Note on EMI Measurement at Open Field Test Site (2) - On the Shortened Dipole Antenna

A shortened dipole antenna is recommended for use as the reference antenna for EMI measurements of electronic equip­ ment in the frequency range below 80 MHz down to 30 MHz by CISPR Publication 16, Amendment 1. Antenna factors for shortened dipople antennas are given by nomographs in the technical requirements. Discrepancies between the technical requirements for shortened dipole antennas and nomographs, and suggested revisions for achieving compatibility are given below. ( EMI Measurement, Shortened Dipole Antenna ) (3) The polarization polarity ratio shall be more than 20 dB. (4) The calibration curves (antenna factors) shall be used to obtain the electrom agnetic field strength from measured value. (5) The internal niose level in receivers shall be below the signal by more than 10 dB, even when the induced voltage becomes low by shortening the antenna elem ent. The nomographs necessary for Item (4) above are given. Fig. 1 shows an example of these nomographs, to be used when the cable and receiver characteristic impedance are 50 Ohms. The CISPR Pub. 16, Amendment 1 also gives the nomographs of 60 and 75 Ohms applicable to the characteristic impedance. As a reference antenna to be used for EMI measurement from electronic equipment, a dipole antenna tuned to half wavelength is specified for the frequency range from 30 MHz to 1 GHz . The CISPR Publication 16, Amendment 1 (Oct. 1980) specifies the use of shortened dipole antenna with antenna elem ent of a length tuned to 80 MHz for measuring the frequency below 80 MHz. For the use of this shortened dipole antenna, it also gives some precautions and nomographs to obtain antenna factors. This report first points out the discrepancies between the precautions and the nomographs, then describes the proposed improvements. 1. Conditions on the shortened dipole The key points stipulated for shortened dipole in the CISPR Pub. 16, Amendment 1 are as follows. (1) The elem ent of a shortened dipole antenna shall be longer than one-tenth of the wavelength. (2) The cable connected to the antenna shall match the input impedance of the receiver, and the VSWR on the cable shall be no more than 2. 2. Use of a shortened dipole antenna The CISPR Pub. 16 specifies the type of shortened dipole antenna in paragraph 13. and the frequency range of 30 MHz to 300 MHz in 13.3 . From this, it is reasonable to understand that the shortened dipole antenna is to be used over a range from 30 MHz to 80 MHz. The 80 MHz wavelength is 3.75 m, and the length of antenna elem ents (X./4) is 0.94 m. This is a length that the lower end of the antenna elem ent does not hit the ground plane when the height of the antenna is varied between 1 meter and 4 meters for the vertically polarized wave measurement. In view of this, this length is reasonable. Also this length of the antenna (which is tw ice the length of the elem ent = 0.9375 x 2 = 1.875 m) well satisfies the requirement for more than 1/10 of the wavelength (10 meters) at the low est frequency, 30 MHz. 3. Discrepancies in the CISPR antenna factor nomographs The antenna factors to be used for calculating the electrom agnetic field strength are given on Fig. 42 to 44 in CISPR Pub. 16 Amendment 1, when a shortened dipole antenna CH2294-7/86/000-0350 $01 .OO©!986 IEEE 350 Fa 2 (d B ) SESSION 5A 5-10-3 10-3 4 5 1 0 '