Making Antenna Gain Measurements
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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 information 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 minimize the effects of reflections from below
or behind the reference antenna.
de Tom N4KG
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