[amsat-bb] 2.4GHz DSS cordless phones

[Scott Townley <nx7u@xxxxxxxx>]

Let's see if I did this right.  Not looking to be utterly exact but close.
Looking at a popular manufacturer's 2.4GHz phone, channel 1 (center 
freq=2407.42MHz), the transmitted "noise floor" is 40dB down at 
2405MHz.  Assume that spectral density continues down to 2401MHz.

The phone's maximum transmit power is +14dBm, with a spreading bandwidth of 
1.366MHz, so that's -47.35dBm/Hz.  In the victim (ham) receiver's 2.5kHz 
bandwidth, that would be -13.4dBm.

With the 40dB out-of-channel rejection, the "noise" signal from the phone 
is -53.4dBm (in 2.5kHz bandwidth).

Say the AO-40 antenna is 50 feet away.  That's 64dB of free-space loss, now 
we're down to -117dBm.

If the receiving system has a noise temperature of 100K, that's -144.6dBm 
noise floor.

So the antenna rejection needs to be -117-(-144.6)=27.6dB.  But that's if 
the AO-40 antenna is isotropic...a 21dBic antenna would  would need 
27.6+21-3=45.6dB "front-to-back" (or front-to-whatever) to knock the 
cordless phone down to the noise floor (the extra 3dB is linear-to-circular 
mismatch--assuming the sidelobes/backlobes of the AO-40 antenna is still 
circularly polarized, which it probably isn't).

And if the cordless noise=noise floor, that's actually a 3dB loss in 
sensitivity for the receive system.

Uuuuuuuuuuuuggggggggggggglllllllllllllllllyyyyyyyyyyyyyyyyyy  very very few 
antennas are that clean...I would need to be making a 20dB mistake to even 
have a chance.  Maybe the out-of-channel emissions continue to roll off 
below 2405 (the manufacturer's submitted occupied bandwidth plot only went 
down to 2405).

Is it really that grim?

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