Front-to-back ratio of a smooth metallic-wall conical horn

By expressing the front fields of the subject conical horn in terms of its gain, and by explicit evaluation of the known GTD expressions for its back field, a concise formula for its front-to-back (F/B) ratio is obtained. The much lower level of back field produced by a corrugated-wall horn as compared to a smooth metallic-wall horn of the same gain is then pointed out.