./..........
.......... /.\..........
........ /.....\........
....TX2 --- RX2....

TX = Transmit Antenna
RX = Receive Antenna

MIMO is quite different from antenna diversity that is found in many radios such as 802.11b/g radios that have 2 antennas. In this case the receiver is using the multiple Rx signals usually from a single transmit antenna and combining them (different ways to combine them) to obtain better SNR. In MIMO you actually use the spatial domain (space) to code the data (in addition to time domain which is far more common) therefore taking advantage of the fact that the transmit signals from the different antennas will follow different paths, have different delays and be distorted differently.

The resulting conclusion is that the more uncorrelated the MIMO channel is (the higher the rank of the channel matrix) the higher the MIMO gain. This somewhat explains why if you had a single high gain, high directivity indoor antenna performance might actually be worse than if you had dual lower gain, lower directivity (or omnidirectional) antennas. This also explains why in the beginning i said if you have a 2nd antenna orient it different from the first to allow it to capture an different signal altogether.

Finally a minor point often overlooked, recall that dB is a logarithmic scale.

dB = 10 x log (Power)

So the 11dB difference between a 16dBi antenna and the factory 5dBi is ~12x. Power doubled every 3dB.

So your absolutely right, even taking into account losses, this is way more than double the power. But as with everything in life too much of anything is not good. Too much input power will cause bad things to happen (input compression, clipping, desensitization of adjacent radios, dynamic range issues, provider mad at you .....etc) .... so for those of you out there blessed with $$ to burn, if your RSS (Received Signal Strength) is already good but need more throughput consider getting another modem and a MultiWAN link aggregation router instead.

[wayne21224]

Thank You

Thank you for the wonderful explanation. I think what I will try is antenna to start. I already have a decent connection, however my throughput isnt as high as it should be. Im hoping that the LoS isnt a major concern because I do have many structures blocking direct sight. I am hoping that with the correct placement with the 23 degree beamwidth, I should be able to improve the connections throughput with a single antenna. As you put it, too much is generally a good thing, however not in this case. I do thank you for the help

Have a wonderful evening/day!


PS
You didnt mention if there was any sort of discount if I did decide to purchase two at the same time

[thuor]

Check with sales on monday on the discount for multiple units 480-218-1877. Doubt it but never hurts to ask.