Have news broadcasts in USA ever been subsidized by the federal government?

This depends on how you define "subsidize", but the tldr is: yes, and they still are.

The first obvious example is the Corporation for Public Broadcasting, which is a non-profit organization founded by Congress and subsidized by the Federal Government annually. The CPB provides about 1/6 of the funds necessary to run that large infrastructure of public broadcasting in the United States (the rest generally coming from private sources via other methods of fundraising). News Hour, shown on PBS, benefits from this public funding.

The second example is more of a stretch, but I would argue definitely qualifies. Airwaves in the United States are owned by the people, in the form of the government. The major broadcast networks (NBC, CBS, Fox, ABC, the CW, etc.) get free access to these airwaves - estimated to be worth some 500 billion dollars - in exchange for their providing content, at least some of the time, that is deemed to be in the public interest. News programming, arts and cultural programming, educational childrens programming, etc are the primary examples of said public interest programming. Given the cost of producing said programming versus the value of the usage of the airwaves, it could easily be argued that this is a subsidy by the government to these private companies. Now the percentage of US households who use over the air TV is steadily decreasing, but none-the-less, the subsidy remains in place.