antenna placement
In a message dated 7/16/2007 3:01:58 AM Eastern Daylight Time,
europa-list@matronics.com writes:
So, we're talking about an average handful of milliwatts here rec'd by our
body.
Like trying to heat food in the microwave by turning it on for just 2 seconds
each hour. In a week, it will still be room temp.
Hello Fred,
You are absolutely correct here. It should also be noted that microwave
ovens use a specifically designed antenna in the roof of the oven to
concentrate
and focus their energy directly into the center of the oven in order to heat
whatever is being heated. The bottom of the oven is a reflector of sorts that
reflects any energy not absorbed initially by the object being heated back
into that object. This focusing of energy and reflection of unabsorbed energy
overcomes "skin effect" of the object being heated. This heating effect by
direct RF exposure was first noted by engineers at Ratheon back in the late
1930's when RADAR was being developed for the military and the first commercial
microwave ovens that were sold by Ratheon in the 1950's were called "Radar
Ranges".
Skin effect, in the case of, but not limited to, living or once living
tissue, is a phenomena that occurs primarily because of the water content in
whatever is being subjected to the radio frequency (RF) radiation. Lenz's Law
states that the density of current in a conductor (including your body) is
least
at the center and increases towards the outer circumference of the conductor
at radio frequencies. RF energy tends to go around bodies with large amounts
of water, iow, the path of least resistance, particularly at higher
frequencies. Hence, the term skin effect. Above about 30 Ghz the rules change
some,
but I won't go into that here. This skin effect is also the reason why no one
has been able to consistently show, and never will show, a link between
non-ionizing radiation and the formation of cancer, despite the trial lawyer's
best, most tireless efforts to prove a link so they can make lots of money.
The amount of RF penetration into organic tissue is primarily dependent of
the power level transmitted, the distance from the antenna and the focal
point, in any, of the antenna. In the case of a microwave oven heating a piece
of
meat, what actually happens is the focused energy heats the water in the meat
tissue, which in turn heats the surrounding tissues, That's why it takes
1000 watts of concentrated microwave energy centered in the focal point of the
antenna to defrost a turkey for dinner or pop a bag of popcorn. It's also why
you can turn a good steak into shoe leather if you let it go too long in the
microwave.
By contrast, your cell phone operates at 6/10ths of a watt at it's highest
power setting (when located farthest from the cell) and radiates
omnidirectionally, and thus there is very little penetration, if any, into your
body.
Similarly, transponders, which typically pulse in "bursts" of around 250-300
watts, broadcast their energy from the antenna, usually a vertically polarized
dipole, in all directions. The pattern of radiated energy is determined by the
type of antenna used. In the case of a vertically polarized dipole the
pattern resembles a bow tie stood on end. Skin effect, coupled by the short
duration of the burst of RF, plus the pattern of radiation, results in very
little,
if any, tissue penetration of RF from your transponder antenna unless you are
sitting on it or happen to inadvertently swallow it.
Without loosing everybody in the math, it's also notable that, due to the
physics of radio frequency transmission in the free space of our atmosphere,
roughly 96% of the radiated energy is dissipated in the first two wave lengths
---From the source of the radiation, i.e. the antenna. The wave length is
roughly determined by the speed of light divided by the frequency of operation.
There are other minor factors that determine specific wave length and before
anybody jumps up and down, please note, I am ignoring the effects of modulation
on wave length and field strength here for simplicity.
In the case of a 1090 Mhz transponder, the peak to peak wave length is about
11 inches (10.83"). So, if you have your transponder antenna more than 11"
away from your body, you aren't gong to receive enough RF radiation to make
any difference in your reproductive efforts, so don't count on it as a method
of birth control. Similarly, to minimize shadowing, you should locate your
transponder antenna at least 11" away from reflective (metallic) objects for
best results. 3 wave lengths is even better.
FWIW, I put my transponder antenna on the right side of the fuselage, just
in front of the joggle at the tail section. Fiberglass is invisible to RF. In
fact, most commercially available microwave radomes (covers that shield feed
horns from bird poop and ice falling off the tower) and most antenna masts
are made of fiberglass for that reason. Therefore, there is no reason to locate
any RF antenna on the outside of a fiberglass airplane unless directed
(coerced) by your gubmint to do so.
I used an Antenna Specialists dipole antenna. I shaped a piece of balsa wood
to fill in the curve of the inside of the fuselage, bonded the balsa to the
fuselage side with Redux, then bonded the antenna to the balsa with Redux. I
have not had any ATC report any shadowing effects whatsoever and, at
altitude, I've been "seen" as far away as 80 nm from the radar transceiver. It
did
take a rather expensive ($225) 14' piece of precisely tuned, mil spec,
precision coax, with precision connectors, to achieve the maximum of 1 dB of
cable/connector loss (as required by Garmin) to locate the antenna where I did,
though. My primary reason for locating my transponder antenna where I did was
more to get it away from other devices in the cockpit that it might interfere
with, and several wave lengths away from the COM antenna, not to prevent the
pilot and copilot from receiving RF radiation from the transponder antenna.
After commissioning over 7,000 cellular telephone sites in 16 countries,
along with about 1800 co-located terrestrial microwave sites, and about 25
years
as an amateur radio operator, if RF radiation were going to make anybody
glow in the dark it'd be me........and I still have to use a flashlight (torch)
to find my way in the dark......
Regards,
John Lawton
Whitwell, TN (TN89)
N245E - Down for annual inspection after finally receiving my Repairman's
certificate 9 months after application to the FAA!
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