It took some time but Google finally bellied up to the bar: The search giant's move to offer banners for mobile extends the life of an ad unit that refuses to die.

As reported last week (see story), Google AdWords for mobile banners will be just like PC image ads, only smaller to fit on mobile screens. The banner ads will be keyword-targeted, priced on a cost-per-click basis and link to a mobile Web site.

It is validation of a unit that has been much derided but underrated for its branding and direct response potential.

The banner ad is about 15 years old -- some may say even older -- and it has essentially looked the same on the Internet since ad sizes were standardized a while back.

What's the charm of a banner ad?

First, it gives the advertiser an opportunity to display graphics with call to action that's contextual to the content on the page. Clever banner ads draw reader attention with a strong headline, eye-catching static or rich media creative and compelling offer. Bad ads simply go on blinking.

Second, running banner ads consistently on a site, be it online or mobile, associates the advertiser with the publisher's values and audience profile. Upscale jeweler Tiffany & Co. has mastered this art by regularly running button banners flanking the New York Times logo on the publication's site homepage.

Third, the banner serves as much purpose as a billboard, print or television ad does: reinforcing the advertiser's message in a subliminal way that may bear fruit when the brand is in the running to meet a consumer or business need.

Finally, it is one of the many tools in the marketer's online and mobile advertising kit, including search-based text ads, commercials, affiliate links, emails and text messages.

It might be self-serving for a publication to tout the banner ad, but really, what advertising options do marketers have that are so simple, cost-effective and easy to execute?

A good marketing strategy will always include a mix of brand advertising with direct response, public relations and events. Rare is the occasion where one channel or tactic fulfills the needs of a branding or customer acquisition or customer retention plan.

Banner banter
But while the online banner benefits from a larger screen, the mobile unit is constrained by screen size, browser or on-deck portal features.

On mobile, the advertiser has about six or seven words to convey the message next to an appealing image that reflects the brand values. A "Click here" link to a well-optimized mobile landing page would wrap up the message.

So what should the copy include? The brand name and an offer in a manner compelling enough to spur a click-through. Run A/B tests to see which headline, copy and image works. Don't over-promise, don't mislead. Match word with desired emotion.

Maintain the same consistency in promise once the user clicks through to the landing page. The offer or message should be relevant and strike a chord. Make the registration process simple, if the idea is to offer a white paper or elicit an inquiry.

As for the banner's image, keep in mind the mobile screen size and pixel resolution. Primary colors look good on the mobile screen. But run tests to make sure the ads are visible outdoors -- after all, it is a mobile phone that's used almost anywhere.

One more thing: Not all mobile phones have the Apple iPhone's sophisticated Safari browser, so test on different browsers for the BlackBerry, Nokia, Motorola, LG, Samsung, Helio and Palm models.

This is basic stuff, and most smart marketers are aware of the pros and cons of mobile banner ads. The best practices that apply to online banner ads are equally useful for mobile. Still, read up the Mobile Marketing Association's latest Global Mobile Advertising Guidelines.

If Google has thrown its weight behind display advertising online and in mobile -- don't forget it bought the No. 1 online display advertising giant, DoubleClick -- then we know that the banner has won another lease to life.