On 22 Dec 2004 03:53:39 -0800, (icelord) wrote:

(whip antennas are out of the question, since the suit can't
have any holes).

Hi OM,

I don't see what holes have to do with your problem unless the suit is
not of fabric but made of laminar plastic, say (very uncomfortable if
so).

The main problem is the human body effect and the
size constraints. Gain is also a major requirement since the range of
the operations will be 400 meters with line of sight.
(Server side: 10 dBm Tx Power and a 6 dB whip antenna, Suit side: -96
dBm sensibility- freq:433 MHz)

Size is not much of a constraint (for a dipole size of 34 cM). 400
meters is not that far for 10dBm (neglecting the "6 dB whip antenna").
Sensitivity suggests you can tolerate 106 dB path loss which would not
be found in 400 meters distance (about 80dB path loss, less with "6 dB
whip antenna"). This leaves something like 20 to 30dB headroom before
you consider body proximity and its attendant loss.

I've also found a
dipole body worn antenna ( http://www.panorama.co.uk )., but I'm not
sure of its performance.

It is simple enough to build and test certainly. There is absolutely
nothing remarkable about it, and the molded plastic T contains only
wire connections of the two conductors (coax) coming to it. Given
your constraints, you don't have much choice in how it performs. If
it doesn't work, you will have to move either the suit closer to your
server, or the server closer to the suit (or increase
power/sensitivity).

Currently I'm trying to develop my own
antenna, which consists of a pcb small loop antenna. I'm asking for
sugestions and maybe antenna references for this type of application.

A small loop is not going to give you gain and putting it on a PCB is
not a solution with any advantage. Such claims to the contrary
(should you encounter them in the marketplace) are merely advertising
hyperbole with no particular merit. The dipole your link leads to is
the common reference against which all antennas are compared to and
any loss suffered by its proximity to the body will be suffered by all
others equally from the identical problem.

If this loss is intolerable, you must put some distance between it and
the body, but aside from your "pack," you have painted such solutions
out of the picture through your constraints.

As an aside. Your biggest problem will be with multi-path reception.
The problem here is multiple reflections between and around the two
antennas that will cancel the signal through any movement of your suit
across a quarter wavelength (17 cM or so). This could be a mild
problem, or it could be severe, or it could go unnoticed. This is all
a function of reflective surfaces within that same range. The
solution is multiple antennas performing in what is called "space
diversity." Research this term as a key phrase in combination with
the key word antenna. The rules are very simple and still within your
implementation.

73's
Richard Clark, KB7QHC