Target Market
Marketing without defining your target market whether you are using free advertising or paid advertising is an utter waste of resources. No matter how amazing your product or service is, there is no way it can appeal to every person out there. That is just reality. You must break your target down into a definable and, most importantly, reachable group.
Now I know, you're likely worrying that if you don't try to reach everyone who could possibly be interested in your business, that you will lose out on sales and opportunites. The strange thing about defining your target market is that you make more sales in less time than if you don't define it. I could go into a long explanation about why that is, but the truth won't change. You can check this out amongst any successful business, online or off and consult as many experts as are out there. In the end, if your desire is to succeed without wasting time or money - you'll come to the point where you will know that defining your target market is essential, not optional.
With radio advertising, you need to know who your target market is so that you can choose the right station in your market. For example, you likely wouldn't want to be advertising dentures on a hip-hop station. It just woudn't make sense.
With internet radio or podcast advertising, it's even more important to have a clear idea of who your target market is. Most podcasts appeal to a smalll niche of listeners. If you're selling a product that is meant for moms, advertising on a podcast for motorcycle enthusiasts isn't likely to bring you the results you desire. That's not to say that no moms will be listening to the show, but the laws of numbers and averages will be working against you rather than for you.