Hello Tom, My reaction is may be a little offset from your discussion topic, but deals with the roots of C63.4
The method in C63.4 is based on half wave tuned dipole antenna's because they: - Easy to calculate (mathematically) - Emulate a monopole radiator / receiver as close as possible The height variation prescribed deals with the problem of wave interference between the path directly from send antenna to receive antenna and the path that reflects by earth plane. The vector addition of these 2 waves has dependent of the signal frequency peaks and zero's in the transmission path, that can be cancelled out by changing the height of one of the two antenna's. For the full frequency range from 30 MHz off this requires a max height of 4 meters. For higher frequencies less height will be sufficient. The math example below this theory of height change works for mono-poles only. For broad band antennas the point of reception (especially. at 30 MHz) is not well defined, because of the physical size of the antenna. Path lengths are different for left tips and center of the antenna, the angles in the left and right wing of the antenna create different path lengths and for the LPA antenna the center of reception is different for high then for low frequencies. Anyway ! broad band antennas do not have such clear responses in peak and zero's, as dipoles will. Even more, most test engineers find that in practice -were the send antenna is replaced by a EUT, even more a diffuse radiator then a broad band antenna- this variation in height never leads to finding a peak response. The increase in path length however makes that the values measured decrease when the antenna rises. For calibration purposes however C63.4 prescribes the height variation. One has to realize however that the fatter the antenna is compared to a tuned dipole, the lesser this method of calibration gives the expected response. One way of eliminating the problem is reducing ground reflection by rising the total set-up to 20 meters above ground. This way no height variation is required, as the path via the ground is much longer as the measurement distance between the antenna pair. At Holland Signaal in the Netherlands such experiments have lead to acceptable results. The problem as indicated by you about correlation in the vertical plane makes it very clear why correlation is difficult. The path lengths for right and left tips of the receiving antenna are VERY different when its orientation is vertical. This is true also for dipoles, so this method is principally incorrect for vertical oriented antenna calibration Hope this gives the group some thoughts ...... Regards, Ing. Gert Gremmen == CE-test, qualified testing, Consultancy, Compliance tests for EMC and Electrical Safety 15 Great EMC-design tips available ! Visit our site : http://www.cetest.nl The Dutch Electronics Directory http://www.cetest.nl/electronics.htm == -----Oorspronkelijk bericht----- Van: TDonnelly [SMTP:[email protected]] Verzonden: dinsdag 20 januari 1998 17:00 Aan: PSTC Onderwerp: Antenna Correlation In ANSI C63.4 the required antenna is a half wave tuned dipole. There is an allowance that lets a broadband antenna be used provided the results can be correlated to the half wave tuned dipole. When performing radiated emissions testing to CFR 47, Part 15, using ANSI C63.4 the requirement is to height scan the receiving antenna from 1 to 4 meters. My question concerns the instance where vertical measurements are made at the lower frequencies (below 80 MHz). If the received antenna is height scanned from 1 to 4 meters at these lower frequencies the measured levels will not correlate to a half wave tuned dipole as the worst case vertical levels are typically found at lower antenna heights. It has been my experience that the height scan of the broadband antenna must be limited to the range that the half wave tuned dipole can be scanned over to demonstrate correlation. I would like to have some discussion on this issue to get a feel for how this is being addressed by others. Tom Donnelly EMC Engineer Lucent Technologies [email protected]
