無線局やレーダーから発射されるスプリアス等のEMI測定に用いるパラボラアンテナの利得を校正する際に必要となるアンテナ間距離（校正距離）について，物理光学近似を用いた計算機シミュレーションによつて検討した.その結果，パラボラアンテナの一次放射器がパラボラの焦点にある場合には，アンテナ開口寸法と波長から求まる遠方界条件(R=2D~2/λ)程度の校正距離で，アンテナ利得を誤差0.1dB以内で決定できるが，焦点から外れた位置に一次放射器がある場合には，校正誤差が極めて大きくなることが分かった.さらに，一次放射器として対数周期ダイポールアレイアンテナ(LPDA)を用いる市販の広帯域測定用パラボラアンテナについて，利得校正の誤差と校正距離との関係を検討した.%A parabolic antenna incorporating a broadband feed antenna is often used for low-level and broadband EMI measurements such as spurious measurements of radars. Accordingly, calibration of parabolic measuring antennas is requested to determine their gain. In this connection, the present paper theoretically investigates the separation distance between transmit and receive antennas required for accurate calibration of parabolic antennas. Numerical simulation is carried out using the physical optics approximation method (PO). It evidently shows that, if the feed antenna is set at the focal point of the reflector, calibration with an antenna separation greater than R=2D~2/λ can yield the antenna gain with an error less than 0.1dB. However, if the feed antenna is away from the focal point, calibration error remarkably increases and a larger antenna separation is required. Furthermore, a commercially-available parabolic antenna using a log periodic dipole array (LPDA) as a feed is numerically studied to determine the separation distance required for the frequency range from 3 GHz to 18 GHz.
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