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## [TowerTalk] Antenna Gain Measurements

 To: [TowerTalk] Antenna Gain Measurements n4kg@juno.com (T. A. Russell) Sun, 6 Jul 1997 08:27:10 -0600
 Making Antenna Gain Measurements Measuring antenna gain is much more difficult than first meets the eye. dBi (dB relative to an isotropic source in free space) is, to my mind, the best reference for computer modeling. Conversion to dBd (dB relative to a Dipole in FREE SPACE) merely involves subtracting 2.2 dB. The problem is that many, if not most, people then ASS-U-ME that they can measure the difference between a dipole over gound and another antenna at the same height and use that number as the gain in dBd. Unfortunately, this is NOT the case (except perhaps at some specific height where this relationship just happens to hold true). The reason that gain relative to a dipole at some height above earth is NOT equal to the actual antenna gain in dBd is that the gain of a dipole over earth is NOT constant with height. The radiation resistance of a dipole varies with height above ground due to the reflection from the earth coming back and combining in phase or out of phase, depending on the specific antenna height in wavelengths. This variation in radiation resistance is plotted in many antenna books, including the ARRL antenna books. The range of variation is from approximately 45 to 97 Ohms. Also, plotted in many of these books is the GAIN VARIATION due to this effect, which amounts to about 2 dB. Unfortuantely, the gain variation curve has been OMITTED in the latest (18th edition) of the ARRL ANTENNA BOOK. To my mind, it is crucial to understand this dipole GAIN variation as well as the impedance variation when making antenna comparisons. I hope that ARRL will see fit to reinstate this curve in future editions of the ANTENNA BOOK. Neglecting this important variable obfuscates the truth in measuring antenna gain. (Yagi's are less susceptible to gain and impedance variations with height due to the pattern rejections below the antenna.) To put this 2 db gain variation in perspective, recall that 2 dB is the usual difference between a 3 and 5 element Yagi (assuming adequate boom length in both cases). Also, 2 dB is usually the net gain obtained by properly stacking two identical Yagi antennas. A 2 dB variation in reference antenna gain can artificially inflate (or deflate) the "measured" gain of the antenna under test. The CURRENT in a dipole is maximized when the radiation resistance is minimized (P = I^2*R, for constant power I is inversely proportional to the square root of R). One such point occurs at approximately 5/8 wavelength. This corresponds to 80 ft on 40 meters (40 ft on 20 meters) which indeed have proven to be very effective heights for dipoles. While increasing height above these levels lowers the angle of radiation, the gain of a DIPOLE will fall, thus minimizing the benefit of additonal height (for a dipole) until over 1 wavelength is reached. Again, this imformation was available in older antenna books but has been dropped from the latest ARRL ANTENNA BOOK. It is important to have a complete understanding of the fundamentals to see the whole picture in making antenna comparisons. Another potential problem with using a dipole as the reference antenna is that it radiates equally in the forward and backward directions. If there are any conductive objects BEHIND the dipole (such as power lines, house wiring, gutters, metal backed insulation, fences, vehicles, water towers, hills or mountains, etc.) then reflections from these objects can also alter the apparent gain from the dipole. A Yagi of KNOWN gain vs. frequency is probably a better reference antenna since it will minize the effects of reflections from below or behind the reference antenna. de Tom N4KG -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm
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