The thing is Mary, what will be the purpose of the alarm ? To scare intruders away ? To attract the attention of neighbours or passers by ? To bring to your attention that you have intruders ? People take very little notice of alarms these days.
Unless the alarm is going to have a worthwhile effect, you're wasting your money. You'd be better off making the property secure by installing more secure deadlocks and deterrents like high fencing, barbed wire, prickly bushes, non-drying paint, gravel on paths and drives etc.