Antenna Performance

An 868MHz project I am involved with uses a Xemics 1205 Tranceiver
taken from a reference design. The output of which is fed to a PCB
mounted SMA connector at the end of about 1.5" of PCB trace. The
trace was calculated to be 50Ohms but not measured although a
transmission test from the point of the reference design and the SMA
connector shows a 2db drop.

Unfortunately the whole thing is placed in a metal box with the
Antenna passing through the box onto the board. The aerial
manufacturer has told us that most energy is emmitted in the box and I
guess we also loose receiver sensitivity. Performance wise we set
the transmitter to transmit at 15db and get a an ERP of .2mw remote
devices with a PCB aerial in plastic cases have an ERP of

We decided to change the SMA connector to a bulk head mounted sma
connector, However the range now goes down from a reasonably
respectable indoor range of 25m+ (can't get further without passing
through a metal wall) to a less inspiring 4 feet. It appear that
adding 2-3" of coax with the SMA on the end is enough to do this. We
have managed to get about 20Feet if we use a Yagi aerial.

The Xemics part is a direct conversion device and produces IQ signals
to monitor. With the extra cable the signals look good with full
amplitude at 2 feet but at 4 feet either the I or Q signal vanish but
strangely not both as I believe the same input signal is mixed with
the local oscillator and a 90 degree phase shifted on to produce the
IQ signals.

Could the affects I'm seeing be a reflection of the local oscillator
off a poorly matched PCB trace or is there some other strange things I
should be looking for, apart from a good RF engineer to work it out
for me.

Many thanks for any help or pointers

Paul

.

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