Transponder antenna radiation

[[email protected]]

My transponder installation instructions suggest to place the antenna
over 6 feet away from human body to keep the radiation away. I have a
composite fuselage and use an aluminum disc with the standard quarter
wave antenna. If I place the antenna and disc below me so the antenna
is on the side of the disc away from the body but not 6 feet away, is
that OK?

[Ron Webb]

Makes no sense to me. I've never seen anything in any transponder book about
antenna mounting with regard to RF personnel hazards. Any reasonable danger
zone would be much less than 6 feet.

To answer the question, mounting the antenna straight below you so you are
in one of the nulls of a quarter wave ground plane antenna would reduce
exposure a lot (a factor of 100 or so is sometimes used for other purposes).

The average power output from a transponder is variable according to how
many interrogators it is replying to at any one time, and what code it is
replying with. Even assuming an environment like Oklahoma City, where there
are 20 or so interrogators running, thanks to the FAA training academy, the
airport, the air force base, and maybe other stuff. The frequency in use is
1030/1090mhz. Assume 100 Watts peak power and a 1:1000 duty cycle for each
interrogator. The average power would still be only around 2 Watts. The
antenna gain is a maximum of +2 dBi. That gives a power density at 2 meters
(~6 feet) of 2W/(4*pi*(200cm)^2)=4 microWatts/cm^2. That's one 250th of
what's considered safe. (~1MilliWatt/cm^2)

As a comparison, your cell phone is close, in both power and frequency, yet
you think nothing of pressing it against your brain, and talking for
extended periods.

These are back of the envelope calcs, and I don't guarantee them even a
little, but if I were to go flyin' with you when your airplane is done, The
transponder would be on page 4 of my list of things I'd worry about...








> wrote in message
oups.com...
> My transponder installation instructions suggest to place the antenna
> over 6 feet away from human body to keep the radiation away. I have a
> composite fuselage and use an aluminum disc with the standard quarter
> wave antenna. If I place the antenna and disc below me so the antenna
> is on the side of the disc away from the body but not 6 feet away, is
> that OK?
>

[[email protected]]

Thanks for your reply. Appreciate the length you went to.
My plane is already flying since 2004 but am thinking of moving the
transponder antenna from under the main wing closer to the front to
cut down on the cable length/weight and have a clearer shot at ATC
with the transponder and probably
get more radiated power out of it due to shorter cable. The
transponder is rated
at 200 watts.
--------------------------------------------------------------------------
SQ2000 canard http://www.abri.com/sq2000

On Mar 10, 9:08 pm, "Ron Webb" > wrote:
...... but if I were to go flyin' with you when your airplane is done,
The
> transponder would be on page 4 of my list of things I'd worry about...
> ...

[COLIN LAMB]

If the antenna is under you, and it is vertically polarized, then you need a
ground plane. That can be a metal disc 1/2 wave in each direction from the
coax. 1/4 wave is the distance it is tall.

I made a plate about 3/4 wavelength and that should reduce radiation
substantially.

I was at a meeting regarding ham radio antennas. Some woman testified that
she was afraid the ham radio antenna down the street was frying her brain.
Her proof that radio waves were bad was that most of the ham operators had
gray hari. During the break, she went outside, lit up a cigarette and got
on her cell phone.

Colin

[COLIN LAMB]

I see I had a typo. The ground plane (disc) must be 1/4 wave in each
direction minimum. Greater diameter can be used, but is not necessary for
proper operation. It may be helptul in reducing readiation above the
inverted antenna.

Colin

[RST Engineering]

That's not quite true. It has to be an odd integer multiple of a quarter
wave to operate properly...that's 1/4, 3/4, 5/4, and so on. The antenna
itself is a quarter wave long, so if you want to use that as a template that
would work. A quarter wave in air is about 2.75 inches long.

Jim


"COLIN LAMB" > wrote in message
ink.net...

>I see I had a typo. The ground plane (disc) must be 1/4 wave in each
>direction minimum. Greater diameter can be used, but is not necessary for
>proper operation.

[COLIN LAMB]

The ideal ground plane should be perfectly conducting and infinite
(according to John Kraus in his book Antennas). It is true that the
"measured impedance of a stub antenna varies appreciably as a function of
the ground-plane-diameter", but the variation diminishes as the diameter is
increased. Large square ground planes result in less variation than round
ones.

From the standpoint of radiation shielding, the larger plane should be
better than small ones.

Colin