[CUWiN-Dev] H-Pol antennas (was Re: Client/Mesh Domains)

[Bill Comisky]

On Thu, 20 Jan 2005, Quantum Scientific wrote:

[ snip ]
>
> In the List archive I noticed reference to interference from other 
> sources.  We are planning on using *horizontal* polarization, with 
> waveguide (travelling-wave) antennas.  Even if on the same channel as a 
> noise source, there is a 20dB difference.  Unfortunately these antennae 
> cannot be made for less than $100 each, although this should be a safe 
> method of bandwidth isolation indefinitely, particularly in urban areas. 
> And they're even quite safe from lightning *inherently*, as lightning is 
> vertically-polarized, and also the enclosure is grounded aluminum. 
> OTOH, this higher node cost is why it would be best if clients could 
> connect to a node wirelessly.

Carl,

Polarization purity is degraded by multipath and scattering.  From what 
I've seen in urban environments, the signal from local APs is all over the 
place.  Just monitor the signal levels as you rotate a linearly polarized 
antenna, and you'll often find the signal is strongest off-vertical, and 
depending on where you are, even horizontal.  Going H-pol will help you if
have good line of sight to the V-pol noise source you want to isolate, but 
don't count on it to eliminate interference from local APs inside nearby 
buildings.

Also, any lightning protection of an antenna is not due it's polarization. 
Rotate your waveguide antenna 90 degrees and it's now vertically polarized 
(though with a narrow azimuth beamwidth).. does this change its 
susceptibility to lightning?  The antenna is not "receiving" the 
lightning.  Any lightning protection is due to proper grounding, and I 
suspect in the case of your waveguide antenna, having the feed inside a 
conducting grounded structure isolates the path to the electronics.  This 
type of isolation would be difficult for a typical monopole, but again 
it's not the polarization, it's the construction.

Bill

--
Bill Comisky
bcomisky at pobox.com