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Re: 2.4 gHzCordless Phones



I've been looking at it with a spectrum analzyer recently to evaluate 
it and other interfering signals on the 802.11 networking we are 
doing at 2.4 ghz on campus and around town.

Some ovens may turn the magnetron on and off at a once a second rate 
depending on the duty cycle of the heating they are doing, but while 
they are ON the wave that we see is a typically a rising set of 
spikes that are apparently 60 hz and usually sweeping up in 
frequency. I believe from looking inside a few that they are 
basically a half wave rectifier/voltage doubler circuit that provides 
power for half a cycle, and none for the other half of the cycle, and 
that looks exactly like what we see on the analzyer. The frequency 
appears to drift upwards fairly rapidly with heating, and then go 
back down again with those on/off periods for duty cycle. It's my 
understanding that the 802.11 gear and the phones both use short 
packets to fit in the half cycle off periods of the 60hz power 
pulses. When we test in an apartment complex, we can see ovens from 
several apartments away - kind of makes you wonder how much is 
getting out of them into the occupants nearby!

For what it's worth, the 802.11 stuff and the 2.4ghz spreadspectrum 
phones don't seem to play well together - the phones are strong 
radiators and occupy about a mhz of bandwidth. The networking gear 
seems to hear the channel busy and backs off - till the call is over 
in many cases. If you plan an in house network with these, you 
probably want to stay with 900 mhz phones.

I haven't listened to what the 802.11 stuff or the phones sound like 
on an SSB receiver, if I can get in on that group buy of the drake 
2880's I'll report after that. I can say that surveying our town for 
noise on 2.4 ghz with the analzyer, it's increased substantially with 
several companies offering 2.4 ghz 802.11 networking for laptops and 
palmtops around the core of the town, and with other long range links 
out as far as 8 miles. The campus is wiring about 100 POPs on the 
campus for it inside buildings with common areas. So there's going to 
be a lot of this stuff around. The good news is that a high gain 
antenna pointed up at the sky probably will ignore most of it - I 
hope.

Kevin, WB2EMS



>Actually the pulses were about once per second.  But
>the interference does have a "AC hash" character to it.
>
>Doug
>NA1DB
>
>
>At 02:59 PM 1/19/01 , you wrote:
>>At 11:36 AM 1/19/2001, Doug Braun NA1DB wrote:
>>>running my receiver when the microwave on.  I got pretty strong 
>>>interference ... shows up in SSB as a pulsating noise level 
>>>(probably due to the thingy in the oven that "stirs" the microwave 
>>>energy).  The noise blanker on my rig reduced it significantly.
>>
>>I think you will find that the pulses are synchronous with the AC 
>>line.  One short pulse of microwave energy per AC cycle.  Therefore 
>>I believe it is due to the way the magnetron is powered.
>>
>>I looked into this a few years back when we were designing the 
>>Globalstar downlink, which is also on 2.4 GHz.  I was quite worried 
>>about interference from microwave ovens, so we characterized it, 
>>and made sure we designed a front end for the handheld phone that 
>>would be able to survive it.  We succeeded, as microwave 
>>interference hasn't turned out to be a problem.
>
>
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F. Kevin Feeney
Senior Information Technology Engineer
Cornell University, Ithaca New York
607-255-5186, fkf1@cornell.edu

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