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 From:  Douglas Stringer <dougstringer at mac dot com>
 To:  Jim Thompson <jim at netgate dot com>
 Cc:  Administrator at mac dot com, Justin Wilson <j2sw at mtin dot net>, m0n0wall at lists dot m0n0 dot ch
 Subject:  Re: [m0n0wall] Wireless Questions
 Date:  Sun, 13 Nov 2005 21:15:56 -0500
I agree, however it doesn't stop lots of vendors from doing it  
(besides Vivato) i.e. Tropos!
Even with 22+ MHZ separation between channels (1 and 11), there are  
issues that I've seen... though I'm not sure what they are.
Is this a case of harmonic frequencies.... or is it the radiation  
mask extending down and beyond the 22MHz channel (and across the  
band), or something else??


Doug


On Nov 13, 2005, at 7:00 PM, Jim Thompson wrote:

> Michael Mee wrote:
>
>> Douglas Stringer wrote:
>> > On Nov 11, 2005, at 11:00 PM, Mike Mee wrote:
>> >> I've done this many times and it does work quite well (though  
>> you lose
>> >> approx 1/2 the bandwidth due to the radios overloading each other,
>> >> even if they're on channels 1 and 11..)
>>
>>> What do you mean by "radios overloading each other." Could you  
>>> be  more specific?
>>
>>
>> The radio frequencies are close enough for the broadcast on one to  
>> effect the receive on the other. This affects throughput.
>>
>> The only real solution is two radios mounted 2-3 meters apart,  
>> including antennas (Or notch filters tuned to the specific channel  
>> (expensive) which still won't help much if the miniPCI cards are  
>> physically proximate).
>
> 2-3 meters isn't enough if the directivity of the antenna (gain) is  
> large and/or pointed toward each other (or a common third point).
> I've never tested the 2511MP cards in a lab, but they don't 'leak'  
> that much, the biggest problem is probably injection locking via  
> the LO if the radios are operated co-channel (or close).  Ths is  
> something we battled a lot on the first-gen Vivato
> switch, which uses the Agere (Lucent/Orinico) chipset, a (ahem)  
> highly similar design.
>
> If you can take care of the other issues (to your satisfaction,  
> anyway), then running the two radios on ch1 and ch11 should be fine  
> in terms of intermod and injection locking (inside the case).   If  
> you care about performance (range, throughput, whatever), then two  
> radios (and antennas) nearby each other in the same band "doesn't  
> work".
>
> For instance, the little "bridge/router" I did for Vivato uses two  
> 2511MP (see previous message) cards on a little MIPS-powered  
> board.   The applications for this are essoteric at best, but it  
> does pass FCC (and wouldn't with
> any real amount of intermod.)   We did have to put some little  
> ferrite coil "chokes" on the pigtails through, as the FCC wants to  
> arrange the antennas in the "worst" orientation (this ends up being  
> with them alongside each other at a distance of about 1mm on that  
> product), and the chokes were required to pass FCC in this  
> configuration.
>
>> This is a well recognised though often ignored (and often debated)  
>> effect. Its easy enough to verify however!
>
> Yeah, lets not go there.   I've been int too many on-line fights  
> where my simple math gets labeled "theory", and the other side is  
> finally forced to admit that "it works" really translates to "I get  
> some packets through under some circumstances".
>
> I'd rather not make a (bigger) ass of myself here.
>
> Jim
>
>
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