GARMIN20.txt          INTERFACING THE GPS-20 TO APRS


     The GPS-20 is an OEM GPS circuit card which only needs three wires 
and an antenna to work.  Connect Ground, +5 volts (regulated) and then 
run the Serial data output to your PC, TNC, or other RS-232 serial device.  
It will power up and begin sending data immediately.  It is ideal for the
APRS MIC-ENCODER or other applications where you want a power-on startup.
Packaging is your option...


       --------------------------
       |       GPS-20 CARD      |
       |                      *-|---< pin 10   +5 volts
       |                      *-|---> pin 4    TXD to serial port
       |                        |
       |                      *-|---* pin 8    GROUND (common)
       --------------------------


ANTENNA:  Any HAM should be able to build some kind of antenna that 
works well.  Just scale it to 1575 MHZ (1.8 inch quarter wave).  
CAUTION:  The antenna jack has +5 volts on the center pin, and your GPS 
could be damaged if you short it out.  To see it work, stick a tiny 1.8 
inch piece of wire in the antenna jack, (Do NOT force a fat wire in and 
ruin your ant jack).  Take it outside you should have no problem getting 
a fix...  I use a piece of wire the size of a tiny signal diode.  Even
the leads on a 1/4 watt resistor are too big...


PACKAGING:  The reason you bought this unit was for flexibility in 
implementation.  Here are some suggestions:

   Put the GPS and antenna in a WX proof box, run three wires to APRS
   Put the GPS inside your TNC,  run an external antenna
   Put the GPS inside the APRS MIC encoder box, place it on your dash
       board, plug it into your 2 meter MIC jack.*

       * The MIC encoder is a complete TNC on a chip which will place
       your GPS posit in a 0.2 second tone burst on the end of your
       voice transmissions on ANY radio.  See VOX-RPTR.txt

NOTES:  1) It will take the card up to 20 minutes to get its FIRST ever
           fix.  From then on, it should take only a minute or two..

        2) The 5 volt power supply must be stiff (low impedance). 
           Otherwise, the GPS may not initialize on power up.  If 
           you dont get NMEA data within a second of turning it on,
           then dont panic, change power supplies.

        3) If you want to run both a TNC and the GPS on the same COMM
           port with APRS, see the GPS.txt file on how to build the
           Hardware Single Port (HSP) switch.
           
        4) The GPS-20 comes with a complete tech manual.  If you want
           additional information, refer to that document.  Also be 
           sure to read GPS.txt in the APRS files.

SIMPLE PATCH ANTENNA:

Here is how to take the GPS-20 card available from TAPR and integrate 
it into a single package with the patch antenna design in the OCT 95 
issue of QST.  The GPS-20 mounts directly to the bottom of the ground 
plane PC board (using standoffs).  A 1/8 inch piece of tubing and a 
3/8 inch piece of tiny wire plug directly into the antenna connector 
on the GPS-20 as shown in the following microscopic detail:

    patch antenna
    ------------------------|*|--------------------------------
    ************************\*/********************************
                           | * |                      |
                           | * |                      |            
                 pin ------->* |                     5 mm
                           | * |<--- insulation       |
    Gnd plane              | * |                      |
    ********************/| | * | |\****************************
    ---------------------| | * | |-----------------------------
                         |   *   |
            sleeve ----> |   *   |
                         |   *   | <- PLUG this into the GPS-20

                             
NOTES:  The copper side of the boards is shown with asterixs (*).

SLEEVE:  Standard 1/8 inch hobby brass tubing.  It is a *LOOSE* fit
and must be built up with a little solder.  YOu may even have to tack 
on two very thin strands of wire.  I took two strands of very fine wire 
from the shield of a piece of RG-58 coax and soldered one to each side 
of the sleve to get a good tight fit.  The sleve sticks out the bottom 
about 1/8 inch and will plug directly into the GPS-20.

PIN:  Use the lead from a tiny signal diode or transistor.  Even the 
leads from a 1/4 w resistor are too large.  If it is too large, then
you will stress the connector on the GPS.  Notice how the pin is flush
with the bottom of the sleeve.

INSULATOR:  Its only function is to center the wire in the sleeve.  
It can only stick into the sleeve about 1/16 inch , since the
GPS connector also has an insulating sleeve around its center pin...
Use a scrap of wire insulation and/or layers of heat shrink..

The under copper surface of the patch antenna will have 5 volts on it.
DO NOT SHORT THIS ANTENNA.  Once the antenna is assembled, the chance
of a short are minimal since the dielectric side of the patch sticks
up.  This reason this surface MUST be up, is so that there is only
air in the space between the antenna and the ground plane copper.  This
whole antenna (with GPS) forms the TOP cover of my APRS MIC-ENCODER box 
which will sit on my dash board and connnect into my 2m radio...
