> I was thinking of 433MHz with a simple decoder chip, similar to
> a garage door opener. The arena is enclosed with sheet iron
> panelling, and the closest parking is probably 50 metres outside
> which should limit interference from car remotes

I use the 433MHz ZW3100 and ZW3102 modules, originally
from Jaycar/Electus but now I get them in quantity from Taiwan

A 12F629 is used at either end to handle Manchester encoding of
a number unique to each pair. Time delay for this is about 200ms

200m line-of-sight is possible with a 1/4 wave (34cm) aerial in good
conditions. 100m is very reliable outside, even in heavy rain through
two wet buildings. Range is still OK with 1/16th, or even no aerial.

I've not experienced any interference problems in several years

Joe

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