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??
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
> 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
>> 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.
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