What a beautiful cake to behold! And so tasty with so few calories.
Preheat oven to 375°F. Double check that your tube pan is clean and dry. (Any trace amounts of oil could cause your whipped egg whites to deflate).
Stir flour, 1/4 cup of the sugar, and sugar substitute together in medium bowl and set aside.
In a large mixing bowl, whip the egg whites with the vanilla, cream of tartar, and salt together with mixer until medium-stiff peaks form. Gradually add the remaining sugar (3/4 cup) while continuing to whip to stiff peaks. When you think the egg whites have reached maximum volume, gradually fold in the flour mixture, one third at a time.
Stir in 1 cup fresh cranberries or 1/2 cup dried cranberries and orange zest. Be careful not to overmix.
Pour the batter in the prepared tube pan and bake 40-45 minutes or until cake springs back when touched. Balance the tube pan upside down on the top of a bottle to prevent decompression while cooling. When completely cool, run a knife around the edge of the pan and invert onto a serving plate.