> I'm transmitting data from PIC to PIC via separate 433MHz rx and tx
> boards.
> Does anyone know a source of antenna relays that can be switched
> rapidly, say two or four times a second, and have a long life span at
> that rate?
> Is there any such thing as solid sate relays that would do this?
> Power is lo, 2mW.
> Regards Howard.


This may be overkill but a small reed relay would work well - as would most
small relays.
Reeds may be purchased bare and have suitable coils added (Jaycar sell some)
or small complete reed relays are available. These have very fast switching
times and will handle power levels well above what you need. Using a double
pole changeover relay would allow grounding of the receiver input during
transmit.

You could also make a PIN diode TR switch (see amateur radio handbooks for
details) - these should be very easy at the power levels you are dealing
with. I suspect that using 1N4148 diodes would work OK.
Basically these consist of two diodes in series with the transmit and
receive lines with the eg cathodes of both diodes facing each other .
Positive DC is fed to the two anodes via RFCs. RF is also fed to one diode's
anode and the aerial connected to the other anode.
When the centre points (two cathodes) are not connected to any other device
the diodes block and the switch is "off". If you now ground the centre point
via an RFC BOTH diodes conduct DC and the RF can flow through BOTH diodes.
The DC bias voltage has to be in excess of the peak RF voltage (easy in this
case) so that the diodes are not placed into reverse blocking by the RF
voltage. For better isolation it is also possible to shunt the receiver
input to ground with a similar arrangement during transmit. With such an
arrangement it is possible to make receivers that can listen BETWEEN the
Morse code dots when sending Morse - this gives some idea of the achievable
response times.





        Russell McMahon

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