Ù�Easy way to defense index

What is the way to increase the defense index with the possible lowest cost?

Purchasing weapons.

ammunition has no effect on the index.
Also, military units do not count in the index.
They should but currently they do not.

Thanks
but all weapons have the same effect on the defense index?

From what I recall, the most efficient ways to raise your D index in terms of costs are radar planes, missile interceptor batteries, and interceptors.

Note: Batteries and interceptors use ammo on a monthly basis for training - so you'll need to make sure you're stocked up if you buy some of those.

In general, more expensive weapons have more effect on the index than simpler ones.
The defensive index counts for 50% of the defense index.
The offensive index and the strategic index count for 25% each.

It seems the E O studied the issue.
The are differences and it could be that some weapons deliver more in terms of index points.