[RFI] cell phone interference on airplanes

Tim Groat tcgroat at mesanetworks.net
Thu Mar 2 22:51:39 EST 2006

(1) The flight crew is in control of the system. If it is causing 
interference, they can (and will!) shut it down, and write it up for the 
maintenance crew.

(2) Air Fones aren't "cell phones" in the popular sense. They use dedicated 
frequency assignments and an infrastructure designed for one purpose: low 
volume calls from aircraft at cruising altitude. Unobstructed line-of-site 
paths means ERP can be low even for relatively long paths. There's no 
signal loss from tall buildings, bridges, large trucks, etc.

(3) The antennas are outside the big aluminum pipe, not inside. The 
shielding efficiency of the fuselage isn't really high at these frequencies 
(window dimensions are the significant limitation), but it does give some 
protection to the plane's avionics and cables. With the antenna inside, the 
consumer phone's automatic power control goes to QRO mode to overcome the 
shielding loss. Adaptive power control potentially doubles the shielding: 
for example, 10dB of shielding yields 10 dB of ingress attenuation plus 
10dB less transmit power = 20dB less signal strength at the electrical cables.

(4) The fuselage is an RF-reverberant enclosure. A signal inside the 
enclosure does not decrease at 20dB/decade (distance). Hot spots can occur 
far from the culprit transmitter, and are hard to reproduce or measure 
accurately. The field distribution is far from uniform, and the distance 
between peaks and nulls is small (a couple of inches at 1.9GHz).

--Tim (KR0U)

>"WX5L" <wx5l at charter.net>:
>Would someone in the know explain why technically the cell phones in the 
>headrest(Verizon?) Airphones don't cause the same problems as customer 
>owned cell phones. Why are they immune from causing RFI to the aircraft 
>and also from causing massive interference to cell sites below?

More information about the RFI mailing list