When you create a new ad unit in AdSense control panel –Google gives you an option of specifying what it should do if there are no targeted ads available. The default option is “Show blank space”… but it is much better idea to use backup ads instead of just blanking the precious ad space. Why to leave it blank if you can show something else there?
Backup ads are images created by you. You create an image and upload it on your website. Then, while creating a new ad unit, you should choose “Show other ads from another URL” option instead of “Show blank space”… Google will ask you for a URL where from it can fetch an alternative ad if it does not have any other ad to show. You should provide full URL of the image that you have created. For example:
Remember, you should create different images for different types of ads. The size of the ad and your image must match. If you are creating a 350×250 pixel ad then you should give a URL that is pointing towards a 350×250 pixel image.
Note: THIS POST BELONGS TO OUR GOOGLE ADSENSE BIBLE (a must read for AdSense Success aspirants)
Moreover, you can also give URL of an HTML page whose content shall be shown if there is no relevant ad available. If you plan to give a link in your HTML page; make sure you use “target=_top” because Google serves its ads in an iframe. When your HTML page will be displayed inside iframe the link should load in the top window.
Backup ads are very useful especially if your pages do not attract much of targeted ads from Google. Such pages most show blank ad space. Such blank space can be cleverly utilized to your advantage through backup ads.