TELEMETRY PROBLEM

I am currently trying to set up a low baud rate RF link between two PIC's
(a 16C74 and a 16C84) using 2 VHF 173.225MHz radio modules - a matching
pair from Radio-Tech in the UK (RXM-173-225-4666-60 and TXM-173-4689UK-03).
The application is for model rocket telemetry, although a 1-2km link probably
has wide interest to the readers of this list. I am using an encoder and
decoder between the PIC's and the RF units (M145026 and M145028 from SGS-
Thomson). These use a 5 bit address and enable a nibble of data to be tagged
on the end - the data is only presented after two successful transmissions
of the correct address (the address pins are hardwired).

PROBLEM:

I have managed to transmit data down a wire between the units.
However, I am struggling to do it via the RF link. When the RF unit is
powered up, the output data pins from the decoder all go high from an
initial low state. This shows that the RF has been picked up but the
data is wrong. When I listen to the audio output from the RX, the
tones cease abruptly when the TX is powered. I have ground planed the
circuits from a single point to avoid current loops and have minimal
lengths of unscreened RF wiring. To get the RF link working at all I
had to use a pull up resistor on the data output pin from the decoder
(4k7).

I would be delighted to hear from anyone on the list who can offer any help
or suggestions. It is particularly hard to debug this problem as RF diagnostics
are difficult (I can get access to a scope) and I am new to RF.

                      Waiting hopefully!
                              Steve Parker.