Chicago, Sept. 4 (BridgeNews) - The Federal Reserve Bank of Chicago reported its national activity index improved in July to a reading of -0.50 from a revised June level of -1.10. The monthly index, a weighted average of 85 indicators of national economic activity, is constructed to have an average of zero and a standard deviation of one. The three-month moving average index also improved, at -0.82 from -1.02, but was still below zero for the 13th consecutive month, indicating the economy has been growing below trend for more than a year.