Hello Bozet and welcome to the forum.
Unfortunately Microsoft doesn't do "free" when it comes to their Operating Systems. Actually they seem to have a particularly strong aversion to free with respect to most of their products.
As Jim has pointed out above they do offer a pretty large range of evals (sort of a try before you buy deal) but they are all time bombed with an expiration date that can vary from 30 to in some cases 180 days. So in most cases these are not generally viable options for the typical home users.
In the end you're likely going to have to shell out some bucks, how much will likely depend on how good a shopper you are and what deals you can find. Just beware that if it seems too good to be true, then it likely is.
Being a retiree on fixed income I can certainly appreciate your position. I just want to say that I'm sure Pat intended no insult or offense. If you cruise this and other forums for long enough you'll likely see that a new poster, asking for a free copy of anything but especially an operating system, never, ever, ends well for anyone and can even sometimes result in claims of DMCA violations by the software manufacturer against the site facilitating such a thread and that is why everyone is a bit on egg shells about discussions that might potentially lead to posts that might be construed as software piracy discussions.
Thank you for your understanding and for taking the time to join our community.
Regards
Randy