which wireless router?

Billie Walsh bilwalsh at swbell.net
Tue Feb 26 14:07:12 GMT 2008


Nils Kassube wrote:
> Billie Walsh wrote:
>> Larry Hartman wrote:
>>> It has been 
>>> pointed out to me along the way that if two antennas are operating at
>>> the same frequency at the same time, that when they are a given
>>> distance apart, the signal strength perpendicular to the axis of the
>>> two antennas greatly increases, while the signal strength along that
>>> axis reduces.
> 
> That is correct, as long as you have equal cable lengths between the 
> transmitter and the two antennas. The best distance would be multiple of 
> 0.5 wavelengths.
> 
>>> This theory is why truck drivers employ two antennas for their CB's. 
>>> In their example the typical width of a tractor cab is about the
>>> right width necessary for signal strengthening in front of and behind
>>> the truck.  As was explained to me, the improvement can sometimes be
>>> measured in terms of miles.
> 
> Nice idea - the distance would be 5.5 m.
> 
>>> Depending on the electronic environment, the employment of two
>>> antennas may not always generate interference, but may be
>>> complimentary.
> 
> The term is still "interference", constructive interference in 
> forward/backward direction and destructive interference to the sides. See 
> <http://en.wikipedia.org/wiki/Interference>.
> 
>>> Dunno about Linksys or any other vendor, but it is a 
>>> consideration.
>> Considering the difference between 27MHz, CB frequency, and the 2.4GHz
>> of wifi the spacing on a Linksys may be just about the equivalent
>> distance.
> 
> Sure, that could be done at the WLAN frequencies as well, but then the 
> antennas would have to be fixed and mechanically parallel. However, you 
> can rotate the antennas in 2 axes and the distance is not right. While 
> you could compensate for the distance with the cable lengths, I don't 
> think the antennas are connected in parallel. The distance of the 
> antennas is 0.15 m, which is 1.25 wavelengths. For antennas in parallel 
> it would be more useful to have a distance of 1 or 1.5 wavelengths. But 
> the distance is best suited for diversity reception. With a given 
> distance of 1.25 wavelengths, if one antenna is in a position of 
> destructive interference, the other antenna usually is in a position of 
> constructive interference.
> 
> 
> Nils
> 

Take a look at:

http://en.wikipedia.org/wiki/Fresnel_zone

-- 
Life is what happens while your busy making other plans.



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