Help with an invisible fence circuit?

Get off your lazy ass and use a push mower.

Put a peg in the middle of your yard and tie a rope from it to your power mower. Makes a nice spiral. This will likely mulch similar number of small mammals as the RF one you propose.

Does not have to be RF, one or two kHz AC will work just fine. The real trick is in the sensor, you have to sense approximately which direction you "hit" the wire from in order to "bounce off" the field.

I would go crude on this and use 3 open-core inductors mounted in a triangle pattern with experimentally-determined-spacing as sensors, amplify & threshold and then use a lookup table to perform an action based on what combination of sensors triggered first.

In article , Frithiof Andreas Jensen wrote: [...]

To extend:

The amplifier should have a filter or two as part of it. You don't want anything below the signal frequency getting through. There is likely to be a fair amount of 60Hz and 180Hz (NA assumed).

You also want to prevent the local radio stations from getting in. This should be a fairly simple low pass.

You can series resonate the loop with a largish capacitor so that the power amplifier driving it see nearly a pure resistance.

An autonomous, engine-powered machine with whirling blades? Better put up a good wall, so that it doesn't eat the neighbor kids.

Good Luck! Rich