Google has reached an agreement with broadcaster Clear Channel Radio to power the station's radio ads. 

The deal will give Google the ability to broker ads for over 675 radio stations with its Google Audio Ads system.

The 30-second advertising spots will be offered to retailers through Google's AdSense programme, and delivered directly to the stations through content management software.

Google and Clear Channel hope that the new system will open the door for online advertisers that had not previously considered radio advertising or had been unable to advertise in local markets outside their own.

The move also marks the second expansion of Google's advertising platform in as many weeks. The search giant announced plans earlier this month to sell ads on the DISH Network satellite TV system through AdSense.  

Google has been trying to move into the radio advertising business since the purchase of dMarc in January 2006.  

Online advertising has always been the backbone of Google's business model, accounting for virtually all of the company's $10.6bn in revenue last year. Expanding to radio and TV offers a way to diversity the company's revenue stream.

Google has also been expanding its traditional advertising system. Last Friday the company announced that it would spend $3.1bn to acquire banner ad firm DoubleClick, the largest acquisition in Google's history.  

Clear Channel is the largest radio company in the US, claiming over 110 million listeners. The company also operates TV stations and billboards, which will not be part of the Google programme.