Something interesting happened in the latest (Spring 2011) Arbitron Radio Ratings for the Wilmington, DE radio market.  The #1 and #3 most listened to radio stations in Wilmington weren’t from Wilmington.  They were Philadelphia stations, sending their signals 30 miles south to shake up the radio community.

Urban AC WDAS-FM was #1 with a 8.3 share of all measured radio listeners.  Wilmington Adult Contemporary WJBR-FM ranked second with a 7.8 share, followed by Philly heritage rocker WMMR-FM with a 5.5 share.

So how did this happen and what does this mean for advertisers?  First off, this phenomenon is not unique to Wilmington and Philadelphia.  Many medium and large markets sit in the shadows of a major market whose stations infiltrate their market share.  San Francisco and San Jose.  Miami and West Palm Beach.  Boston and Providence.

The reasons why people tap into out-of-market radio stations is as simple as one might imagine.  There’s something unique about the out-of-market programming content that simply isn’t available in their home market.

Wilmington’s hometown radio stations don’t include an Urban AC, so WDAS-FM out of Philly fills that available programming niche.  San Jose sports fans get their SF Giants fix by tuning in to SF’s KNBR, flagship station for the World Champions of baseball.  Or the reason might just be that a nationally-syndicated talk show host is on the air in one market, but not the other.

The challenge is how to effectively buy the smaller market without paying the high rates of advertising in the larger market.  For example, WDAS-FM may be #1 in Wilmington, but the overwhelming majority of their listeners reside in the eight-county Philadelphia radio metro – none of which include Wilmington.

So how do you tackle these problems?  Here are a few ideas and idea starters.

• Take advantage of the county-by-county listener breakdowns that are available to Arbitron ratings subscribers.  Ask the station to share data on which dayparts score highest in the neighboring market.  You can minimize waste by maximizing your effective spend.

• Consider a test on the big city signal’s station online.  You’ll get a much lower rate and be able to leverage banners and other digital assets to help make your campaign work.

• Look into Personalized Radio Services like Pandora.  If a market (like Wilmington) is missing a key format (like Urban AC), chances are there are a lot of users of Pandora, Slacker (and others) who type in an Urban AC artists’ name to create their own Urban AC stations.  Here you can leverage your :15 or :30 audio ad with online and mobile banners and “click to action” tools that connect you with the demo you’re targeting in precisely the zip codes where you want to reach them.

Radio continues to blur the lines of market and marketplace.  Local becomes regional, while the Internet makes local stations international.  Keep asking questions of your agency, your consultants and your network of friends in the media.  The rules may be changing, but these new media platforms often bring greater opportunities and results than the options of just a few years ago.

Mark Lipsky is the President and CEO of The Radio Agency. Please follow The Radio Agency’s Blog “Sounding Board” by subscribing to the email or RSS links above. Visit our website TheRadioAgency.com