"RJ" wrote:
We'll be moving to southern Arizona.

I'd like to continue MW-DX
without interference from Mexican AM transmitters.

Is there any way to set up an outside antenna
to receive "from the north" only ?

Not easily. A phased vertical array with a cardiod pattern could give a
sharp null in one direction, but is not practical for a backyard
installation.

For loopstick radios,
I was thinking of a grounded metal plate
on the back of the radio....

Remember, the signal is coming in off the ionosphere at a pretty high angle.

I don't think a loop would work for this application...

I think it might if you could steer the loop to notch out the offending
station.

Art N2AH