By Chad Finn, Boston.com/Globe Staff

An oft-cited reason for WEEI’s recent struggles in its sports-radio showdown with 98.5 The Sports Hub is its lack of an FM signal.

As of Monday, that reason will no longer exist. And neither will Mike FM.

Industry sources have confirmed that Entercom, WEEI’s parent company, will announce tomorrow morning that WEEI (850-AM) will begin simulcasting on WMKK (93.7), better known as the pop-rock station Mike FM.

Entercom, WEEI’s parent company, owns four stations in the Boston market: WEEI (850), WMKK (93.7), hard-rock WAAF (97.7/107.3 FM), and talk station WRKO (680 AM).
WMKK’s pop-music format -- its slogan is ‘‘We play everything’’ -- will not move elsewhere on the dial.

While WEEI executives have occasionally denied that a simulcast or outright switch to FM was imminent, the news is not a surprise. The Sports Hub’s advantage in having a potent FM signal has often been noted by its competition as a significant factor in its rise up the Arbitron ratings since launching an all-sports format in August 2009.

Entercom’s decision to implement the simulcast now comes two months after The Sports Hub’s sizeable victory in Arbiton’s spring ratings book. The Sports Hub earned an 8.8 share, finishing first in the coveted men 25-54 demographic. WEEI-AM was tied for sixth (5.1), but moved up to fourth (5.6) when the Boston share from its Providence-based FM station was included.

WEEI’s move is similar to a recent one made by WIP in Philadelphia, which began simulcasting on an FM signal in mid-August. WIP, coincidentally, is owned by CBS Radio, which also owns The Sports Hub.

Entercom said in a press release last night that it will have a major announcement at 8 a.m. tomorrow morning on the ‘‘Dennis and Callahan Show.’’