GARMIN20.TXT      INTERFACING THE GPS-20 TO APRS
===========================================================================
Document version: 8.3.5
Document dated:   10 Mar 99
Author(s):        Bob Bruninga, WB4APR <bruninga@nadn.navy.mil>
ABSTRACT
GARMIN20.TXT      Interfacing a Garmin GPS-20, and making a patch antenna.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


     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.

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

ANTENNA:  Just scale any favorite antenna to 1575 MHZ (1.8" quarter wave).
CAUTION:  The antenna jack has +5 volts on the center pin, Dont short it!


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.  The sleeve 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.  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...