Re: terrorists use microwave guns against planes

Hi Christophe

> Well I did not want to fry something at this distance. I just thought
> if a mobile phone with 4 mW output can cause problems to the
> electronic of an airplane what could a 1.000 W emission do? And when
> it is focused with a parabolic dish on something it should cause even
> more problems provided that the distance is not too great.

>From a personal perspective I'm really glad that they do ban mobile phones
on planes! Nothing so obnoxious than someone so self important that they
feel the need to shout down the telephone so everyone else can hear their
conversation.

The reality is that you average mobile phone will pump out a lot more than
4mW in weaker signal areas, where a couple of watts is common.

The reason they don't want you to use your mobile phones on aircraft is that
there is a minute perceived risk. I would hazard a guess that most
commercial passenger aircraft flying around the globe have at least one
person who accidentally forgot to turn off their phone. (I believe there's a
company looking at a way to allow passengers to use their mobile phones in
the air - perish the thought).

One of the perceived problems in aviation of mobile phones is that some
unwanted mixing products in the aircraft electronics may upset the
instrumentation, giving unreliable or false readings. I believe that there
are a couple of documented cases of this happening, although nothing
crashed. Another vague possibility is that a cell phone may appear to be
working, but is slightly faulty, inadvertently radiating significant RF of
its own within the aviation navigation and radar bands.

There has recently been some concern over the expansion of the Band II
broadcast band in some regions, and its compatibility with the aircraft
allocation just above it. Band II is your usual FM stereo broadcast band,
with many 100kW transmitters.

You might want to consider the enormously powerful (1MW) terrestrial
transmitters used by TV stations across many bands throughout the world
adjacent to the cellular GSM and analogue 800MHz allocation. After all,
planes don't avoid these terrestrial TV transmitters dumping megawatts into
the atmosphere.

What's really important is that inverse square law applies to elctromagnetic
signals. Double the distance from an electromagnetic source and the relative
power will be one quarter. One thousand times the distance, one millionth
the power.

In order to achieve sufficient effective radiated electromagnetic power to
cause mayhem at a distance, it would be necessary to either use a very
directional antenna (for example, as you suggest, a dish) to concentrate the
power into a tighter beam or increase the frequency. So as an example let's
take a 60cm dish, similar in size to a TV dish. At 2.4GHz this has a half
power beamwidth of about 17 degrees. Even with a 1kW microwave oven
magnetron, within a distance of 12m the beam has already dissipated out to
safe controlled human limits (as per FCC EM limits).

Microwave weapons on the other hand would operate at higher frequency. For
example 95GHz would produce half power beamwidths of about 0.4 degrees for a
60cm dish - significantly smaller than the 17 degrees at 2.4GHz, and would
concentrate the same amount of power into a beam 2,000 times stronger than
at 2.4GHz. To provide the equivalent 0.4 degree beamwidth at 2.4GHz you
would need a 23.7m diameter dish. That's a very big dish. And we haven't
even thought about building the mechanics to point it at anything yet.

In contrast to the 12m safe distance for a 60cm dish at 2.4GHz, at 95GHz
you'd have to walk over 500m away from the antenna to be within safe
controlled human limits for the same 1kW power.

In conclusion I think that (a) a microwave weapon uses much higher
frequencies than that of a microwave oven and (b) probably uses quite a bit
more power too.

Kind Regards, Howard