Arendt is going to form 7West with Ricki Lee, co-founder of radio technology company G Media, in order to provide digital products to radio stations.
He will not be directly replaced at Planet Rock and Malcolm Bluemel, owner and chairman of Planet Rock, will resume the role of chief executive in March. Planet Rock's national ad sales are handled by Absolute Radio.
Arendt joined Planet Rock in June 2010 with a brief to help the station maximise its growth potential and build on audience gains as the radio industry moves towards a digital future.
Bluemel said: "Jonathan will be missed at Planet Rock. He has driven audience and revenue to record levels and overseen a complete re-image of the station. I wish him well and look forward to working with 7West in the future."
Prior to Planet Rock, Arendt was a founder of radio marketing and research agency Hallett Arendt. He was previously a director of London speech station LBC, alternative music stations XFM and NME Radio, CanWest Media UK, and a number of independent stations.
Arendt said: "The last 18 months have been brilliant, working with truly talented people and great rock music – what could have been better? Malcolm has big plans for the station and I know it will go from strength to strength."
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