Gain of electromagnetic horns

The absolute gain of a standard horn is often measured by determining the transmission loss versus separation between two identical standard horns. Correction ratios are needed because the usual criterion for separation (2a2λ) may not justify the use of the far-zone power transmission formula. Using the near-field power transmission formula, the ratio between the Fraunhofer and Fresnel gain of a pyramidal electromagnetic horn has been computed as a function of horn dimensions and separation distance. The calculated corrections have been applied in the absolute gain measurement of a standard horn which was used as a calibration reference in a recent 4080-mc gain measurement of a large horn-reflector antenna. The measured gain of the standard horn at 4080 mc is 20.11 db with an accuracy of ±0.035 db. The calculated gain is 20.15 db.