Karmarama has been briefed to raise the profile of Heart and Capital, nationwide and in London respectively.
Global's last heavyweight push, which ran earlier this year, backed the expansion of the Heart network to 33 stations. It was created by Beattie McGuinness Bungay.
Separately, Global has appointed Social UK as the retained branding agency for its radio brands and charities. It had previously worked for Global on a project basis.
The review was led by marketing director Giles Pearman, who replaced Nicola Thomson in June.