On Mon, 10 Aug 2009 16:41:08 +0000, Rob wrote:

Stuart Longland wrote:
Ahh right... I had a look around, looks like Minikits[1] sell the power
modules (and even a PCB). This looks promising.

Maybe with some more searching you can find PCBs that include the tx/rx
switching, preferably with PIN diodes. That will save you a lot of
money and construction effort.
(with a bare amplifier PCB as shown on that site, you will need relays
and a circuit to drive them when transmitting, in addition to the
amplifier PCB)

The handheld can be wound right back to 50mW when running off
batteries... otherwise 0.5W. A pi network isn't hard though. That
said... I won't make the mistake I made last time -- last time I tried
making it with 5W wire wound resistors... and wondered why it didn't
work.

It is better not to rely solely on lowpower settings of the handheld, as
sooner or later you'll forget to set it to lowpower, it will drive 5W
into the module, and destroy it.
So it is best to have some attenuation in front of the module, such that
it will survive accidental input of the full power of the handheld. You
can then still use lower power in normal use, so the handheld will not
run too hot. Find the maximum allowable input power for the module and
design the pi network so it will attenuate the highest output power of
the handheld sufficiently. Then calculate the nominal power to the
module (for driving it to e.g. 90% of its max output) and use the
handheld at that power level. That could be 0.5 W or so.

If one were very clever one may be able to make the last leg of the
attenuator a pair of back-to-back diodes that would clip at a moderately
low level, protecting the module while still allowing it to work when you
_do_ set the radio to low power.

I'd have to work through the details to see if it's feasible, but it sure
sounds cool.

--
www.wescottdesign.com