In a message dated 1/25/2009 3:11:55 A.M. Eastern Standard Time,
europa-list@matronics.com writes:
>>>>>For sure the peak power is up to 250 watts but the mean power is very
low.
If you worry about that consider your cell phone working at 900 Mhz or
so, its in your shirt pocket most of the time responding to updates from
the base station every so often, and then you get it out and place it
next to your ear and make continuous transmission's (its true duplex
operation !) for several tens of minutes. And if you are on the fringes
of a cell the power is ramped up to maintain communication.<<<<<
Hey Tim,
You are correct that transponders pulses are short bursts. You are also
correct that your cell phone does update periodically from the overlaying
cellular system and does in fact transmit and receive from the controlling cell
routinely, regardless of whether or not you are making a phone call. Often, you
will hear the update happening on your car radio speakers, for incidence. This
updating is done to keep track of where the mobile is in the system and to
verify the mobile's authenticity and registration within the system, among
other reasons.
However, you are incorrect about "true duplex operation" of cellular
telephones. This is my primary area of expertise with RF and one with which I
am
intimately familiar. Cellular, PCS and SMR operate half duplex. The switch
between transmission and reception on your phone happens extremely fast, thus
is
usually imperceptible to your ear in most cases. Occasionally, though, it is
perceptible if you listen closely. Ever noticed how two people talking at the
same time on a cell to cell call can't hear both sides of the conversation as
you would, say with a land line? Very often you have to repeat what was just
said if two people talk at the same time on a cell to cell call because only
half of the audio of the conversation is being transmitted. This happens
because your phone is operating half duplex, not full duplex. This half duplex
operation done for a variety of reasons, but the primary reason these days is
to extend the battery life of your mobile. This scheme has been used since
day one in the USA when the original cellular protocol was implemented in the
mid 1980's. I would imagine it is the same in the UK since USA based GSM
mobiles will work in your country provided a local SIM chip is installed.
Another point to consider is your hand held cell phone operates at 6/10ths
of a watt at it's maximum power level. The "ramping up" you mention is
referred to in the industry as power stepping of the mobile. In urban areas
where
you are in close proximity to the controlling cellular base station indeed your
phone will often operate at considerably lower power settings, then step up
to higher power levels as either you move farther away from the cell, or as
your mobile's signal as "seen" from the controlling cell drops below
predetermined levels because of physical or atmospheric issues blocking or
partially
blocking your phone's signal. With some digital transmission schemes,
particularly GSM and CDMA, power stepping is also employed in areas of dense RF
saturation.
With this in mind, and coupled with skin effect, which is the tendency for
weak signals to pass around your body rather than through it because your body
is made up primarily of water, you are in no danger of ill effects from a
cell phone's non-ionizing radiation, even when held next to your head.
Remember, it takes 600-1000 watts of highly concentrated microwave energy to
pop
a
bag of popcorn or defrost tonight's roast for dinner. Your cell phone radiates
omni-directionally, so only a small portion of the weak signal being
transmitted is directed towards your head when in use. Skin effect causes that
weak
signal to pass around, not into, your head.
Transponder antennae, on the other hand, in close proximity, say closer than
two wave lengths, do, IMHO, pose a significant RF hazard when the exposure
occurs repeatedly over a long period of time, even with the short burst
duration. This is particularly true for those of us, like myself, who are
routinely exposed to high levels of RF. While the nominal power output of a
typical
transponder is around 250 watts, peak power is usually rated in the two
kilowatt range and this does not take into account the gain of the antenna.
Granted, these are indeed short bursts, but why expose yourself to this power
level
unnecessarily, even for short bursts? (unless of course, you are beyond child
bearing years or have no desire to father children) If you keep the
transponder antenna at least two wave lengths away from your body roughly 96% of
the
energy is dissipated within this distance. Skin effect takes care of the rest.
There are certainly more pressing issues to consider when building and
flying our wonderful little birds. I didn't intend to sound as if being exposed
to
1090 Mhz transponder signals will cause you to grow extra appendages. It
certainly won't. However, unnecessary exposure to RF certainly should not be
ignored, Repeated exposure can damage your retinas, among other things. I know
several long time HAM radio operators who wish they had heeded the warning. In
this day and age we are all exposed to a wide variety of RF. In fact, most
people would be very concerned if they knew first hand what they are exposed
to in their day to day lives. In my humble opinion, there is absolutely
nothing wrong with being cautious.
Regards,
John Lawton
Whitwell, TN (TN89)
N245E - Flying
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