Western Australia FOR
Aha! I'm in the southern province on the other side of the dotted line or "rabbit fence". So we can talk about what's available (and legal) in OZ
I guess one of the prime requirements is simplicity of operation as well as reliability over the given signal path.
MJC2's suggestion is a good one, and some of the Uniden series of home phones in Australia have this feature. I'm a bit nervous about a reliable 50+ metre range - it may be OK if there isn't a lot of material (walls fences etc) between the two units, however it needs to be tested. We have
these here - I'll do a bit of a walk-around when it cools down tonight and see what might be expected in a suburban situation.
UHF CB can be an option - 80 channels are now available, and very low power units running CTCSS on one of the new channels might be OK - freedom from intrusion by others is a reasonable expectation under that option, however others would be able to hear your conversations and that's probably ruled that option out. A pair of
these on low power and using one of the upper 40 channels will meet the communication options with ease, however as stated, others will be able to eavesdrop. Voice scramblers appear to only be available on some of the advanced mobile transceivers and these become expensive and need power supplies and antennas.
There may be other options altogether but these are the two most immediately obvious ones for Australian usage. I'll give it some more thought and see what might emerge.
FOR
PS It's a bit hot around this part of the world at the moment
but I think you are probably well familiar with that .....